Switch the core library to be non-null by default

See comments in c8c128d335 for further
details. This requires /relatively/ few changes - mostly cases we were
missing @Nullable annotations.

buildSrc/src/main/kotlin/cc-tweaked.java-convention.gradle.kts

@@ -6,6 +6,7 @@ import java.nio.charset.StandardCharsets
 
 plugins {
     `java-library`
+    idea
     jacoco
     checkstyle
     id("com.diffplug.spotless")
@@ -139,3 +140,12 @@ spotless {
         ktlint().editorConfigOverride(ktlintConfig)
     }
 }
+
+idea.module {
+    excludeDirs.addAll(project.files("run", "out", "logs").files)
+
+    // Force Gradle to write to inherit the output directory from the parent, instead of writing to out/xxx/classes.
+    // This is required for Loom, and we patch Forge's run configurations to work there.
+    // TODO: Submit a patch to Forge to support ProjectRootManager.
+    inheritOutputDirs = true
+}

projects/core-api/build.gradle.kts

@@ -18,6 +18,7 @@ dependencies {
     compileOnly(project(":mc-stubs"))
     compileOnlyApi(libs.jsr305)
     compileOnlyApi(libs.checkerFramework)
+    compileOnlyApi(libs.jetbrainsAnnotations)
 
     "docApi"(project(":"))
 }

projects/core-api/src/main/java/dan200/computercraft/api/lua/IArguments.java

@@ -5,6 +5,8 @@
  */
 package dan200.computercraft.api.lua;
 
+import org.jetbrains.annotations.Contract;
+
 import javax.annotation.Nullable;
 import java.nio.ByteBuffer;
 import java.util.Map;
@@ -387,7 +389,9 @@ public interface IArguments {
      * @return The argument's value, or {@code def} if none was provided.
      * @throws LuaException If the value is not a string.
      */
-    default String optString(int index, String def) throws LuaException {
+    @Nullable
+    @Contract("_, !null -> !null")
+    default String optString(int index, @Nullable String def) throws LuaException {
         return optString(index).orElse(def);
     }
 
@@ -399,7 +403,9 @@ public interface IArguments {
      * @return The argument's value, or {@code def} if none was provided.
      * @throws LuaException If the value is not a table.
      */
-    default Map<?, ?> optTable(int index, Map<Object, Object> def) throws LuaException {
+    @Nullable
+    @Contract("_, !null -> !null")
+    default Map<?, ?> optTable(int index, @Nullable Map<Object, Object> def) throws LuaException {
         return optTable(index).orElse(def);
     }
 }

projects/core-api/src/main/java/dan200/computercraft/api/peripheral/IComputerAccess.java

@@ -86,6 +86,7 @@ public interface IComputerAccess {
      * @see #unmount(String)
      * @see IMount
      */
+    @Nullable
     String mountWritable(String desiredLocation, IWritableMount mount, String driveName);
 
     /**

projects/core/build.gradle.kts

@@ -5,6 +5,7 @@ plugins {
     id("cc-tweaked.kotlin-convention")
     id("cc-tweaked.java-convention")
     id("cc-tweaked.publishing")
+    id("cc-tweaked.errorprone")
     id("cc-tweaked")
 }
 

projects/core/src/main/java/dan200/computercraft/core/apis/ApiFactories.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.apis;
 
 import dan200.computercraft.api.lua.ILuaAPIFactory;
 
-import javax.annotation.Nonnull;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashSet;
@@ -20,7 +19,7 @@ public final class ApiFactories {
     private static final Collection<ILuaAPIFactory> factories = new LinkedHashSet<>();
     private static final Collection<ILuaAPIFactory> factoriesView = Collections.unmodifiableCollection(factories);
 
-    public static synchronized void register(@Nonnull ILuaAPIFactory factory) {
+    public static synchronized void register(ILuaAPIFactory factory) {
         Objects.requireNonNull(factory, "provider cannot be null");
         factories.add(factory);
     }

projects/core/src/main/java/dan200/computercraft/core/apis/ComputerAccess.java

@@ -13,7 +13,7 @@ import dan200.computercraft.core.filesystem.FileSystemException;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.util.HashSet;
 import java.util.Objects;
 import java.util.Set;
@@ -40,8 +40,9 @@ public abstract class ComputerAccess implements IComputerAccess {
         mounts.clear();
     }
 
+    @Nullable
     @Override
-    public synchronized String mount(@Nonnull String desiredLoc, @Nonnull IMount mount, @Nonnull String driveName) {
+    public synchronized String mount(String desiredLoc, IMount mount, String driveName) {
         Objects.requireNonNull(desiredLoc, "desiredLocation cannot be null");
         Objects.requireNonNull(mount, "mount cannot be null");
         Objects.requireNonNull(driveName, "driveName cannot be null");
@@ -49,14 +50,14 @@ public abstract class ComputerAccess implements IComputerAccess {
         // Mount the location
         String location;
         var fileSystem = environment.getFileSystem();
-        if (fileSystem == null) throw new IllegalStateException("File system has not been created");
 
         synchronized (fileSystem) {
             location = findFreeLocation(desiredLoc);
             if (location != null) {
                 try {
                     fileSystem.mount(driveName, location, mount);
-                } catch (FileSystemException ignored) {
+                } catch (FileSystemException e) {
+                    throw new IllegalArgumentException(e.getMessage());
                 }
             }
         }
@@ -65,8 +66,9 @@ public abstract class ComputerAccess implements IComputerAccess {
         return location;
     }
 
+    @Nullable
     @Override
-    public synchronized String mountWritable(@Nonnull String desiredLoc, @Nonnull IWritableMount mount, @Nonnull String driveName) {
+    public synchronized String mountWritable(String desiredLoc, IWritableMount mount, String driveName) {
         Objects.requireNonNull(desiredLoc, "desiredLocation cannot be null");
         Objects.requireNonNull(mount, "mount cannot be null");
         Objects.requireNonNull(driveName, "driveName cannot be null");
@@ -74,14 +76,14 @@ public abstract class ComputerAccess implements IComputerAccess {
         // Mount the location
         String location;
         var fileSystem = environment.getFileSystem();
-        if (fileSystem == null) throw new IllegalStateException("File system has not been created");
 
         synchronized (fileSystem) {
             location = findFreeLocation(desiredLoc);
             if (location != null) {
                 try {
                     fileSystem.mountWritable(driveName, location, mount);
-                } catch (FileSystemException ignored) {
+                } catch (FileSystemException e) {
+                    throw new IllegalArgumentException(e.getMessage());
                 }
             }
         }
@@ -91,7 +93,7 @@ public abstract class ComputerAccess implements IComputerAccess {
     }
 
     @Override
-    public void unmount(String location) {
+    public void unmount(@Nullable String location) {
         if (location == null) return;
         if (!mounts.contains(location)) throw new IllegalStateException("You didn't mount this location");
 
@@ -105,17 +107,17 @@ public abstract class ComputerAccess implements IComputerAccess {
     }
 
     @Override
-    public void queueEvent(@Nonnull String event, Object... arguments) {
+    public void queueEvent(String event, @Nullable Object... arguments) {
         Objects.requireNonNull(event, "event cannot be null");
         environment.queueEvent(event, arguments);
     }
 
-    @Nonnull
     @Override
     public IWorkMonitor getMainThreadMonitor() {
         return environment.getMainThreadMonitor();
     }
 
+    @Nullable
     private String findFreeLocation(String desiredLoc) {
         try {
             var fileSystem = environment.getFileSystem();

projects/core/src/main/java/dan200/computercraft/core/apis/FSAPI.java

@@ -17,6 +17,7 @@ import dan200.computercraft.core.filesystem.FileSystem;
 import dan200.computercraft.core.filesystem.FileSystemException;
 import dan200.computercraft.core.metrics.Metrics;
 
+import javax.annotation.Nullable;
 import java.nio.file.attribute.FileTime;
 import java.util.HashMap;
 import java.util.Map;
@@ -58,7 +59,7 @@ import java.util.function.Function;
  */
 public class FSAPI implements ILuaAPI {
     private final IAPIEnvironment environment;
-    private FileSystem fileSystem = null;
+    private @Nullable FileSystem fileSystem = null;
 
     public FSAPI(IAPIEnvironment env) {
         environment = env;
@@ -79,6 +80,12 @@ public class FSAPI implements ILuaAPI {
         fileSystem = null;
     }
 
+    private FileSystem getFileSystem() {
+        var filesystem = fileSystem;
+        if (filesystem == null) throw new IllegalStateException("File system is not mounted");
+        return filesystem;
+    }
+
     /**
      * Returns a list of files in a directory.
      *
@@ -97,7 +104,7 @@ public class FSAPI implements ILuaAPI {
     public final String[] list(String path) throws LuaException {
         environment.observe(Metrics.FS_OPS);
         try {
-            return fileSystem.list(path);
+            return getFileSystem().list(path);
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -178,7 +185,7 @@ public class FSAPI implements ILuaAPI {
     @LuaFunction
     public final long getSize(String path) throws LuaException {
         try {
-            return fileSystem.getSize(path);
+            return getFileSystem().getSize(path);
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -193,7 +200,7 @@ public class FSAPI implements ILuaAPI {
     @LuaFunction
     public final boolean exists(String path) {
         try {
-            return fileSystem.exists(path);
+            return getFileSystem().exists(path);
         } catch (FileSystemException e) {
             return false;
         }
@@ -208,7 +215,7 @@ public class FSAPI implements ILuaAPI {
     @LuaFunction
     public final boolean isDir(String path) {
         try {
-            return fileSystem.isDir(path);
+            return getFileSystem().isDir(path);
         } catch (FileSystemException e) {
             return false;
         }
@@ -223,7 +230,7 @@ public class FSAPI implements ILuaAPI {
     @LuaFunction
     public final boolean isReadOnly(String path) {
         try {
-            return fileSystem.isReadOnly(path);
+            return getFileSystem().isReadOnly(path);
         } catch (FileSystemException e) {
             return false;
         }
@@ -239,7 +246,7 @@ public class FSAPI implements ILuaAPI {
     public final void makeDir(String path) throws LuaException {
         try {
             environment.observe(Metrics.FS_OPS);
-            fileSystem.makeDir(path);
+            getFileSystem().makeDir(path);
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -258,7 +265,7 @@ public class FSAPI implements ILuaAPI {
     public final void move(String path, String dest) throws LuaException {
         try {
             environment.observe(Metrics.FS_OPS);
-            fileSystem.move(path, dest);
+            getFileSystem().move(path, dest);
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -277,7 +284,7 @@ public class FSAPI implements ILuaAPI {
     public final void copy(String path, String dest) throws LuaException {
         try {
             environment.observe(Metrics.FS_OPS);
-            fileSystem.copy(path, dest);
+            getFileSystem().copy(path, dest);
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -296,7 +303,7 @@ public class FSAPI implements ILuaAPI {
     public final void delete(String path) throws LuaException {
         try {
             environment.observe(Metrics.FS_OPS);
-            fileSystem.delete(path);
+            getFileSystem().delete(path);
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -363,32 +370,32 @@ public class FSAPI implements ILuaAPI {
             switch (mode) {
                 case "r" -> {
                     // Open the file for reading, then create a wrapper around the reader
-                    var reader = fileSystem.openForRead(path, EncodedReadableHandle::openUtf8);
+                    var reader = getFileSystem().openForRead(path, EncodedReadableHandle::openUtf8);
                     return new Object[]{ new EncodedReadableHandle(reader.get(), reader) };
                 }
                 case "w" -> {
                     // Open the file for writing, then create a wrapper around the writer
-                    var writer = fileSystem.openForWrite(path, false, EncodedWritableHandle::openUtf8);
+                    var writer = getFileSystem().openForWrite(path, false, EncodedWritableHandle::openUtf8);
                     return new Object[]{ new EncodedWritableHandle(writer.get(), writer) };
                 }
                 case "a" -> {
                     // Open the file for appending, then create a wrapper around the writer
-                    var writer = fileSystem.openForWrite(path, true, EncodedWritableHandle::openUtf8);
+                    var writer = getFileSystem().openForWrite(path, true, EncodedWritableHandle::openUtf8);
                     return new Object[]{ new EncodedWritableHandle(writer.get(), writer) };
                 }
                 case "rb" -> {
                     // Open the file for binary reading, then create a wrapper around the reader
-                    var reader = fileSystem.openForRead(path, Function.identity());
+                    var reader = getFileSystem().openForRead(path, Function.identity());
                     return new Object[]{ BinaryReadableHandle.of(reader.get(), reader) };
                 }
                 case "wb" -> {
                     // Open the file for binary writing, then create a wrapper around the writer
-                    var writer = fileSystem.openForWrite(path, false, Function.identity());
+                    var writer = getFileSystem().openForWrite(path, false, Function.identity());
                     return new Object[]{ BinaryWritableHandle.of(writer.get(), writer) };
                 }
                 case "ab" -> {
                     // Open the file for binary appending, then create a wrapper around the reader
-                    var writer = fileSystem.openForWrite(path, true, Function.identity());
+                    var writer = getFileSystem().openForWrite(path, true, Function.identity());
                     return new Object[]{ BinaryWritableHandle.of(writer.get(), writer) };
                 }
                 default -> throw new LuaException("Unsupported mode");
@@ -404,7 +411,7 @@ public class FSAPI implements ILuaAPI {
      * @param path The path to get the drive of.
      * @return The name of the drive that the file is on; e.g. {@code hdd} for local files, or {@code rom} for ROM files.
      * @throws LuaException If the path doesn't exist.
-     * @cc.treturn string The name of the drive that the file is on; e.g. {@code hdd} for local files, or {@code rom} for ROM files.
+     * @cc.treturn string|nil The name of the drive that the file is on; e.g. {@code hdd} for local files, or {@code rom} for ROM files.
      * @cc.usage Print the drives of a couple of mounts:
      *
      * <pre>{@code
@@ -412,10 +419,11 @@ public class FSAPI implements ILuaAPI {
      * print("/rom/: " .. fs.getDrive("rom"))
      * }</pre>
      */
+    @Nullable
     @LuaFunction
     public final Object[] getDrive(String path) throws LuaException {
         try {
-            return fileSystem.exists(path) ? new Object[]{ fileSystem.getMountLabel(path) } : null;
+            return getFileSystem().exists(path) ? new Object[]{ getFileSystem().getMountLabel(path) } : null;
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -435,7 +443,7 @@ public class FSAPI implements ILuaAPI {
     @LuaFunction
     public final Object getFreeSpace(String path) throws LuaException {
         try {
-            var freeSpace = fileSystem.getFreeSpace(path);
+            var freeSpace = getFileSystem().getFreeSpace(path);
             return freeSpace >= 0 ? freeSpace : "unlimited";
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
@@ -459,7 +467,7 @@ public class FSAPI implements ILuaAPI {
     public final String[] find(String path) throws LuaException {
         try {
             environment.observe(Metrics.FS_OPS);
-            return fileSystem.find(path);
+            return getFileSystem().find(path);
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
@@ -476,10 +484,11 @@ public class FSAPI implements ILuaAPI {
      * @cc.since 1.87.0
      * @see #getFreeSpace To get the free space available on this drive.
      */
+    @Nullable
     @LuaFunction
     public final Object getCapacity(String path) throws LuaException {
         try {
-            var capacity = fileSystem.getCapacity(path);
+            var capacity = getFileSystem().getCapacity(path);
             return capacity.isPresent() ? capacity.getAsLong() : null;
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
@@ -508,21 +517,21 @@ public class FSAPI implements ILuaAPI {
     @LuaFunction
     public final Map<String, Object> attributes(String path) throws LuaException {
         try {
-            var attributes = fileSystem.getAttributes(path);
+            var attributes = getFileSystem().getAttributes(path);
             Map<String, Object> result = new HashMap<>();
             result.put("modification", getFileTime(attributes.lastModifiedTime()));
             result.put("modified", getFileTime(attributes.lastModifiedTime()));
             result.put("created", getFileTime(attributes.creationTime()));
             result.put("size", attributes.isDirectory() ? 0 : attributes.size());
             result.put("isDir", attributes.isDirectory());
-            result.put("isReadOnly", fileSystem.isReadOnly(path));
+            result.put("isReadOnly", getFileSystem().isReadOnly(path));
             return result;
         } catch (FileSystemException e) {
             throw new LuaException(e.getMessage());
         }
     }
 
-    private static long getFileTime(FileTime time) {
+    private static long getFileTime(@Nullable FileTime time) {
         return time == null ? 0 : time.toMillis();
     }
 }

projects/core/src/main/java/dan200/computercraft/core/apis/HTTPAPI.java

@@ -18,7 +18,6 @@ import io.netty.handler.codec.http.HttpHeaderNames;
 import io.netty.handler.codec.http.HttpHeaders;
 import io.netty.handler.codec.http.HttpMethod;
 
-import javax.annotation.Nonnull;
 import java.util.Collections;
 import java.util.Locale;
 import java.util.Map;
@@ -35,7 +34,7 @@ import static dan200.computercraft.core.apis.TableHelper.*;
 public class HTTPAPI implements ILuaAPI {
     private final IAPIEnvironment apiEnvironment;
 
-    private final ResourceGroup<CheckUrl> checkUrls = new ResourceGroup<>(ResourceGroup.DEFAULT);
+    private final ResourceGroup<CheckUrl> checkUrls = new ResourceGroup<>(() -> ResourceGroup.DEFAULT_LIMIT);
     private final ResourceGroup<HttpRequest> requests = new ResourceQueue<>(() -> CoreConfig.httpMaxRequests);
     private final ResourceGroup<Websocket> websockets = new ResourceGroup<>(() -> CoreConfig.httpMaxWebsockets);
 
@@ -155,8 +154,7 @@ public class HTTPAPI implements ILuaAPI {
         }
     }
 
-    @Nonnull
+    private HttpHeaders getHeaders(Map<?, ?> headerTable) throws LuaException {
-    private HttpHeaders getHeaders(@Nonnull Map<?, ?> headerTable) throws LuaException {
         HttpHeaders headers = new DefaultHttpHeaders();
         for (Map.Entry<?, ?> entry : headerTable.entrySet()) {
             var value = entry.getValue();

projects/core/src/main/java/dan200/computercraft/core/apis/IAPIEnvironment.java

@@ -14,7 +14,6 @@ import dan200.computercraft.core.filesystem.FileSystem;
 import dan200.computercraft.core.metrics.Metric;
 import dan200.computercraft.core.terminal.Terminal;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
 public interface IAPIEnvironment {
@@ -27,16 +26,12 @@ public interface IAPIEnvironment {
 
     int getComputerID();
 
-    @Nonnull
     ComputerEnvironment getComputerEnvironment();
 
-    @Nonnull
     GlobalEnvironment getGlobalEnvironment();
 
-    @Nonnull
     IWorkMonitor getMainThreadMonitor();
 
-    @Nonnull
     Terminal getTerminal();
 
     FileSystem getFileSystem();
@@ -45,7 +40,7 @@ public interface IAPIEnvironment {
 
     void reboot();
 
-    void queueEvent(String event, Object... args);
+    void queueEvent(String event, @Nullable Object... args);
 
     void setOutput(ComputerSide side, int output);
 
@@ -73,7 +68,7 @@ public interface IAPIEnvironment {
 
     void cancelTimer(int id);
 
-    void observe(@Nonnull Metric.Event event, long change);
+    void observe(Metric.Event event, long change);
 
-    void observe(@Nonnull Metric.Counter counter);
+    void observe(Metric.Counter counter);
 }

projects/core/src/main/java/dan200/computercraft/core/apis/LuaDateTime.java

@@ -7,6 +7,7 @@ package dan200.computercraft.core.apis;
 
 import dan200.computercraft.api.lua.LuaException;
 
+import javax.annotation.Nullable;
 import java.time.Instant;
 import java.time.LocalDateTime;
 import java.time.ZoneId;
@@ -117,6 +118,7 @@ final class LuaDateTime {
         return def;
     }
 
+    @Nullable
     private static Boolean getBoolField(Map<?, ?> table, String field) throws LuaException {
         var value = table.get(field);
         if (value instanceof Boolean || value == null) return (Boolean) value;

projects/core/src/main/java/dan200/computercraft/core/apis/OSAPI.java

@@ -13,7 +13,7 @@ import dan200.computercraft.core.util.StringUtil;
 import it.unimi.dsi.fastutil.ints.Int2ObjectMap;
 import it.unimi.dsi.fastutil.ints.Int2ObjectOpenHashMap;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.time.Instant;
 import java.time.ZoneId;
 import java.time.ZoneOffset;
@@ -40,7 +40,7 @@ public class OSAPI implements ILuaAPI {
 
     private record Alarm(double time, int day) implements Comparable<Alarm> {
         @Override
-        public int compareTo(@Nonnull Alarm o) {
+        public int compareTo(Alarm o) {
             var t = day * 24.0 + time;
             var ot = day * 24.0 + time;
             return Double.compare(t, ot);
@@ -241,9 +241,10 @@ public class OSAPI implements ILuaAPI {
      * Returns the label of the computer, or {@code nil} if none is set.
      *
      * @return The label of the computer.
-     * @cc.treturn string The label of the computer.
+     * @cc.treturn string|nil The label of the computer.
      * @cc.since 1.3
      */
+    @Nullable
     @LuaFunction({ "getComputerLabel", "computerLabel" })
     public final Object[] getComputerLabel() {
         var label = apiEnvironment.getLabel();
@@ -258,7 +259,7 @@ public class OSAPI implements ILuaAPI {
      */
     @LuaFunction
     public final void setComputerLabel(Optional<String> label) {
-        apiEnvironment.setLabel(StringUtil.normaliseLabel(label.orElse(null)));
+        apiEnvironment.setLabel(label.map(StringUtil::normaliseLabel).orElse(null));
     }
 
     /**

projects/core/src/main/java/dan200/computercraft/core/apis/PeripheralAPI.java

@@ -18,7 +18,6 @@ import dan200.computercraft.core.computer.ComputerSide;
 import dan200.computercraft.core.metrics.Metrics;
 import dan200.computercraft.core.util.LuaUtil;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.util.*;
 
@@ -99,20 +98,23 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
         }
 
         // IComputerAccess implementation
+
+        @Nullable
         @Override
-        public synchronized String mount(@Nonnull String desiredLoc, @Nonnull IMount mount, @Nonnull String driveName) {
+        public synchronized String mount(String desiredLoc, IMount mount, String driveName) {
             if (!attached) throw new NotAttachedException();
             return super.mount(desiredLoc, mount, driveName);
         }
 
+        @Nullable
         @Override
-        public synchronized String mountWritable(@Nonnull String desiredLoc, @Nonnull IWritableMount mount, @Nonnull String driveName) {
+        public synchronized String mountWritable(String desiredLoc, IWritableMount mount, String driveName) {
             if (!attached) throw new NotAttachedException();
             return super.mountWritable(desiredLoc, mount, driveName);
         }
 
         @Override
-        public synchronized void unmount(String location) {
+        public synchronized void unmount(@Nullable String location) {
             if (!attached) throw new NotAttachedException();
             super.unmount(location);
         }
@@ -124,19 +126,17 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
         }
 
         @Override
-        public void queueEvent(@Nonnull String event, Object... arguments) {
+        public void queueEvent(String event, @Nullable Object... arguments) {
             if (!attached) throw new NotAttachedException();
             super.queueEvent(event, arguments);
         }
 
-        @Nonnull
         @Override
         public String getAttachmentName() {
             if (!attached) throw new NotAttachedException();
             return side;
         }
 
-        @Nonnull
         @Override
         public Map<String, IPeripheral> getAvailablePeripherals() {
             if (!attached) throw new NotAttachedException();
@@ -153,7 +153,7 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
 
         @Nullable
         @Override
-        public IPeripheral getAvailablePeripheral(@Nonnull String name) {
+        public IPeripheral getAvailablePeripheral(String name) {
             if (!attached) throw new NotAttachedException();
 
             for (var wrapper : peripherals) {
@@ -164,7 +164,6 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
             return null;
         }
 
-        @Nonnull
         @Override
         public IWorkMonitor getMainThreadMonitor() {
             if (!attached) throw new NotAttachedException();
@@ -185,7 +184,7 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
     // IPeripheralChangeListener
 
     @Override
-    public void onPeripheralChanged(ComputerSide side, IPeripheral newPeripheral) {
+    public void onPeripheralChanged(ComputerSide side, @Nullable IPeripheral newPeripheral) {
         synchronized (peripherals) {
             var index = side.ordinal();
             if (peripherals[index] != null) {
@@ -253,6 +252,7 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
         return false;
     }
 
+    @Nullable
     @LuaFunction
     public final Object[] getType(String sideName) {
         var side = ComputerSide.valueOfInsensitive(sideName);
@@ -264,6 +264,7 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
         }
     }
 
+    @Nullable
     @LuaFunction
     public final Object[] hasType(String sideName, String type) {
         var side = ComputerSide.valueOfInsensitive(sideName);
@@ -278,6 +279,7 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
         return null;
     }
 
+    @Nullable
     @LuaFunction
     public final Object[] getMethods(String sideName) {
         var side = ComputerSide.valueOfInsensitive(sideName);

projects/core/src/main/java/dan200/computercraft/core/apis/RedstoneAPI.java

@@ -10,6 +10,8 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.core.computer.ComputerSide;
 
+import java.util.List;
+
 /**
  * Get and set redstone signals adjacent to this computer.
  * <p>
@@ -72,7 +74,7 @@ public class RedstoneAPI implements ILuaAPI {
      * @cc.since 1.2
      */
     @LuaFunction
-    public final String[] getSides() {
+    public final List<String> getSides() {
         return ComputerSide.NAMES;
     }
 

projects/core/src/main/java/dan200/computercraft/core/apis/TableHelper.java

@@ -8,7 +8,6 @@ package dan200.computercraft.core.apis;
 import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaValues;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.util.Map;
 
@@ -22,17 +21,15 @@ public final class TableHelper {
         throw new IllegalStateException("Cannot instantiate singleton " + getClass().getName());
     }
 
-    @Nonnull
+    public static LuaException badKey(String key, String expected, @Nullable Object actual) {
-    public static LuaException badKey(@Nonnull String key, @Nonnull String expected, @Nullable Object actual) {
         return badKey(key, expected, LuaValues.getType(actual));
     }
 
-    @Nonnull
+    public static LuaException badKey(String key, String expected, String actual) {
-    public static LuaException badKey(@Nonnull String key, @Nonnull String expected, @Nonnull String actual) {
         return new LuaException("bad field '" + key + "' (" + expected + " expected, got " + actual + ")");
     }
 
-    public static double getNumberField(@Nonnull Map<?, ?> table, @Nonnull String key) throws LuaException {
+    public static double getNumberField(Map<?, ?> table, String key) throws LuaException {
         var value = table.get(key);
         if (value instanceof Number) {
             return ((Number) value).doubleValue();
@@ -41,7 +38,7 @@ public final class TableHelper {
         }
     }
 
-    public static int getIntField(@Nonnull Map<?, ?> table, @Nonnull String key) throws LuaException {
+    public static int getIntField(Map<?, ?> table, String key) throws LuaException {
         var value = table.get(key);
         if (value instanceof Number) {
             return (int) ((Number) value).longValue();
@@ -50,11 +47,11 @@ public final class TableHelper {
         }
     }
 
-    public static double getRealField(@Nonnull Map<?, ?> table, @Nonnull String key) throws LuaException {
+    public static double getRealField(Map<?, ?> table, String key) throws LuaException {
         return checkReal(key, getNumberField(table, key));
     }
 
-    public static boolean getBooleanField(@Nonnull Map<?, ?> table, @Nonnull String key) throws LuaException {
+    public static boolean getBooleanField(Map<?, ?> table, String key) throws LuaException {
         var value = table.get(key);
         if (value instanceof Boolean) {
             return (Boolean) value;
@@ -63,8 +60,7 @@ public final class TableHelper {
         }
     }
 
-    @Nonnull
+    public static String getStringField(Map<?, ?> table, String key) throws LuaException {
-    public static String getStringField(@Nonnull Map<?, ?> table, @Nonnull String key) throws LuaException {
         var value = table.get(key);
         if (value instanceof String) {
             return (String) value;
@@ -74,8 +70,7 @@ public final class TableHelper {
     }
 
     @SuppressWarnings("unchecked")
-    @Nonnull
+    public static Map<Object, Object> getTableField(Map<?, ?> table, String key) throws LuaException {
-    public static Map<Object, Object> getTableField(@Nonnull Map<?, ?> table, @Nonnull String key) throws LuaException {
         var value = table.get(key);
         if (value instanceof Map) {
             return (Map<Object, Object>) value;
@@ -84,7 +79,7 @@ public final class TableHelper {
         }
     }
 
-    public static double optNumberField(@Nonnull Map<?, ?> table, @Nonnull String key, double def) throws LuaException {
+    public static double optNumberField(Map<?, ?> table, String key, double def) throws LuaException {
         var value = table.get(key);
         if (value == null) {
             return def;
@@ -95,7 +90,7 @@ public final class TableHelper {
         }
     }
 
-    public static int optIntField(@Nonnull Map<?, ?> table, @Nonnull String key, int def) throws LuaException {
+    public static int optIntField(Map<?, ?> table, String key, int def) throws LuaException {
         var value = table.get(key);
         if (value == null) {
             return def;
@@ -106,11 +101,11 @@ public final class TableHelper {
         }
     }
 
-    public static double optRealField(@Nonnull Map<?, ?> table, @Nonnull String key, double def) throws LuaException {
+    public static double optRealField(Map<?, ?> table, String key, double def) throws LuaException {
         return checkReal(key, optNumberField(table, key, def));
     }
 
-    public static boolean optBooleanField(@Nonnull Map<?, ?> table, @Nonnull String key, boolean def) throws LuaException {
+    public static boolean optBooleanField(Map<?, ?> table, String key, boolean def) throws LuaException {
         var value = table.get(key);
         if (value == null) {
             return def;
@@ -121,7 +116,8 @@ public final class TableHelper {
         }
     }
 
-    public static String optStringField(@Nonnull Map<?, ?> table, @Nonnull String key, String def) throws LuaException {
+    @Nullable
+    public static String optStringField(Map<?, ?> table, String key, @Nullable String def) throws LuaException {
         var value = table.get(key);
         if (value == null) {
             return def;
@@ -133,7 +129,7 @@ public final class TableHelper {
     }
 
     @SuppressWarnings("unchecked")
-    public static Map<Object, Object> optTableField(@Nonnull Map<?, ?> table, @Nonnull String key, Map<Object, Object> def) throws LuaException {
+    public static Map<Object, Object> optTableField(Map<?, ?> table, String key, Map<Object, Object> def) throws LuaException {
         var value = table.get(key);
         if (value == null) {
             return def;
@@ -144,7 +140,7 @@ public final class TableHelper {
         }
     }
 
-    private static double checkReal(@Nonnull String key, double value) throws LuaException {
+    private static double checkReal(String key, double value) throws LuaException {
         if (!Double.isFinite(value)) throw badKey(key, "number", getNumericType(value));
         return value;
     }

projects/core/src/main/java/dan200/computercraft/core/apis/TermAPI.java

@@ -12,7 +12,6 @@ import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.core.terminal.Terminal;
 import dan200.computercraft.core.util.Colour;
 
-import javax.annotation.Nonnull;
 
 /**
  * Interact with a computer's terminal or monitors, writing text and drawing
@@ -51,7 +50,6 @@ public class TermAPI extends TermMethods implements ILuaAPI {
         return new Object[]{ c.getR(), c.getG(), c.getB() };
     }
 
-    @Nonnull
     @Override
     public Terminal getTerminal() {
         return terminal;

projects/core/src/main/java/dan200/computercraft/core/apis/TermMethods.java

@@ -12,7 +12,6 @@ import dan200.computercraft.core.terminal.Palette;
 import dan200.computercraft.core.terminal.Terminal;
 import dan200.computercraft.core.util.StringUtil;
 
-import javax.annotation.Nonnull;
 import java.nio.ByteBuffer;
 
 /**
@@ -30,7 +29,6 @@ public abstract class TermMethods {
         return bit;
     }
 
-    @Nonnull
     public abstract Terminal getTerminal() throws LuaException;
 
     /**

projects/core/src/main/java/dan200/computercraft/core/apis/handles/BinaryReadableHandle.java

@@ -8,7 +8,9 @@ package dan200.computercraft.core.apis.handles;
 import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.core.filesystem.TrackingCloseable;
+import dan200.computercraft.core.util.Nullability;
 
+import javax.annotation.Nullable;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.nio.ByteBuffer;
@@ -28,10 +30,10 @@ public class BinaryReadableHandle extends HandleGeneric {
     private static final int BUFFER_SIZE = 8192;
 
     private final ReadableByteChannel reader;
-    final SeekableByteChannel seekable;
+    final @Nullable SeekableByteChannel seekable;
     private final ByteBuffer single = ByteBuffer.allocate(1);
 
-    BinaryReadableHandle(ReadableByteChannel reader, SeekableByteChannel seekable, TrackingCloseable closeable) {
+    BinaryReadableHandle(ReadableByteChannel reader, @Nullable SeekableByteChannel seekable, TrackingCloseable closeable) {
         super(closeable);
         this.reader = reader;
         this.seekable = seekable;
@@ -59,6 +61,7 @@ public class BinaryReadableHandle extends HandleGeneric {
      * @cc.treturn [3] string The bytes read as a string. This is returned when the {@code count} is given.
      * @cc.changed 1.80pr1 Now accepts an integer argument to read multiple bytes, returning a string instead of a number.
      */
+    @Nullable
     @LuaFunction
     public final Object[] read(Optional<Integer> countArg) throws LuaException {
         checkOpen();
@@ -130,6 +133,7 @@ public class BinaryReadableHandle extends HandleGeneric {
      * @cc.treturn string|nil The remaining contents of the file, or {@code nil} if we are at the end.
      * @cc.since 1.80pr1
      */
+    @Nullable
     @LuaFunction
     public final Object[] readAll() throws LuaException {
         checkOpen();
@@ -164,6 +168,7 @@ public class BinaryReadableHandle extends HandleGeneric {
      * @cc.since 1.80pr1.9
      * @cc.changed 1.81.0 `\r` is now stripped.
      */
+    @Nullable
     @LuaFunction
     public final Object[] readLine(Optional<Boolean> withTrailingArg) throws LuaException {
         checkOpen();
@@ -230,10 +235,11 @@ public class BinaryReadableHandle extends HandleGeneric {
          * @cc.treturn string The reason seeking failed.
          * @cc.since 1.80pr1.9
          */
+        @Nullable
         @LuaFunction
         public final Object[] seek(Optional<String> whence, Optional<Long> offset) throws LuaException {
             checkOpen();
-            return handleSeek(seekable, whence, offset);
+            return handleSeek(Nullability.assertNonNull(seekable), whence, offset);
         }
     }
 }

projects/core/src/main/java/dan200/computercraft/core/apis/handles/BinaryWritableHandle.java

@@ -10,7 +10,9 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.api.lua.LuaValues;
 import dan200.computercraft.core.filesystem.TrackingCloseable;
+import dan200.computercraft.core.util.Nullability;
 
+import javax.annotation.Nullable;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.nio.channels.FileChannel;
@@ -26,10 +28,10 @@ import java.util.Optional;
  */
 public class BinaryWritableHandle extends HandleGeneric {
     private final WritableByteChannel writer;
-    final SeekableByteChannel seekable;
+    final @Nullable SeekableByteChannel seekable;
     private final ByteBuffer single = ByteBuffer.allocate(1);
 
-    protected BinaryWritableHandle(WritableByteChannel writer, SeekableByteChannel seekable, TrackingCloseable closeable) {
+    protected BinaryWritableHandle(WritableByteChannel writer, @Nullable SeekableByteChannel seekable, TrackingCloseable closeable) {
         super(closeable);
         this.writer = writer;
         this.seekable = seekable;
@@ -86,7 +88,8 @@ public class BinaryWritableHandle extends HandleGeneric {
         try {
             // Technically this is not needed
             if (writer instanceof FileChannel channel) channel.force(false);
-        } catch (IOException ignored) {
+        } catch (IOException e) {
+            throw new LuaException(e.getMessage());
         }
     }
 
@@ -114,10 +117,11 @@ public class BinaryWritableHandle extends HandleGeneric {
          * @cc.treturn string The reason seeking failed.
          * @cc.since 1.80pr1.9
          */
+        @Nullable
         @LuaFunction
         public final Object[] seek(Optional<String> whence, Optional<Long> offset) throws LuaException {
             checkOpen();
-            return handleSeek(seekable, whence, offset);
+            return handleSeek(Nullability.assertNonNull(seekable), whence, offset);
         }
     }
 }

projects/core/src/main/java/dan200/computercraft/core/apis/handles/EncodedReadableHandle.java

@@ -9,7 +9,7 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.core.filesystem.TrackingCloseable;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.io.BufferedReader;
 import java.io.IOException;
 import java.nio.channels.Channels;
@@ -30,12 +30,12 @@ public class EncodedReadableHandle extends HandleGeneric {
 
     private final BufferedReader reader;
 
-    public EncodedReadableHandle(@Nonnull BufferedReader reader, @Nonnull TrackingCloseable closable) {
+    public EncodedReadableHandle(BufferedReader reader, TrackingCloseable closable) {
         super(closable);
         this.reader = reader;
     }
 
-    public EncodedReadableHandle(@Nonnull BufferedReader reader) {
+    public EncodedReadableHandle(BufferedReader reader) {
         this(reader, new TrackingCloseable.Impl(reader));
     }
 
@@ -48,6 +48,7 @@ public class EncodedReadableHandle extends HandleGeneric {
      * @cc.treturn string|nil The read line or {@code nil} if at the end of the file.
      * @cc.changed 1.81.0 Added option to return trailing newline.
      */
+    @Nullable
     @LuaFunction
     public final Object[] readLine(Optional<Boolean> withTrailingArg) throws LuaException {
         checkOpen();
@@ -73,6 +74,7 @@ public class EncodedReadableHandle extends HandleGeneric {
      * @throws LuaException If the file has been closed.
      * @cc.treturn nil|string The remaining contents of the file, or {@code nil} if we are at the end.
      */
+    @Nullable
     @LuaFunction
     public final Object[] readAll() throws LuaException {
         checkOpen();
@@ -102,6 +104,7 @@ public class EncodedReadableHandle extends HandleGeneric {
      * @cc.treturn string|nil The read characters, or {@code nil} if at the of the file.
      * @cc.since 1.80pr1.4
      */
+    @Nullable
     @LuaFunction
     public final Object[] read(Optional<Integer> countA) throws LuaException {
         checkOpen();

projects/core/src/main/java/dan200/computercraft/core/apis/handles/EncodedWritableHandle.java

@@ -11,7 +11,6 @@ import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.core.filesystem.TrackingCloseable;
 import dan200.computercraft.core.util.StringUtil;
 
-import javax.annotation.Nonnull;
 import java.io.BufferedWriter;
 import java.io.IOException;
 import java.nio.channels.Channels;
@@ -28,7 +27,7 @@ import java.nio.charset.StandardCharsets;
 public class EncodedWritableHandle extends HandleGeneric {
     private final BufferedWriter writer;
 
-    public EncodedWritableHandle(@Nonnull BufferedWriter writer, @Nonnull TrackingCloseable closable) {
+    public EncodedWritableHandle(BufferedWriter writer, TrackingCloseable closable) {
         super(closable);
         this.writer = writer;
     }
@@ -80,7 +79,8 @@ public class EncodedWritableHandle extends HandleGeneric {
         checkOpen();
         try {
             writer.flush();
-        } catch (IOException ignored) {
+        } catch (IOException e) {
+            throw new LuaException(e.getMessage());
         }
     }
 

projects/core/src/main/java/dan200/computercraft/core/apis/handles/HandleGeneric.java

@@ -10,16 +10,16 @@ import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.core.filesystem.TrackingCloseable;
 import dan200.computercraft.core.util.IoUtil;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.io.IOException;
 import java.nio.channels.Channel;
 import java.nio.channels.SeekableByteChannel;
 import java.util.Optional;
 
 public abstract class HandleGeneric {
-    private TrackingCloseable closeable;
+    private @Nullable TrackingCloseable closeable;
 
-    protected HandleGeneric(@Nonnull TrackingCloseable closeable) {
+    protected HandleGeneric(TrackingCloseable closeable) {
         this.closeable = closeable;
     }
 
@@ -57,6 +57,7 @@ public abstract class HandleGeneric {
      * @throws LuaException If the arguments were invalid
      * @see <a href="https://www.lua.org/manual/5.1/manual.html#pdf-file:seek">{@code file:seek} in the Lua manual.</a>
      */
+    @Nullable
     protected static Object[] handleSeek(SeekableByteChannel channel, Optional<String> whence, Optional<Long> offset) throws LuaException {
         long actualOffset = offset.orElse(0L);
         try {
@@ -75,6 +76,7 @@ public abstract class HandleGeneric {
         }
     }
 
+    @Nullable
     protected static SeekableByteChannel asSeekable(Channel channel) {
         if (!(channel instanceof SeekableByteChannel seekable)) return null;
 

projects/core/src/main/java/dan200/computercraft/core/apis/http/CheckUrl.java

@@ -7,6 +7,7 @@ package dan200.computercraft.core.apis.http;
 
 import dan200.computercraft.core.apis.IAPIEnvironment;
 
+import javax.annotation.Nullable;
 import java.net.URI;
 import java.util.concurrent.Future;
 
@@ -18,7 +19,7 @@ import java.util.concurrent.Future;
 public class CheckUrl extends Resource<CheckUrl> {
     private static final String EVENT = "http_check";
 
-    private Future<?> future;
+    private @Nullable Future<?> future;
 
     private final IAPIEnvironment environment;
     private final String address;
@@ -47,7 +48,7 @@ public class CheckUrl extends Resource<CheckUrl> {
 
             if (tryClose()) environment.queueEvent(EVENT, address, true);
         } catch (HTTPRequestException e) {
-            if (tryClose()) environment.queueEvent(EVENT, address, false, e.getMessage());
+            if (tryClose()) environment.queueEvent(EVENT, address, false, NetworkUtils.toFriendlyError(e));
         }
     }
 

projects/core/src/main/java/dan200/computercraft/core/apis/http/NetworkUtils.java

@@ -25,7 +25,7 @@ import io.netty.handler.traffic.GlobalTrafficShapingHandler;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import javax.net.ssl.SSLException;
 import javax.net.ssl.SSLHandshakeException;
 import javax.net.ssl.TrustManagerFactory;
@@ -65,10 +65,11 @@ public final class NetworkUtils {
     }
 
     private static final Object sslLock = new Object();
-    private static TrustManagerFactory trustManager;
+    private static @Nullable TrustManagerFactory trustManager;
-    private static SslContext sslContext;
+    private static @Nullable SslContext sslContext;
     private static boolean triedSslContext = false;
 
+    @Nullable
     private static TrustManagerFactory getTrustManager() {
         if (trustManager != null) return trustManager;
         synchronized (sslLock) {
@@ -86,6 +87,7 @@ public final class NetworkUtils {
         }
     }
 
+    @Nullable
     public static SslContext getSslContext() throws HTTPRequestException {
         if (sslContext != null || triedSslContext) return sslContext;
         synchronized (sslLock) {
@@ -171,10 +173,10 @@ public final class NetworkUtils {
         return bytes;
     }
 
-    @Nonnull
+    public static String toFriendlyError(Throwable cause) {
-    public static String toFriendlyError(@Nonnull Throwable cause) {
         if (cause instanceof WebSocketHandshakeException || cause instanceof HTTPRequestException) {
-            return cause.getMessage();
+            var message = cause.getMessage();
+            return message == null ? "Could not connect" : message;
         } else if (cause instanceof TooLongFrameException) {
             return "Message is too large";
         } else if (cause instanceof ReadTimeoutException || cause instanceof ConnectTimeoutException) {

projects/core/src/main/java/dan200/computercraft/core/apis/http/Resource.java

@@ -8,6 +8,7 @@ package dan200.computercraft.core.apis.http;
 import dan200.computercraft.core.util.IoUtil;
 import io.netty.channel.ChannelFuture;
 
+import javax.annotation.Nullable;
 import java.io.Closeable;
 import java.lang.ref.Reference;
 import java.lang.ref.ReferenceQueue;
@@ -94,12 +95,14 @@ public abstract class Resource<T extends Resource<T>> implements Closeable {
         return limiter.queue(thisT, () -> task.accept(thisT));
     }
 
-    protected static <T extends Closeable> T closeCloseable(T closeable) {
+    @Nullable
+    protected static <T extends Closeable> T closeCloseable(@Nullable T closeable) {
         IoUtil.closeQuietly(closeable);
         return null;
     }
 
-    protected static ChannelFuture closeChannel(ChannelFuture future) {
+    @Nullable
+    protected static ChannelFuture closeChannel(@Nullable ChannelFuture future) {
         if (future != null) {
             future.cancel(false);
             var channel = future.channel();
@@ -109,7 +112,8 @@ public abstract class Resource<T extends Resource<T>> implements Closeable {
         return null;
     }
 
-    protected static <T extends Future<?>> T closeFuture(T future) {
+    @Nullable
+    protected static <T extends Future<?>> T closeFuture(@Nullable T future) {
         if (future != null) future.cancel(true);
         return null;
     }

projects/core/src/main/java/dan200/computercraft/core/apis/http/ResourceGroup.java

@@ -18,9 +18,6 @@ import java.util.function.Supplier;
  */
 public class ResourceGroup<T extends Resource<T>> {
     public static final int DEFAULT_LIMIT = 512;
-    public static final IntSupplier DEFAULT = () -> DEFAULT_LIMIT;
-
-    private static final IntSupplier ZERO = () -> 0;
 
     final IntSupplier limit;
 
@@ -32,10 +29,6 @@ public class ResourceGroup<T extends Resource<T>> {
         this.limit = limit;
     }
 
-    public ResourceGroup() {
-        limit = ZERO;
-    }
-
     public void startup() {
         active = true;
     }

projects/core/src/main/java/dan200/computercraft/core/apis/http/ResourceQueue.java

@@ -21,9 +21,6 @@ public class ResourceQueue<T extends Resource<T>> extends ResourceGroup<T> {
         super(limit);
     }
 
-    public ResourceQueue() {
-    }
-
     @Override
     public synchronized void shutdown() {
         super.shutdown();

projects/core/src/main/java/dan200/computercraft/core/apis/http/options/Action.java

@@ -5,7 +5,6 @@
  */
 package dan200.computercraft.core.apis.http.options;
 
-import javax.annotation.Nonnull;
 import java.util.OptionalInt;
 import java.util.OptionalLong;
 
@@ -17,7 +16,6 @@ public enum Action {
         this, OptionalLong.empty(), OptionalLong.empty(), OptionalInt.empty(), OptionalInt.empty()
     );
 
-    @Nonnull
     public PartialOptions toPartial() {
         return partial;
     }

projects/core/src/main/java/dan200/computercraft/core/apis/http/options/AddressPredicate.java

@@ -9,6 +9,7 @@ import com.google.common.net.InetAddresses;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import javax.annotation.Nullable;
 import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.util.regex.Pattern;
@@ -51,6 +52,7 @@ interface AddressPredicate {
             return true;
         }
 
+        @Nullable
         public static HostRange parse(String addressStr, String prefixSizeStr) {
             int prefixSize;
             try {
@@ -79,11 +81,11 @@ interface AddressPredicate {
             var size = prefixSize;
             for (var i = 0; i < minBytes.length; i++) {
                 if (size <= 0) {
-                    minBytes[i] &= 0;
+                    minBytes[i] = (byte) 0;
-                    maxBytes[i] |= 0xFF;
+                    maxBytes[i] = (byte) 0xFF;
                 } else if (size < 8) {
-                    minBytes[i] &= 0xFF << (8 - size);
+                    minBytes[i] = (byte) (minBytes[i] & 0xFF << (8 - size));
-                    maxBytes[i] |= ~(0xFF << (8 - size));
+                    maxBytes[i] = (byte) (maxBytes[i] | ~(0xFF << (8 - size)));
                 }
 
                 size -= 8;

projects/core/src/main/java/dan200/computercraft/core/apis/http/options/AddressRule.java

@@ -10,7 +10,6 @@ import dan200.computercraft.core.apis.http.options.AddressPredicate.DomainPatter
 import dan200.computercraft.core.apis.http.options.AddressPredicate.HostRange;
 import dan200.computercraft.core.apis.http.options.AddressPredicate.PrivatePattern;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.net.Inet4Address;
 import java.net.Inet6Address;
@@ -32,14 +31,14 @@ public final class AddressRule {
     private final OptionalInt port;
     private final PartialOptions partial;
 
-    private AddressRule(@Nonnull AddressPredicate predicate, OptionalInt port, @Nonnull PartialOptions partial) {
+    private AddressRule(AddressPredicate predicate, OptionalInt port, PartialOptions partial) {
         this.predicate = predicate;
         this.partial = partial;
         this.port = port;
     }
 
     @Nullable
-    public static AddressRule parse(String filter, OptionalInt port, @Nonnull PartialOptions partial) {
+    public static AddressRule parse(String filter, OptionalInt port, PartialOptions partial) {
         var cidr = filter.indexOf('/');
         if (cidr >= 0) {
             var addressStr = filter.substring(0, cidr);
@@ -63,7 +62,7 @@ public final class AddressRule {
      * @param ipv4Address An ipv4 version of the address, if the original was an ipv6 address.
      * @return Whether it matches any of these patterns.
      */
-    private boolean matches(String domain, int port, InetAddress address, Inet4Address ipv4Address) {
+    private boolean matches(String domain, int port, InetAddress address, @Nullable Inet4Address ipv4Address) {
         if (this.port.isPresent() && this.port.getAsInt() != port) return false;
         return predicate.matches(domain)
             || predicate.matches(address)

projects/core/src/main/java/dan200/computercraft/core/apis/http/options/Options.java

@@ -5,20 +5,18 @@
  */
 package dan200.computercraft.core.apis.http.options;
 
-import javax.annotation.Nonnull;
 
 /**
  * Options about a specific domain.
  */
 public final class Options {
-    @Nonnull
     public final Action action;
     public final long maxUpload;
     public final long maxDownload;
     public final int timeout;
     public final int websocketMessage;
 
-    Options(@Nonnull Action action, long maxUpload, long maxDownload, int timeout, int websocketMessage) {
+    Options(Action action, long maxUpload, long maxDownload, int timeout, int websocketMessage) {
         this.action = action;
         this.maxUpload = maxUpload;
         this.maxDownload = maxDownload;

projects/core/src/main/java/dan200/computercraft/core/apis/http/options/PartialOptions.java

@@ -5,10 +5,13 @@
  */
 package dan200.computercraft.core.apis.http.options;
 
+import com.google.errorprone.annotations.Immutable;
+
 import javax.annotation.Nullable;
 import java.util.OptionalInt;
 import java.util.OptionalLong;
 
+@Immutable
 public final class PartialOptions {
     public static final PartialOptions DEFAULT = new PartialOptions(
         null, OptionalLong.empty(), OptionalLong.empty(), OptionalInt.empty(), OptionalInt.empty()
@@ -20,6 +23,7 @@ public final class PartialOptions {
     private final OptionalInt timeout;
     private final OptionalInt websocketMessage;
 
+    @SuppressWarnings("Immutable") // Lazily initialised, so this mutation is invisible in the public API
     private @Nullable Options options;
 
     public PartialOptions(@Nullable Action action, OptionalLong maxUpload, OptionalLong maxDownload, OptionalInt timeout, OptionalInt websocketMessage) {

projects/core/src/main/java/dan200/computercraft/core/apis/http/request/HttpRequest.java

@@ -24,6 +24,7 @@ import io.netty.handler.timeout.ReadTimeoutHandler;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import javax.annotation.Nullable;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.nio.charset.StandardCharsets;
@@ -42,9 +43,9 @@ public class HttpRequest extends Resource<HttpRequest> {
 
     private static final int MAX_REDIRECTS = 16;
 
-    private Future<?> executorFuture;
+    private @Nullable Future<?> executorFuture;
-    private ChannelFuture connectFuture;
+    private @Nullable ChannelFuture connectFuture;
-    private HttpRequestHandler currentRequest;
+    private @Nullable HttpRequestHandler currentRequest;
 
     private final IAPIEnvironment environment;
 
@@ -55,7 +56,10 @@ public class HttpRequest extends Resource<HttpRequest> {
 
     final AtomicInteger redirects;
 
-    public HttpRequest(ResourceGroup<HttpRequest> limiter, IAPIEnvironment environment, String address, String postText, HttpHeaders headers, boolean binary, boolean followRedirects) {
+    public HttpRequest(
+        ResourceGroup<HttpRequest> limiter, IAPIEnvironment environment, String address, @Nullable String postText,
+        HttpHeaders headers, boolean binary, boolean followRedirects
+    ) {
         super(limiter);
         this.environment = environment;
         this.address = address;
@@ -171,7 +175,7 @@ public class HttpRequest extends Resource<HttpRequest> {
             // Do an additional check for cancellation
             checkClosed();
         } catch (HTTPRequestException e) {
-            failure(e.getMessage());
+            failure(NetworkUtils.toFriendlyError(e));
         } catch (Exception e) {
             failure(NetworkUtils.toFriendlyError(e));
             LOG.error(Logging.HTTP_ERROR, "Error in HTTP request", e);

projects/core/src/main/java/dan200/computercraft/core/apis/http/request/HttpRequestHandler.java

@@ -20,6 +20,7 @@ import io.netty.handler.codec.http.*;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import javax.annotation.Nullable;
 import java.io.Closeable;
 import java.net.URI;
 import java.net.URISyntaxException;
@@ -27,6 +28,7 @@ import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Objects;
 
 import static dan200.computercraft.core.apis.http.request.HttpRequest.getHeaderSize;
 
@@ -47,10 +49,10 @@ public final class HttpRequestHandler extends SimpleChannelInboundHandler<HttpOb
     private final HttpMethod method;
     private final Options options;
 
-    private Charset responseCharset;
+    private @Nullable Charset responseCharset;
     private final HttpHeaders responseHeaders = new DefaultHttpHeaders();
-    private HttpResponseStatus responseStatus;
+    private @Nullable HttpResponseStatus responseStatus;
-    private CompositeByteBuf responseBody;
+    private @Nullable CompositeByteBuf responseBody;
 
     HttpRequestHandler(HttpRequest request, URI uri, HttpMethod method, Options options) {
         this.request = request;
@@ -112,7 +114,7 @@ public final class HttpRequestHandler extends SimpleChannelInboundHandler<HttpOb
                         HttpRequest.checkUri(redirect);
                     } catch (HTTPRequestException e) {
                         // If we cannot visit this uri, then fail.
-                        request.failure(e.getMessage());
+                        request.failure(NetworkUtils.toFriendlyError(e));
                         return;
                     }
 
@@ -167,6 +169,9 @@ public final class HttpRequestHandler extends SimpleChannelInboundHandler<HttpOb
     }
 
     private void sendResponse() {
+        Objects.requireNonNull(responseStatus, "Status has not been set");
+        Objects.requireNonNull(responseCharset, "Charset has not been set");
+
         // Read the ByteBuf into a channel.
         var body = responseBody;
         var bytes = body == null ? EMPTY_BYTES : NetworkUtils.toBytes(body);
@@ -203,6 +208,7 @@ public final class HttpRequestHandler extends SimpleChannelInboundHandler<HttpOb
      * @param headers The headers of the HTTP response.
      * @return The URI to redirect to, or {@code null} if no redirect should occur.
      */
+    @Nullable
     private URI getRedirect(HttpResponseStatus status, HttpHeaders headers) {
         var code = status.code();
         if (code < 300 || code > 307 || code == 304 || code == 306) return null;

projects/core/src/main/java/dan200/computercraft/core/apis/http/request/HttpResponseHandle.java

@@ -13,7 +13,6 @@ import dan200.computercraft.core.apis.handles.EncodedReadableHandle;
 import dan200.computercraft.core.apis.handles.HandleGeneric;
 import dan200.computercraft.core.asm.ObjectSource;
 
-import javax.annotation.Nonnull;
 import java.util.Collections;
 import java.util.Map;
 
@@ -31,7 +30,7 @@ public class HttpResponseHandle implements ObjectSource {
     private final String responseStatus;
     private final Map<String, String> responseHeaders;
 
-    public HttpResponseHandle(@Nonnull HandleGeneric reader, int responseCode, String responseStatus, @Nonnull Map<String, String> responseHeaders) {
+    public HttpResponseHandle(HandleGeneric reader, int responseCode, String responseStatus, Map<String, String> responseHeaders) {
         this.reader = reader;
         this.responseCode = responseCode;
         this.responseStatus = responseStatus;

projects/core/src/main/java/dan200/computercraft/core/apis/http/websocket/Websocket.java

@@ -29,6 +29,7 @@ import io.netty.handler.codec.http.websocketx.WebSocketVersion;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import javax.annotation.Nullable;
 import java.lang.ref.WeakReference;
 import java.net.URI;
 import java.net.URISyntaxException;
@@ -51,9 +52,9 @@ public class Websocket extends Resource<Websocket> {
     static final String CLOSE_EVENT = "websocket_closed";
     static final String MESSAGE_EVENT = "websocket_message";
 
-    private Future<?> executorFuture;
+    private @Nullable Future<?> executorFuture;
-    private ChannelFuture connectFuture;
+    private @Nullable ChannelFuture connectFuture;
-    private WeakReference<WebsocketHandle> websocketHandle;
+    private @Nullable WeakReference<WebsocketHandle> websocketHandle;
 
     private final IAPIEnvironment environment;
     private final URI uri;
@@ -73,12 +74,14 @@ public class Websocket extends Resource<Websocket> {
         try {
             uri = new URI(address);
         } catch (URISyntaxException ignored) {
+            // Fall through to the case below
         }
 
         if (uri == null || uri.getHost() == null) {
             try {
                 uri = new URI("ws://" + address);
             } catch (URISyntaxException ignored) {
+                // Fall through to the case below
             }
         }
 
@@ -152,7 +155,7 @@ public class Websocket extends Resource<Websocket> {
             // Do an additional check for cancellation
             checkClosed();
         } catch (HTTPRequestException e) {
-            failure(e.getMessage());
+            failure(NetworkUtils.toFriendlyError(e));
         } catch (Exception e) {
             failure(NetworkUtils.toFriendlyError(e));
             LOG.error(Logging.HTTP_ERROR, "Error in websocket", e);

projects/core/src/main/java/dan200/computercraft/core/apis/http/websocket/WebsocketHandle.java

@@ -15,7 +15,7 @@ import io.netty.channel.Channel;
 import io.netty.handler.codec.http.websocketx.BinaryWebSocketFrame;
 import io.netty.handler.codec.http.websocketx.TextWebSocketFrame;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.io.Closeable;
 import java.util.Arrays;
 import java.util.Optional;
@@ -36,7 +36,7 @@ public class WebsocketHandle implements Closeable {
     private final Options options;
     private boolean closed = false;
 
-    private Channel channel;
+    private @Nullable Channel channel;
 
     public WebsocketHandle(Websocket websocket, Options options, Channel channel) {
         this.websocket = websocket;
@@ -127,7 +127,6 @@ public class WebsocketHandle implements Closeable {
             this.timeoutId = timeoutId;
         }
 
-        @Nonnull
         @Override
         public MethodResult resume(Object[] event) {
             if (event.length >= 3 && Objects.equal(event[0], MESSAGE_EVENT) && Objects.equal(event[1], websocket.address())) {

projects/core/src/main/java/dan200/computercraft/core/asm/Generator.java

@@ -21,7 +21,6 @@ import org.objectweb.asm.Type;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
@@ -77,8 +76,7 @@ public final class Generator<T> {
         this.methodDesc = methodDesc.toString();
     }
 
-    @Nonnull
+    public List<NamedMethod<T>> getMethods(Class<?> klass) {
-    public List<NamedMethod<T>> getMethods(@Nonnull Class<?> klass) {
         try {
             return classCache.get(klass);
         } catch (ExecutionException e) {
@@ -87,7 +85,6 @@ public final class Generator<T> {
         }
     }
 
-    @Nonnull
     private List<NamedMethod<T>> build(Class<?> klass) {
         ArrayList<NamedMethod<T>> methods = null;
         for (var method : klass.getMethods()) {
@@ -121,7 +118,7 @@ public final class Generator<T> {
         return Collections.unmodifiableList(methods);
     }
 
-    private void addMethod(List<NamedMethod<T>> methods, Method method, LuaFunction annotation, PeripheralType genericType, T instance) {
+    private void addMethod(List<NamedMethod<T>> methods, Method method, LuaFunction annotation, @Nullable PeripheralType genericType, T instance) {
         var names = annotation.value();
         var isSimple = method.getReturnType() != MethodResult.class && !annotation.mainThread();
         if (names.length == 0) {
@@ -133,7 +130,6 @@ public final class Generator<T> {
         }
     }
 
-    @Nonnull
     private Optional<T> build(Method method) {
         var name = method.getDeclaringClass().getName() + "." + method.getName();
         var modifiers = method.getModifiers();
@@ -259,6 +255,7 @@ public final class Generator<T> {
         return cw.toByteArray();
     }
 
+    @Nullable
     private Boolean loadArg(MethodVisitor mw, Class<?> target, Method method, boolean unsafe, java.lang.reflect.Type genericArg, int argIndex) {
         if (genericArg == target) {
             mw.visitVarInsn(ALOAD, 1);

projects/core/src/main/java/dan200/computercraft/core/asm/GenericMethod.java

@@ -12,7 +12,7 @@ import dan200.computercraft.api.peripheral.PeripheralType;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.util.ArrayList;
@@ -30,12 +30,12 @@ public class GenericMethod {
     final Method method;
     final LuaFunction annotation;
     final Class<?> target;
-    final PeripheralType peripheralType;
+    final @Nullable PeripheralType peripheralType;
 
     private static final List<GenericSource> sources = new ArrayList<>();
-    private static List<GenericMethod> cache;
+    private static @Nullable List<GenericMethod> cache;
 
-    GenericMethod(Method method, LuaFunction annotation, Class<?> target, PeripheralType peripheralType) {
+    GenericMethod(Method method, LuaFunction annotation, Class<?> target, @Nullable PeripheralType peripheralType) {
         this.method = method;
         this.annotation = annotation;
         this.target = target;
@@ -52,7 +52,7 @@ public class GenericMethod {
         return cache = sources.stream().flatMap(GenericMethod::getMethods).toList();
     }
 
-    public static synchronized void register(@Nonnull GenericSource source) {
+    public static synchronized void register(GenericSource source) {
         Objects.requireNonNull(source, "Source cannot be null");
 
         if (cache != null) {

projects/core/src/main/java/dan200/computercraft/core/asm/LuaMethod.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.asm;
 
 import dan200.computercraft.api.lua.*;
 
-import javax.annotation.Nonnull;
 import java.util.Collections;
 
 public interface LuaMethod {
@@ -21,6 +20,5 @@ public interface LuaMethod {
 
     String[] EMPTY_METHODS = new String[0];
 
-    @Nonnull
+    MethodResult apply(Object target, ILuaContext context, IArguments args) throws LuaException;
-    MethodResult apply(@Nonnull Object target, @Nonnull ILuaContext context, @Nonnull IArguments args) throws LuaException;
 }

projects/core/src/main/java/dan200/computercraft/core/asm/NamedMethod.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.asm;
 
 import dan200.computercraft.api.peripheral.PeripheralType;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
 public final class NamedMethod<T> {
@@ -15,21 +14,19 @@ public final class NamedMethod<T> {
     private final T method;
     private final boolean nonYielding;
 
-    private final PeripheralType genericType;
+    private final @Nullable PeripheralType genericType;
 
-    NamedMethod(String name, T method, boolean nonYielding, PeripheralType genericType) {
+    NamedMethod(String name, T method, boolean nonYielding, @Nullable PeripheralType genericType) {
         this.name = name;
         this.method = method;
         this.nonYielding = nonYielding;
         this.genericType = genericType;
     }
 
-    @Nonnull
     public String getName() {
         return name;
     }
 
-    @Nonnull
     public T getMethod() {
         return method;
     }

projects/core/src/main/java/dan200/computercraft/core/asm/PeripheralMethod.java

@@ -12,7 +12,6 @@ import dan200.computercraft.api.lua.MethodResult;
 import dan200.computercraft.api.peripheral.IComputerAccess;
 import dan200.computercraft.api.peripheral.IDynamicPeripheral;
 
-import javax.annotation.Nonnull;
 import java.util.Arrays;
 
 public interface PeripheralMethod {
@@ -24,6 +23,5 @@ public interface PeripheralMethod {
         method -> (instance, context, computer, args) -> ((IDynamicPeripheral) instance).callMethod(computer, context, method, args)
     );
 
-    @Nonnull
+    MethodResult apply(Object target, ILuaContext context, IComputerAccess computer, IArguments args) throws LuaException;
-    MethodResult apply(@Nonnull Object target, @Nonnull ILuaContext context, @Nonnull IComputerAccess computer, @Nonnull IArguments args) throws LuaException;
 }

projects/core/src/main/java/dan200/computercraft/core/asm/ResultHelpers.java

@@ -7,10 +7,13 @@ package dan200.computercraft.core.asm;
 
 import dan200.computercraft.api.lua.MethodResult;
 
+import javax.annotation.Nullable;
+
 final class ResultHelpers {
     private ResultHelpers() {
     }
 
+    @Nullable
     static Object[] checkNormalResult(MethodResult result) {
         if (result.getCallback() != null) {
             // Due to how tasks are implemented, we can't currently return a MethodResult. This is an

projects/core/src/main/java/dan200/computercraft/core/computer/Computer.java

@@ -16,6 +16,7 @@ import dan200.computercraft.core.computer.mainthread.MainThreadScheduler;
 import dan200.computercraft.core.filesystem.FileSystem;
 import dan200.computercraft.core.terminal.Terminal;
 
+import javax.annotation.Nullable;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicLong;
 
@@ -37,7 +38,7 @@ public class Computer {
 
     // Various properties of the computer
     private final int id;
-    private String label = null;
+    private @Nullable String label = null;
 
     // Read-only fields about the computer
     private final GlobalEnvironment globalEnvironment;
@@ -112,7 +113,7 @@ public class Computer {
         executor.queueStop(false, true);
     }
 
-    public void queueEvent(String event, Object[] args) {
+    public void queueEvent(String event, @Nullable Object[] args) {
         executor.queueEvent(event, args);
     }
 
@@ -134,11 +135,12 @@ public class Computer {
         return id;
     }
 
+    @Nullable
     public String getLabel() {
         return label;
     }
 
-    public void setLabel(String label) {
+    public void setLabel(@Nullable String label) {
         if (!Objects.equal(label, this.label)) {
             this.label = label;
             externalOutputChanged.set(true);

projects/core/src/main/java/dan200/computercraft/core/computer/ComputerExecutor.java

@@ -19,10 +19,10 @@ import dan200.computercraft.core.metrics.Metrics;
 import dan200.computercraft.core.metrics.MetricsObserver;
 import dan200.computercraft.core.util.Colour;
 import dan200.computercraft.core.util.IoUtil;
+import dan200.computercraft.core.util.Nullability;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.io.InputStream;
 import java.util.ArrayDeque;
@@ -63,9 +63,9 @@ final class ComputerExecutor {
     private final ComputerThread scheduler;
     final TimeoutState timeout;
 
-    private FileSystem fileSystem;
+    private @Nullable FileSystem fileSystem;
 
-    private ILuaMachine machine;
+    private @Nullable ILuaMachine machine;
 
     /**
      * Whether the computer is currently on. This is set to false when a shutdown starts, or when turning on completes
@@ -126,7 +126,7 @@ final class ComputerExecutor {
      * Note, if command is not {@code null}, then some command is scheduled to be executed. Otherwise it is not
      * currently in the queue (or is currently being executed).
      */
-    private volatile StateCommand command;
+    private volatile @Nullable StateCommand command;
 
     /**
      * The queue of events which should be executed when this computer is on.
@@ -150,7 +150,7 @@ final class ComputerExecutor {
      */
     private boolean closed;
 
-    private IWritableMount rootMount;
+    private @Nullable IWritableMount rootMount;
 
     /**
      * The thread the executor is running on. This is non-null when performing work. We use this to ensure we're only
@@ -193,6 +193,8 @@ final class ComputerExecutor {
     }
 
     FileSystem getFileSystem() {
+        var fileSystem = this.fileSystem;
+        if (fileSystem == null) throw new IllegalStateException("FileSystem has not been created yet");
         return fileSystem;
     }
 
@@ -276,7 +278,7 @@ final class ComputerExecutor {
      * @param event The event's name
      * @param args  The event's arguments
      */
-    void queueEvent(@Nonnull String event, @Nullable Object[] args) {
+    void queueEvent(String event, @Nullable Object[] args) {
         // Events should be skipped if we're not on.
         if (!isOn) return;
 
@@ -320,19 +322,28 @@ final class ComputerExecutor {
         }
     }
 
+    @Nullable
     private IMount getRomMount() {
         return computer.getGlobalEnvironment().createResourceMount("computercraft", "lua/rom");
     }
 
+    @Nullable
     private IWritableMount getRootMount() {
         if (rootMount == null) rootMount = computerEnvironment.createRootMount();
         return rootMount;
     }
 
+    @Nullable
     private FileSystem createFileSystem() {
         FileSystem filesystem = null;
         try {
-            filesystem = new FileSystem("hdd", getRootMount());
+            var mount = getRootMount();
+            if (mount == null) {
+                displayFailure("Cannot mount computer mount", null);
+                return null;
+            }
+
+            filesystem = new FileSystem("hdd", mount);
 
             var romMount = getRomMount();
             if (romMount == null) {
@@ -351,12 +362,14 @@ final class ComputerExecutor {
         }
     }
 
+    @Nullable
     private ILuaMachine createLuaMachine() {
         // Load the bios resource
         InputStream biosStream = null;
         try {
             biosStream = computer.getGlobalEnvironment().createResourceFile("computercraft", "lua/bios.lua");
-        } catch (Exception ignored) {
+        } catch (Exception e) {
+            LOG.error("Failed to load BIOS", e);
         }
 
         if (biosStream == null) {
@@ -562,7 +575,7 @@ final class ComputerExecutor {
         if (machine != null) machine.printExecutionState(out);
     }
 
-    private void displayFailure(String message, String extra) {
+    private void displayFailure(String message, @Nullable String extra) {
         var terminal = computer.getTerminal();
         terminal.reset();
 
@@ -583,8 +596,8 @@ final class ComputerExecutor {
         terminal.write("ComputerCraft may be installed incorrectly");
     }
 
-    private void resumeMachine(String event, Object[] args) throws InterruptedException {
+    private void resumeMachine(@Nullable String event, @Nullable Object[] args) throws InterruptedException {
-        var result = machine.handleEvent(event, args);
+        var result = Nullability.assertNonNull(machine).handleEvent(event, args);
         interruptedEvent = result.isPause();
         if (!result.isError()) return;
 
@@ -600,6 +613,6 @@ final class ComputerExecutor {
         ERROR,
     }
 
-    private record Event(String name, Object[] args) {
+    private record Event(String name, @Nullable Object[] args) {
     }
 }

projects/core/src/main/java/dan200/computercraft/core/computer/ComputerSide.java

@@ -5,8 +5,8 @@
  */
 package dan200.computercraft.core.computer;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import java.util.List;
 
 /**
  * A side on a computer. This is relative to the direction the computer is facing.
@@ -19,7 +19,7 @@ public enum ComputerSide {
     RIGHT("right"),
     LEFT("left");
 
-    public static final String[] NAMES = new String[]{ "bottom", "top", "back", "front", "right", "left" };
+    public static final List<String> NAMES = List.of("bottom", "top", "back", "front", "right", "left");
 
     public static final int COUNT = 6;
 
@@ -31,13 +31,12 @@ public enum ComputerSide {
         this.name = name;
     }
 
-    @Nonnull
     public static ComputerSide valueOf(int side) {
         return VALUES[side];
     }
 
     @Nullable
-    public static ComputerSide valueOfInsensitive(@Nonnull String name) {
+    public static ComputerSide valueOfInsensitive(String name) {
         for (var side : VALUES) {
             if (side.name.equalsIgnoreCase(name)) return side;
         }

projects/core/src/main/java/dan200/computercraft/core/computer/ComputerSystem.java

@@ -13,7 +13,6 @@ import dan200.computercraft.api.peripheral.IPeripheral;
 import dan200.computercraft.core.apis.ComputerAccess;
 import dan200.computercraft.core.apis.IAPIEnvironment;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.util.Collections;
 import java.util.Map;
@@ -33,7 +32,6 @@ public class ComputerSystem extends ComputerAccess implements IComputerSystem {
         this.environment = environment;
     }
 
-    @Nonnull
     @Override
     public String getAttachmentName() {
         return "computer";
@@ -52,7 +50,6 @@ public class ComputerSystem extends ComputerAccess implements IComputerSystem {
         return environment.getLabel();
     }
 
-    @Nonnull
     @Override
     public Map<String, IPeripheral> getAvailablePeripherals() {
         // TODO: Should this return peripherals on the current computer?
@@ -61,7 +58,7 @@ public class ComputerSystem extends ComputerAccess implements IComputerSystem {
 
     @Nullable
     @Override
-    public IPeripheral getAvailablePeripheral(@Nonnull String name) {
+    public IPeripheral getAvailablePeripheral(String name) {
         return null;
     }
 }

projects/core/src/main/java/dan200/computercraft/core/computer/ComputerThread.java

@@ -13,7 +13,6 @@ import dan200.computercraft.core.util.ThreadUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.util.Objects;
 import java.util.TreeSet;
@@ -53,6 +52,7 @@ import java.util.concurrent.locks.ReentrantLock;
  * @see TimeoutState For how hard timeouts are handled.
  * @see ComputerExecutor For how computers actually do execution.
  */
+@SuppressWarnings("GuardedBy") // FIXME: Hard to know what the correct thing to do is.
 public final class ComputerThread {
     private static final Logger LOG = LoggerFactory.getLogger(ComputerThread.class);
     private static final ThreadFactory monitorFactory = ThreadUtils.factory("Computer-Monitor");
@@ -537,7 +537,7 @@ public final class ComputerThread {
         /**
          * The thread this runner runs on.
          */
-        final @Nonnull Thread owner;
+        final Thread owner;
 
         /**
          * Whether this runner is currently executing. This may be set to false when this worker terminates, or when

projects/core/src/main/java/dan200/computercraft/core/computer/Environment.java

@@ -16,7 +16,7 @@ import dan200.computercraft.core.terminal.Terminal;
 import it.unimi.dsi.fastutil.ints.Int2ObjectMap;
 import it.unimi.dsi.fastutil.ints.Int2ObjectOpenHashMap;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.util.Arrays;
 import java.util.Iterator;
 
@@ -56,7 +56,7 @@ public final class Environment implements IAPIEnvironment {
     private final int[] bundledInput = new int[ComputerSide.COUNT];
 
     private final IPeripheral[] peripherals = new IPeripheral[ComputerSide.COUNT];
-    private IPeripheralChangeListener peripheralListener = null;
+    private @Nullable IPeripheralChangeListener peripheralListener = null;
 
     private final Int2ObjectMap<Timer> timers = new Int2ObjectOpenHashMap<>();
     private int nextTimerToken = 0;
@@ -72,25 +72,21 @@ public final class Environment implements IAPIEnvironment {
         return computer.getID();
     }
 
-    @Nonnull
     @Override
     public ComputerEnvironment getComputerEnvironment() {
         return environment;
     }
 
-    @Nonnull
     @Override
     public GlobalEnvironment getGlobalEnvironment() {
         return computer.getGlobalEnvironment();
     }
 
-    @Nonnull
     @Override
     public IWorkMonitor getMainThreadMonitor() {
         return computer.getMainThreadMonitor();
     }
 
-    @Nonnull
     @Override
     public Terminal getTerminal() {
         return computer.getTerminal();
@@ -112,7 +108,7 @@ public final class Environment implements IAPIEnvironment {
     }
 
     @Override
-    public void queueEvent(String event, Object... args) {
+    public void queueEvent(String event, @Nullable Object... args) {
         computer.queueEvent(event, args);
     }
 
@@ -262,6 +258,7 @@ public final class Environment implements IAPIEnvironment {
         }
     }
 
+    @Nullable
     @Override
     public IPeripheral getPeripheral(ComputerSide side) {
         synchronized (peripherals) {
@@ -269,7 +266,7 @@ public final class Environment implements IAPIEnvironment {
         }
     }
 
-    public void setPeripheral(ComputerSide side, IPeripheral peripheral) {
+    public void setPeripheral(ComputerSide side, @Nullable IPeripheral peripheral) {
         synchronized (peripherals) {
             var index = side.ordinal();
             var existing = peripherals[index];
@@ -283,19 +280,20 @@ public final class Environment implements IAPIEnvironment {
     }
 
     @Override
-    public void setPeripheralChangeListener(IPeripheralChangeListener listener) {
+    public void setPeripheralChangeListener(@Nullable IPeripheralChangeListener listener) {
         synchronized (peripherals) {
             peripheralListener = listener;
         }
     }
 
+    @Nullable
     @Override
     public String getLabel() {
         return computer.getLabel();
     }
 
     @Override
-    public void setLabel(String label) {
+    public void setLabel(@Nullable String label) {
         computer.setLabel(label);
     }
 
@@ -315,12 +313,12 @@ public final class Environment implements IAPIEnvironment {
     }
 
     @Override
-    public void observe(@Nonnull Metric.Event event, long change) {
+    public void observe(Metric.Event event, long change) {
         metrics.observe(event, change);
     }
 
     @Override
-    public void observe(@Nonnull Metric.Counter counter) {
+    public void observe(Metric.Counter counter) {
         metrics.observe(counter);
     }
 

projects/core/src/main/java/dan200/computercraft/core/computer/LuaContext.java

@@ -12,7 +12,6 @@ import dan200.computercraft.core.Logging;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
 
 class LuaContext implements ILuaContext {
     private static final Logger LOG = LoggerFactory.getLogger(LuaContext.class);
@@ -23,7 +22,7 @@ class LuaContext implements ILuaContext {
     }
 
     @Override
-    public long issueMainThreadTask(@Nonnull final ILuaTask task) throws LuaException {
+    public long issueMainThreadTask(final ILuaTask task) throws LuaException {
         // Issue command
         final var taskID = computer.getUniqueTaskId();
         final Runnable iTask = () -> {

projects/core/src/main/java/dan200/computercraft/core/computer/mainthread/NoWorkMainThreadScheduler.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.computer.mainthread;
 
 import dan200.computercraft.core.metrics.MetricsObserver;
 
-import javax.annotation.Nonnull;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -38,7 +37,7 @@ public class NoWorkMainThreadScheduler implements MainThreadScheduler {
         }
 
         @Override
-        public void trackWork(long time, @Nonnull TimeUnit unit) {
+        public void trackWork(long time, TimeUnit unit) {
         }
     }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/ComboMount.java

@@ -1,117 +0,0 @@
-/*
- * This file is part of ComputerCraft - http://www.computercraft.info
- * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
- * Send enquiries to dratcliffe@gmail.com
- */
-package dan200.computercraft.core.filesystem;
-
-import dan200.computercraft.api.filesystem.FileOperationException;
-import dan200.computercraft.api.filesystem.IMount;
-
-import javax.annotation.Nonnull;
-import java.io.IOException;
-import java.nio.channels.ReadableByteChannel;
-import java.nio.file.attribute.BasicFileAttributes;
-import java.util.ArrayList;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Set;
-
-public class ComboMount implements IMount {
-    private final IMount[] parts;
-
-    public ComboMount(IMount[] parts) {
-        this.parts = parts;
-    }
-
-    // IMount implementation
-
-    @Override
-    public boolean exists(@Nonnull String path) throws IOException {
-        for (var i = parts.length - 1; i >= 0; --i) {
-            var part = parts[i];
-            if (part.exists(path)) {
-                return true;
-            }
-        }
-        return false;
-    }
-
-    @Override
-    public boolean isDirectory(@Nonnull String path) throws IOException {
-        for (var i = parts.length - 1; i >= 0; --i) {
-            var part = parts[i];
-            if (part.isDirectory(path)) {
-                return true;
-            }
-        }
-        return false;
-    }
-
-    @Override
-    public void list(@Nonnull String path, @Nonnull List<String> contents) throws IOException {
-        // Combine the lists from all the mounts
-        List<String> foundFiles = null;
-        var foundDirs = 0;
-        for (var i = parts.length - 1; i >= 0; --i) {
-            var part = parts[i];
-            if (part.exists(path) && part.isDirectory(path)) {
-                if (foundFiles == null) {
-                    foundFiles = new ArrayList<>();
-                }
-                part.list(path, foundFiles);
-                foundDirs++;
-            }
-        }
-
-        if (foundDirs == 1) {
-            // We found one directory, so we know it already doesn't contain duplicates
-            contents.addAll(foundFiles);
-        } else if (foundDirs > 1) {
-            // We found multiple directories, so filter for duplicates
-            Set<String> seen = new HashSet<>();
-            for (var file : foundFiles) {
-                if (seen.add(file)) {
-                    contents.add(file);
-                }
-            }
-        } else {
-            throw new FileOperationException(path, "Not a directory");
-        }
-    }
-
-    @Override
-    public long getSize(@Nonnull String path) throws IOException {
-        for (var i = parts.length - 1; i >= 0; --i) {
-            var part = parts[i];
-            if (part.exists(path)) {
-                return part.getSize(path);
-            }
-        }
-        throw new FileOperationException(path, "No such file");
-    }
-
-    @Nonnull
-    @Override
-    public ReadableByteChannel openForRead(@Nonnull String path) throws IOException {
-        for (var i = parts.length - 1; i >= 0; --i) {
-            var part = parts[i];
-            if (part.exists(path) && !part.isDirectory(path)) {
-                return part.openForRead(path);
-            }
-        }
-        throw new FileOperationException(path, "No such file");
-    }
-
-    @Nonnull
-    @Override
-    public BasicFileAttributes getAttributes(@Nonnull String path) throws IOException {
-        for (var i = parts.length - 1; i >= 0; --i) {
-            var part = parts[i];
-            if (part.exists(path) && !part.isDirectory(path)) {
-                return part.getAttributes(path);
-            }
-        }
-        throw new FileOperationException(path, "No such file");
-    }
-}

projects/core/src/main/java/dan200/computercraft/core/filesystem/EmptyMount.java

@@ -1,41 +0,0 @@
-/*
- * This file is part of ComputerCraft - http://www.computercraft.info
- * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
- * Send enquiries to dratcliffe@gmail.com
- */
-package dan200.computercraft.core.filesystem;
-
-import dan200.computercraft.api.filesystem.FileOperationException;
-import dan200.computercraft.api.filesystem.IMount;
-
-import javax.annotation.Nonnull;
-import java.io.IOException;
-import java.nio.channels.ReadableByteChannel;
-import java.util.List;
-
-public class EmptyMount implements IMount {
-    @Override
-    public boolean exists(@Nonnull String path) {
-        return path.isEmpty();
-    }
-
-    @Override
-    public boolean isDirectory(@Nonnull String path) {
-        return path.isEmpty();
-    }
-
-    @Override
-    public void list(@Nonnull String path, @Nonnull List<String> contents) {
-    }
-
-    @Override
-    public long getSize(@Nonnull String path) {
-        return 0;
-    }
-
-    @Nonnull
-    @Override
-    public ReadableByteChannel openForRead(@Nonnull String path) throws IOException {
-        throw new FileOperationException(path, "No such file");
-    }
-}

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileMount.java

@@ -11,7 +11,6 @@ import dan200.computercraft.api.filesystem.IWritableMount;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
 import java.io.File;
 import java.io.IOException;
 import java.nio.ByteBuffer;
@@ -25,7 +24,7 @@ import java.util.Set;
 
 public class FileMount implements IWritableMount {
     private static final Logger LOG = LoggerFactory.getLogger(FileMount.class);
-    private static final int MINIMUM_FILE_SIZE = 500;
+    private static final long MINIMUM_FILE_SIZE = 500;
     private static final Set<OpenOption> READ_OPTIONS = Collections.singleton(StandardOpenOption.READ);
     private static final Set<OpenOption> WRITE_OPTIONS = Sets.newHashSet(StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING);
     private static final Set<OpenOption> APPEND_OPTIONS = Sets.newHashSet(StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.APPEND);
@@ -41,7 +40,7 @@ public class FileMount implements IWritableMount {
         }
 
         @Override
-        public int write(@Nonnull ByteBuffer b) throws IOException {
+        public int write(ByteBuffer b) throws IOException {
             count(b.remaining());
             return inner.write(b);
         }
@@ -129,7 +128,7 @@ public class FileMount implements IWritableMount {
     // IMount implementation
 
     @Override
-    public boolean exists(@Nonnull String path) {
+    public boolean exists(String path) {
         if (!created()) return path.isEmpty();
 
         var file = getRealPath(path);
@@ -137,7 +136,7 @@ public class FileMount implements IWritableMount {
     }
 
     @Override
-    public boolean isDirectory(@Nonnull String path) {
+    public boolean isDirectory(String path) {
         if (!created()) return path.isEmpty();
 
         var file = getRealPath(path);
@@ -145,7 +144,7 @@ public class FileMount implements IWritableMount {
     }
 
     @Override
-    public void list(@Nonnull String path, @Nonnull List<String> contents) throws IOException {
+    public void list(String path, List<String> contents) throws IOException {
         if (!created()) {
             if (!path.isEmpty()) throw new FileOperationException(path, "Not a directory");
             return;
@@ -161,7 +160,7 @@ public class FileMount implements IWritableMount {
     }
 
     @Override
-    public long getSize(@Nonnull String path) throws IOException {
+    public long getSize(String path) throws IOException {
         if (!created()) {
             if (path.isEmpty()) return 0;
         } else {
@@ -172,9 +171,8 @@ public class FileMount implements IWritableMount {
         throw new FileOperationException(path, "No such file");
     }
 
-    @Nonnull
     @Override
-    public ReadableByteChannel openForRead(@Nonnull String path) throws IOException {
+    public ReadableByteChannel openForRead(String path) throws IOException {
         if (created()) {
             var file = getRealPath(path);
             if (file.exists() && !file.isDirectory()) return FileChannel.open(file.toPath(), READ_OPTIONS);
@@ -183,9 +181,8 @@ public class FileMount implements IWritableMount {
         throw new FileOperationException(path, "No such file");
     }
 
-    @Nonnull
     @Override
-    public BasicFileAttributes getAttributes(@Nonnull String path) throws IOException {
+    public BasicFileAttributes getAttributes(String path) throws IOException {
         if (created()) {
             var file = getRealPath(path);
             if (file.exists()) return Files.readAttributes(file.toPath(), BasicFileAttributes.class);
@@ -197,7 +194,7 @@ public class FileMount implements IWritableMount {
     // IWritableMount implementation
 
     @Override
-    public void makeDirectory(@Nonnull String path) throws IOException {
+    public void makeDirectory(String path) throws IOException {
         create();
         var file = getRealPath(path);
         if (file.exists()) {
@@ -224,7 +221,7 @@ public class FileMount implements IWritableMount {
     }
 
     @Override
-    public void delete(@Nonnull String path) throws IOException {
+    public void delete(String path) throws IOException {
         if (path.isEmpty()) throw new FileOperationException(path, "Access denied");
 
         if (created()) {
@@ -252,9 +249,8 @@ public class FileMount implements IWritableMount {
         }
     }
 
-    @Nonnull
     @Override
-    public WritableByteChannel openForWrite(@Nonnull String path) throws IOException {
+    public WritableByteChannel openForWrite(String path) throws IOException {
         create();
         var file = getRealPath(path);
         if (file.exists() && file.isDirectory()) throw new FileOperationException(path, "Cannot write to directory");
@@ -269,9 +265,8 @@ public class FileMount implements IWritableMount {
         return new SeekableCountingChannel(Files.newByteChannel(file.toPath(), WRITE_OPTIONS), MINIMUM_FILE_SIZE);
     }
 
-    @Nonnull
     @Override
-    public WritableByteChannel openForAppend(@Nonnull String path) throws IOException {
+    public WritableByteChannel openForAppend(String path) throws IOException {
         if (!created()) {
             throw new FileOperationException(path, "No such file");
         }
@@ -292,7 +287,6 @@ public class FileMount implements IWritableMount {
         return Math.max(capacity - usedSpace, 0);
     }
 
-    @Nonnull
     @Override
     public OptionalLong getCapacity() {
         return OptionalLong.of(capacity - MINIMUM_FILE_SIZE);

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystem.java

@@ -5,6 +5,7 @@
  */
 package dan200.computercraft.core.filesystem;
 
+import com.google.common.base.Splitter;
 import com.google.common.io.ByteStreams;
 import dan200.computercraft.api.filesystem.IFileSystem;
 import dan200.computercraft.api.filesystem.IMount;
@@ -12,7 +13,6 @@ import dan200.computercraft.api.filesystem.IWritableMount;
 import dan200.computercraft.core.CoreConfig;
 import dan200.computercraft.core.util.IoUtil;
 
-import javax.annotation.Nonnull;
 import java.io.Closeable;
 import java.io.IOException;
 import java.lang.ref.Reference;
@@ -60,16 +60,15 @@ public class FileSystem {
     }
 
     public synchronized void mount(String label, String location, IMount mount) throws FileSystemException {
-        if (mount == null) throw new NullPointerException();
+        Objects.requireNonNull(mount, "mount cannot be null");
         location = sanitizePath(location);
         if (location.contains("..")) throw new FileSystemException("Cannot mount below the root");
         mount(new MountWrapper(label, location, mount));
     }
 
     public synchronized void mountWritable(String label, String location, IWritableMount mount) throws FileSystemException {
-        if (mount == null) {
+        Objects.requireNonNull(mount, "mount cannot be null");
-            throw new NullPointerException();
+
-        }
         location = sanitizePath(location);
         if (location.contains("..")) {
             throw new FileSystemException("Cannot mount below the root");
@@ -313,7 +312,7 @@ public class FileSystem {
             } catch (AccessDeniedException e) {
                 throw new FileSystemException("Access denied");
             } catch (IOException e) {
-                throw new FileSystemException(e.getMessage());
+                throw FileSystemException.of(e);
             }
         }
     }
@@ -327,7 +326,7 @@ public class FileSystem {
         }
     }
 
-    private synchronized <T extends Closeable> FileSystemWrapper<T> openFile(@Nonnull MountWrapper mount, @Nonnull Channel channel, @Nonnull T file) throws FileSystemException {
+    private synchronized <T extends Closeable> FileSystemWrapper<T> openFile(MountWrapper mount, Channel channel, T file) throws FileSystemException {
         synchronized (openFiles) {
             if (CoreConfig.maximumFilesOpen > 0 &&
                 openFiles.size() >= CoreConfig.maximumFilesOpen) {
@@ -355,7 +354,7 @@ public class FileSystem {
         path = sanitizePath(path);
         var mount = getMount(path);
         var channel = mount.openForRead(path);
-        return channel != null ? openFile(mount, channel, open.apply(channel)) : null;
+        return openFile(mount, channel, open.apply(channel));
     }
 
     public synchronized <T extends Closeable> FileSystemWrapper<T> openForWrite(String path, boolean append, Function<WritableByteChannel, T> open) throws FileSystemException {
@@ -364,7 +363,7 @@ public class FileSystem {
         path = sanitizePath(path);
         var mount = getMount(path);
         var channel = append ? mount.openForAppend(path) : mount.openForWrite(path);
-        return channel != null ? openFile(mount, channel, open.apply(channel)) : null;
+        return openFile(mount, channel, open.apply(channel));
     }
 
     public synchronized long getFreeSpace(String path) throws FileSystemException {
@@ -373,7 +372,6 @@ public class FileSystem {
         return mount.getFreeSpace();
     }
 
-    @Nonnull
     public synchronized OptionalLong getCapacity(String path) throws FileSystemException {
         path = sanitizePath(path);
         var mount = getMount(path);
@@ -430,9 +428,8 @@ public class FileSystem {
         path = cleanName.toString();
 
         // Collapse the string into its component parts, removing ..'s
-        var parts = path.split("/");
+        var outputParts = new ArrayDeque<String>();
-        var outputParts = new Stack<String>();
+        for (var part : Splitter.on('/').split(path)) {
-        for (var part : parts) {
             if (part.isEmpty() || part.equals(".") || threeDotsPattern.matcher(part).matches()) {
                 // . is redundant
                 // ... and more are treated as .
@@ -441,37 +438,26 @@ public class FileSystem {
 
             if (part.equals("..")) {
                 // .. can cancel out the last folder entered
-                if (!outputParts.empty()) {
+                if (!outputParts.isEmpty()) {
-                    var top = outputParts.peek();
+                    var top = outputParts.peekLast();
                     if (!top.equals("..")) {
-                        outputParts.pop();
+                        outputParts.removeLast();
                     } else {
-                        outputParts.push("..");
+                        outputParts.addLast("..");
                     }
                 } else {
-                    outputParts.push("..");
+                    outputParts.addLast("..");
                 }
             } else if (part.length() >= 255) {
                 // If part length > 255 and it is the last part
-                outputParts.push(part.substring(0, 255));
+                outputParts.addLast(part.substring(0, 255));
             } else {
                 // Anything else we add to the stack
-                outputParts.push(part);
+                outputParts.addLast(part);
             }
         }
 
-        // Recombine the output parts into a new string
+        return String.join("/", outputParts);
-        var result = new StringBuilder();
-        var it = outputParts.iterator();
-        while (it.hasNext()) {
-            var part = it.next();
-            result.append(part);
-            if (it.hasNext()) {
-                result.append('/');
-            }
-        }
-
-        return result.toString();
     }
 
     public static boolean contains(String pathA, String pathB) {

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemException.java

@@ -5,6 +5,7 @@
  */
 package dan200.computercraft.core.filesystem;
 
+import java.io.IOException;
 import java.io.Serial;
 
 public class FileSystemException extends Exception {
@@ -14,4 +15,12 @@ public class FileSystemException extends Exception {
     FileSystemException(String s) {
         super(s);
     }
+
+    public static FileSystemException of(IOException e) {
+        return new FileSystemException(getMessage(e));
+    }
+
+    public static String getMessage(IOException e) {
+        return e.getMessage() == null ? "Access denied" : e.getMessage();
+    }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemWrapper.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.filesystem;
 
 import dan200.computercraft.core.util.IoUtil;
 
-import javax.annotation.Nonnull;
 import java.io.Closeable;
 import java.io.IOException;
 import java.lang.ref.ReferenceQueue;
@@ -57,7 +56,6 @@ public class FileSystemWrapper<T extends Closeable> implements TrackingCloseable
         return isOpen;
     }
 
-    @Nonnull
     public T get() {
         return closeable.get();
     }

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemWrapperMount.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.filesystem;
 
 import dan200.computercraft.api.filesystem.IFileSystem;
 
-import javax.annotation.Nonnull;
 import java.io.IOException;
 import java.nio.channels.ReadableByteChannel;
 import java.nio.channels.WritableByteChannel;
@@ -23,7 +22,7 @@ public class FileSystemWrapperMount implements IFileSystem {
     }
 
     @Override
-    public void makeDirectory(@Nonnull String path) throws IOException {
+    public void makeDirectory(String path) throws IOException {
         try {
             filesystem.makeDir(path);
         } catch (FileSystemException e) {
@@ -32,7 +31,7 @@ public class FileSystemWrapperMount implements IFileSystem {
     }
 
     @Override
-    public void delete(@Nonnull String path) throws IOException {
+    public void delete(String path) throws IOException {
         try {
             filesystem.delete(path);
         } catch (FileSystemException e) {
@@ -40,9 +39,8 @@ public class FileSystemWrapperMount implements IFileSystem {
         }
     }
 
-    @Nonnull
     @Override
-    public ReadableByteChannel openForRead(@Nonnull String path) throws IOException {
+    public ReadableByteChannel openForRead(String path) throws IOException {
         try {
             // FIXME: Think of a better way of implementing this, so closing this will close on the computer.
             return filesystem.openForRead(path, Function.identity()).get();
@@ -51,9 +49,8 @@ public class FileSystemWrapperMount implements IFileSystem {
         }
     }
 
-    @Nonnull
     @Override
-    public WritableByteChannel openForWrite(@Nonnull String path) throws IOException {
+    public WritableByteChannel openForWrite(String path) throws IOException {
         try {
             return filesystem.openForWrite(path, false, Function.identity()).get();
         } catch (FileSystemException e) {
@@ -61,9 +58,8 @@ public class FileSystemWrapperMount implements IFileSystem {
         }
     }
 
-    @Nonnull
     @Override
-    public WritableByteChannel openForAppend(@Nonnull String path) throws IOException {
+    public WritableByteChannel openForAppend(String path) throws IOException {
         try {
             return filesystem.openForWrite(path, true, Function.identity()).get();
         } catch (FileSystemException e) {
@@ -81,7 +77,7 @@ public class FileSystemWrapperMount implements IFileSystem {
     }
 
     @Override
-    public boolean exists(@Nonnull String path) throws IOException {
+    public boolean exists(String path) throws IOException {
         try {
             return filesystem.exists(path);
         } catch (FileSystemException e) {
@@ -90,7 +86,7 @@ public class FileSystemWrapperMount implements IFileSystem {
     }
 
     @Override
-    public boolean isDirectory(@Nonnull String path) throws IOException {
+    public boolean isDirectory(String path) throws IOException {
         try {
             return filesystem.isDir(path);
         } catch (FileSystemException e) {
@@ -99,7 +95,7 @@ public class FileSystemWrapperMount implements IFileSystem {
     }
 
     @Override
-    public void list(@Nonnull String path, @Nonnull List<String> contents) throws IOException {
+    public void list(String path, List<String> contents) throws IOException {
         try {
             Collections.addAll(contents, filesystem.list(path));
         } catch (FileSystemException e) {
@@ -108,7 +104,7 @@ public class FileSystemWrapperMount implements IFileSystem {
     }
 
     @Override
-    public long getSize(@Nonnull String path) throws IOException {
+    public long getSize(String path) throws IOException {
         try {
             return filesystem.getSize(path);
         } catch (FileSystemException e) {

projects/core/src/main/java/dan200/computercraft/core/filesystem/JarMount.java

@@ -8,12 +8,13 @@ package dan200.computercraft.core.filesystem;
 import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
 import com.google.common.io.ByteStreams;
+import com.google.errorprone.annotations.concurrent.LazyInit;
 import dan200.computercraft.api.filesystem.FileOperationException;
 import dan200.computercraft.api.filesystem.IMount;
 import dan200.computercraft.core.apis.handles.ArrayByteChannel;
 import dan200.computercraft.core.util.IoUtil;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.io.File;
 import java.io.FileNotFoundException;
 import java.io.IOException;
@@ -100,6 +101,7 @@ public class JarMount implements IMount {
         }
     }
 
+    @Nullable
     private FileEntry get(String path) {
         var lastEntry = root;
         var lastIndex = 0;
@@ -139,18 +141,18 @@ public class JarMount implements IMount {
     }
 
     @Override
-    public boolean exists(@Nonnull String path) {
+    public boolean exists(String path) {
         return get(path) != null;
     }
 
     @Override
-    public boolean isDirectory(@Nonnull String path) {
+    public boolean isDirectory(String path) {
         var file = get(path);
         return file != null && file.isDirectory();
     }
 
     @Override
-    public void list(@Nonnull String path, @Nonnull List<String> contents) throws IOException {
+    public void list(String path, List<String> contents) throws IOException {
         var file = get(path);
         if (file == null || !file.isDirectory()) throw new FileOperationException(path, "Not a directory");
 
@@ -158,15 +160,14 @@ public class JarMount implements IMount {
     }
 
     @Override
-    public long getSize(@Nonnull String path) throws IOException {
+    public long getSize(String path) throws IOException {
         var file = get(path);
         if (file != null) return file.size;
         throw new FileOperationException(path, "No such file");
     }
 
-    @Nonnull
     @Override
-    public ReadableByteChannel openForRead(@Nonnull String path) throws IOException {
+    public ReadableByteChannel openForRead(String path) throws IOException {
         var file = get(path);
         if (file != null && !file.isDirectory()) {
             var contents = CONTENTS_CACHE.getIfPresent(file);
@@ -191,9 +192,8 @@ public class JarMount implements IMount {
         throw new FileOperationException(path, "No such file");
     }
 
-    @Nonnull
     @Override
-    public BasicFileAttributes getAttributes(@Nonnull String path) throws IOException {
+    public BasicFileAttributes getAttributes(String path) throws IOException {
         var file = get(path);
         if (file != null) {
             var entry = zip.getEntry(file.path);
@@ -204,8 +204,12 @@ public class JarMount implements IMount {
     }
 
     private static class FileEntry {
+        @LazyInit // TODO: Might be nicer to use @Initializer on setup(...)
         String path;
+
         long size;
+
+        @Nullable
         Map<String, FileEntry> children;
 
         void setup(ZipEntry entry) {
@@ -285,6 +289,7 @@ public class JarMount implements IMount {
             return entry.getSize();
         }
 
+        @Nullable
         @Override
         public Object fileKey() {
             return null;
@@ -292,7 +297,7 @@ public class JarMount implements IMount {
 
         private static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
 
-        private static FileTime orEpoch(FileTime time) {
+        private static FileTime orEpoch(@Nullable FileTime time) {
             return time == null ? EPOCH : time;
         }
     }

projects/core/src/main/java/dan200/computercraft/core/filesystem/MountWrapper.java

@@ -9,7 +9,6 @@ import dan200.computercraft.api.filesystem.FileOperationException;
 import dan200.computercraft.api.filesystem.IMount;
 import dan200.computercraft.api.filesystem.IWritableMount;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.io.IOException;
 import java.nio.channels.ReadableByteChannel;
@@ -24,7 +23,7 @@ class MountWrapper {
     private final String location;
 
     private final IMount mount;
-    private final IWritableMount writableMount;
+    private final @Nullable IWritableMount writableMount;
 
     MountWrapper(String label, String location, IMount mount) {
         this.label = label;
@@ -107,7 +106,6 @@ class MountWrapper {
         }
     }
 
-    @Nonnull
     public BasicFileAttributes getAttributes(String path) throws FileSystemException {
         path = toLocal(path);
         try {
@@ -213,9 +211,9 @@ class MountWrapper {
         return FileSystem.toLocal(path, location);
     }
 
-    private FileSystemException localExceptionOf(@Nullable String localPath, @Nonnull IOException e) {
+    private FileSystemException localExceptionOf(@Nullable String localPath, IOException e) {
         if (!location.isEmpty() && e instanceof FileOperationException ex) {
-            if (ex.getFilename() != null) return localExceptionOf(ex.getFilename(), ex.getMessage());
+            if (ex.getFilename() != null) return localExceptionOf(ex.getFilename(), FileSystemException.getMessage(ex));
         }
 
         if (e instanceof java.nio.file.FileSystemException ex) {
@@ -225,7 +223,7 @@ class MountWrapper {
             return localPath == null ? new FileSystemException(message) : localExceptionOf(localPath, message);
         }
 
-        return new FileSystemException(e.getMessage());
+        return FileSystemException.of(e);
     }
 
     private FileSystemException localExceptionOf(String path, String message) {

projects/core/src/main/java/dan200/computercraft/core/filesystem/SubMount.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.filesystem;
 
 import dan200.computercraft.api.filesystem.IMount;
 
-import javax.annotation.Nonnull;
 import java.io.IOException;
 import java.nio.channels.ReadableByteChannel;
 import java.nio.file.attribute.BasicFileAttributes;
@@ -23,34 +22,32 @@ public class SubMount implements IMount {
     }
 
     @Override
-    public boolean exists(@Nonnull String path) throws IOException {
+    public boolean exists(String path) throws IOException {
         return parent.exists(getFullPath(path));
     }
 
     @Override
-    public boolean isDirectory(@Nonnull String path) throws IOException {
+    public boolean isDirectory(String path) throws IOException {
         return parent.isDirectory(getFullPath(path));
     }
 
     @Override
-    public void list(@Nonnull String path, @Nonnull List<String> contents) throws IOException {
+    public void list(String path, List<String> contents) throws IOException {
         parent.list(getFullPath(path), contents);
     }
 
     @Override
-    public long getSize(@Nonnull String path) throws IOException {
+    public long getSize(String path) throws IOException {
         return parent.getSize(getFullPath(path));
     }
 
-    @Nonnull
     @Override
-    public ReadableByteChannel openForRead(@Nonnull String path) throws IOException {
+    public ReadableByteChannel openForRead(String path) throws IOException {
         return parent.openForRead(getFullPath(path));
     }
 
-    @Nonnull
     @Override
-    public BasicFileAttributes getAttributes(@Nonnull String path) throws IOException {
+    public BasicFileAttributes getAttributes(String path) throws IOException {
         return parent.getAttributes(getFullPath(path));
     }
 

projects/core/src/main/java/dan200/computercraft/core/lua/BasicFunction.java

@@ -28,14 +28,14 @@ class BasicFunction extends VarArgFunction {
     private final LuaMethod method;
     private final Object instance;
     private final ILuaContext context;
-    private final String name;
+    private final String funcName;
 
     BasicFunction(CobaltLuaMachine machine, LuaMethod method, Object instance, ILuaContext context, String name) {
         this.machine = machine;
         this.method = method;
         this.instance = instance;
         this.context = context;
-        this.name = name;
+        funcName = name;
     }
 
     @Override
@@ -47,7 +47,7 @@ class BasicFunction extends VarArgFunction {
         } catch (LuaException e) {
             throw wrap(e);
         } catch (Throwable t) {
-            LOG.error(Logging.JAVA_ERROR, "Error calling {} on {}", name, instance, t);
+            LOG.error(Logging.JAVA_ERROR, "Error calling {} on {}", funcName, instance, t);
             throw new LuaError("Java Exception Thrown: " + t, 0);
         } finally {
             arguments.close();

projects/core/src/main/java/dan200/computercraft/core/lua/CobaltLuaMachine.java

@@ -27,7 +27,6 @@ import org.squiddev.cobalt.debug.DebugState;
 import org.squiddev.cobalt.lib.*;
 import org.squiddev.cobalt.lib.platform.VoidResourceManipulator;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.io.InputStream;
 import java.io.Serial;
@@ -58,11 +57,11 @@ public class CobaltLuaMachine implements ILuaMachine {
     private final TimeoutDebugHandler debug;
     private final ILuaContext context;
 
-    private LuaState state;
+    private @Nullable LuaState state;
-    private LuaTable globals;
+    private @Nullable LuaTable globals;
 
-    private LuaThread mainRoutine = null;
+    private @Nullable LuaThread mainRoutine = null;
-    private String eventFilter = null;
+    private @Nullable String eventFilter = null;
 
     public CobaltLuaMachine(MachineEnvironment environment) {
         timeout = environment.timeout();
@@ -115,7 +114,9 @@ public class CobaltLuaMachine implements ILuaMachine {
     }
 
     @Override
-    public void addAPI(@Nonnull ILuaAPI api) {
+    public void addAPI(ILuaAPI api) {
+        if (globals == null) throw new IllegalStateException("Machine has been closed");
+
         // Add the methods of an API to the global table
         var table = wrapLuaObject(api);
         if (table == null) {
@@ -128,9 +129,9 @@ public class CobaltLuaMachine implements ILuaMachine {
     }
 
     @Override
-    public MachineResult loadBios(@Nonnull InputStream bios) {
+    public MachineResult loadBios(InputStream bios) {
-        // Begin executing a file (ie, the bios)
+        if (mainRoutine != null) throw new IllegalStateException("Already set up the machine");
-        if (mainRoutine != null) return MachineResult.OK;
+        if (state == null || globals == null) throw new IllegalStateException("Machine has been destroyed.");
 
         try {
             var value = LoadState.load(state, bios, "@bios.lua", globals);
@@ -147,8 +148,8 @@ public class CobaltLuaMachine implements ILuaMachine {
     }
 
     @Override
-    public MachineResult handleEvent(String eventName, Object[] arguments) {
+    public MachineResult handleEvent(@Nullable String eventName, @Nullable Object[] arguments) {
-        if (mainRoutine == null) return MachineResult.OK;
+        if (mainRoutine == null || state == null) throw new IllegalStateException("Machine has been closed");
 
         if (eventFilter != null && eventName != null && !eventName.equals(eventFilter) && !eventName.equals("terminate")) {
             return MachineResult.OK;
@@ -232,13 +233,13 @@ public class CobaltLuaMachine implements ILuaMachine {
         try {
             if (table.keyCount() == 0) return null;
         } catch (LuaError ignored) {
+            // next should never throw on nil.
         }
 
         return table;
     }
 
-    @Nonnull
+    private LuaValue toValue(@Nullable Object object, @Nullable IdentityHashMap<Object, LuaValue> values) {
-    private LuaValue toValue(@Nullable Object object, @Nullable Map<Object, LuaValue> values) {
         if (object == null) return Constants.NIL;
         if (object instanceof Number num) return valueOf(num.doubleValue());
         if (object instanceof Boolean bool) return valueOf(bool);
@@ -304,11 +305,11 @@ public class CobaltLuaMachine implements ILuaMachine {
         return Constants.NIL;
     }
 
-    Varargs toValues(Object[] objects) {
+    Varargs toValues(@Nullable Object[] objects) {
         if (objects == null || objects.length == 0) return Constants.NONE;
         if (objects.length == 1) return toValue(objects[0], null);
 
-        Map<Object, LuaValue> result = new IdentityHashMap<>(0);
+        var result = new IdentityHashMap<Object, LuaValue>(0);
         var values = new LuaValue[objects.length];
         for (var i = 0; i < values.length; i++) {
             var object = objects[i];
@@ -317,7 +318,8 @@ public class CobaltLuaMachine implements ILuaMachine {
         return varargsOf(values);
     }
 
-    static Object toObject(LuaValue value, Map<LuaValue, Object> objects) {
+    @Nullable
+    static Object toObject(LuaValue value, @Nullable IdentityHashMap<LuaValue, Object> objects) {
         switch (value.type()) {
             case Constants.TNIL:
             case Constants.TNONE:
@@ -448,6 +450,7 @@ public class CobaltLuaMachine implements ILuaMachine {
         @Serial
         private static final long serialVersionUID = 7954092008586367501L;
 
+        @SuppressWarnings("StaticAssignmentOfThrowable")
         static final HardAbortError INSTANCE = new HardAbortError();
 
         private HardAbortError() {

projects/core/src/main/java/dan200/computercraft/core/lua/ILuaMachine.java

@@ -8,7 +8,6 @@ package dan200.computercraft.core.lua;
 import dan200.computercraft.api.lua.IDynamicLuaObject;
 import dan200.computercraft.api.lua.ILuaAPI;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.io.InputStream;
 
@@ -32,7 +31,7 @@ public interface ILuaMachine {
      *
      * @param api The API to register.
      */
-    void addAPI(@Nonnull ILuaAPI api);
+    void addAPI(ILuaAPI api);
 
     /**
      * Create a function from the provided program, and set it up to run when {@link #handleEvent(String, Object[])} is
@@ -43,7 +42,7 @@ public interface ILuaMachine {
      * @param bios The stream containing the boot program.
      * @return The result of loading this machine. Will either be OK, or the error message when loading the bios.
      */
-    MachineResult loadBios(@Nonnull InputStream bios);
+    MachineResult loadBios(InputStream bios);
 
     /**
      * Resume the machine, either starting or resuming the coroutine.

projects/core/src/main/java/dan200/computercraft/core/lua/MachineResult.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.lua;
 
 import dan200.computercraft.core.computer.TimeoutState;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.io.InputStream;
 
@@ -42,19 +41,19 @@ public final class MachineResult {
 
     private final boolean error;
     private final boolean pause;
-    private final String message;
+    private final @Nullable String message;
 
-    private MachineResult(boolean error, boolean pause, String message) {
+    private MachineResult(boolean error, boolean pause, @Nullable String message) {
         this.pause = pause;
         this.message = message;
         this.error = error;
     }
 
-    public static MachineResult error(@Nonnull String error) {
+    public static MachineResult error(String error) {
         return new MachineResult(true, false, error);
     }
 
-    public static MachineResult error(@Nonnull Exception error) {
+    public static MachineResult error(Exception error) {
         return new MachineResult(true, false, error.getMessage());
     }
 

projects/core/src/main/java/dan200/computercraft/core/lua/ResultInterpreterFunction.java

@@ -17,7 +17,6 @@ import org.squiddev.cobalt.*;
 import org.squiddev.cobalt.debug.DebugFrame;
 import org.squiddev.cobalt.function.ResumableVarArgFunction;
 
-import javax.annotation.Nonnull;
 
 /**
  * Calls a {@link LuaMethod}, and interprets the resulting {@link MethodResult}, either returning the result or yielding
@@ -26,7 +25,6 @@ import javax.annotation.Nonnull;
 class ResultInterpreterFunction extends ResumableVarArgFunction<ResultInterpreterFunction.Container> {
     private static final Logger LOG = LoggerFactory.getLogger(ResultInterpreterFunction.class);
 
-    @Nonnull
     static class Container {
         ILuaCallback callback;
         final int errorAdjust;
@@ -41,14 +39,14 @@ class ResultInterpreterFunction extends ResumableVarArgFunction<ResultInterprete
     private final LuaMethod method;
     private final Object instance;
     private final ILuaContext context;
-    private final String name;
+    private final String funcName;
 
     ResultInterpreterFunction(CobaltLuaMachine machine, LuaMethod method, Object instance, ILuaContext context, String name) {
         this.machine = machine;
         this.method = method;
         this.instance = instance;
         this.context = context;
-        this.name = name;
+        funcName = name;
     }
 
     @Override
@@ -60,7 +58,7 @@ class ResultInterpreterFunction extends ResumableVarArgFunction<ResultInterprete
         } catch (LuaException e) {
             throw wrap(e, 0);
         } catch (Throwable t) {
-            LOG.error(Logging.JAVA_ERROR, "Error calling {} on {}", name, instance, t);
+            LOG.error(Logging.JAVA_ERROR, "Error calling {} on {}", funcName, instance, t);
             throw new LuaError("Java Exception Thrown: " + t, 0);
         } finally {
             arguments.close();
@@ -84,7 +82,7 @@ class ResultInterpreterFunction extends ResumableVarArgFunction<ResultInterprete
         } catch (LuaException e) {
             throw wrap(e, container.errorAdjust);
         } catch (Throwable t) {
-            LOG.error(Logging.JAVA_ERROR, "Error calling {} on {}", name, container.callback, t);
+            LOG.error(Logging.JAVA_ERROR, "Error calling {} on {}", funcName, container.callback, t);
             throw new LuaError("Java Exception Thrown: " + t, 0);
         }
 

projects/core/src/main/java/dan200/computercraft/core/lua/TableImpl.java

@@ -9,7 +9,7 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaValues;
 import org.squiddev.cobalt.*;
 
-import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.util.*;
 
 import static dan200.computercraft.api.lua.LuaValues.badTableItem;
@@ -18,7 +18,7 @@ import static dan200.computercraft.api.lua.LuaValues.getNumericType;
 class TableImpl implements dan200.computercraft.api.lua.LuaTable<Object, Object> {
     private final VarargArguments arguments;
     private final LuaTable table;
-    private Map<Object, Object> backingMap;
+    private @Nullable Map<Object, Object> backingMap;
 
     TableImpl(VarargArguments arguments, LuaTable table) {
         this.arguments = arguments;
@@ -61,7 +61,6 @@ class TableImpl implements dan200.computercraft.api.lua.LuaTable<Object, Object>
         }
     }
 
-    @Nonnull
     private LuaValue getImpl(Object o) {
         checkValid();
         if (o instanceof String) return table.rawget((String) o);
@@ -74,12 +73,12 @@ class TableImpl implements dan200.computercraft.api.lua.LuaTable<Object, Object>
         return !getImpl(o).isNil();
     }
 
+    @Nullable
     @Override
     public Object get(Object o) {
         return CobaltLuaMachine.toObject(getImpl(o), null);
     }
 
-    @Nonnull
     private Map<Object, Object> getBackingMap() {
         checkValid();
         if (backingMap != null) return backingMap;
@@ -93,19 +92,16 @@ class TableImpl implements dan200.computercraft.api.lua.LuaTable<Object, Object>
         return getBackingMap().containsKey(o);
     }
 
-    @Nonnull
     @Override
     public Set<Object> keySet() {
         return getBackingMap().keySet();
     }
 
-    @Nonnull
     @Override
     public Collection<Object> values() {
         return getBackingMap().values();
     }
 
-    @Nonnull
     @Override
     public Set<Entry<Object, Object>> entrySet() {
         return getBackingMap().entrySet();

projects/core/src/main/java/dan200/computercraft/core/lua/VarargArguments.java

@@ -10,7 +10,6 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaValues;
 import org.squiddev.cobalt.*;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.nio.ByteBuffer;
 import java.util.Optional;
@@ -20,7 +19,7 @@ final class VarargArguments implements IArguments {
 
     boolean closed;
     private final Varargs varargs;
-    private Object[] cache;
+    private @Nullable Object[] cache;
 
     private VarargArguments(Varargs varargs) {
         this.varargs = varargs;
@@ -72,7 +71,6 @@ final class VarargArguments implements IArguments {
         return value instanceof LuaInteger ? value.toInteger() : (long) LuaValues.checkFinite(index, value.toDouble());
     }
 
-    @Nonnull
     @Override
     public ByteBuffer getBytes(int index) throws LuaException {
         var value = varargs.arg(index + 1);
@@ -92,7 +90,6 @@ final class VarargArguments implements IArguments {
         return Optional.of(ByteBuffer.wrap(str.bytes, str.offset, str.length).asReadOnlyBuffer());
     }
 
-    @Nonnull
     @Override
     public dan200.computercraft.api.lua.LuaTable<?, ?> getTableUnsafe(int index) throws LuaException {
         if (closed) {
@@ -104,7 +101,6 @@ final class VarargArguments implements IArguments {
         return new TableImpl(this, (LuaTable) value);
     }
 
-    @Nonnull
     @Override
     public Optional<dan200.computercraft.api.lua.LuaTable<?, ?>> optTableUnsafe(int index) throws LuaException {
         if (closed) {

projects/core/src/main/java/dan200/computercraft/core/package-info.java

@@ -0,0 +1,22 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+
+/**
+ * ComputerCraft's core Lua runtime and APIs.
+ * <p>
+ * This is not considered part of the stable API, and so should not be consumed by other Minecraft mods. However,
+ * emulators or other CC-tooling may find this useful.
+ */
+@DefaultQualifier(value = NonNull.class, locations = {
+    TypeUseLocation.RETURN,
+    TypeUseLocation.PARAMETER,
+    TypeUseLocation.FIELD,
+})
+package dan200.computercraft.core;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;

projects/core/src/main/java/dan200/computercraft/core/terminal/Palette.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.terminal;
 
 import dan200.computercraft.core.util.Colour;
 
-import javax.annotation.Nonnull;
 
 public class Palette {
     public static final int PALETTE_SIZE = 16;
@@ -46,7 +45,7 @@ public class Palette {
     }
 
     public double[] getColour(int i) {
-        return i >= 0 && i < PALETTE_SIZE ? colours[i] : null;
+        return colours[i];
     }
 
     /**
@@ -58,7 +57,6 @@ public class Palette {
      * @param i The colour index.
      * @return The number as a tuple of bytes.
      */
-    @Nonnull
     public byte[] getRenderColours(int i) {
         return byteColours[i];
     }

projects/core/src/main/java/dan200/computercraft/core/terminal/Terminal.java

@@ -7,7 +7,6 @@ package dan200.computercraft.core.terminal;
 
 import dan200.computercraft.core.util.Colour;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.nio.ByteBuffer;
 
@@ -36,7 +35,7 @@ public class Terminal {
         this(width, height, colour, null);
     }
 
-    public Terminal(int width, int height, boolean colour, Runnable changedCallback) {
+    public Terminal(int width, int height, boolean colour, @Nullable Runnable changedCallback) {
         this.width = width;
         this.height = height;
         this.colour = colour;
@@ -163,7 +162,6 @@ public class Terminal {
         return cursorBackgroundColour;
     }
 
-    @Nonnull
     public Palette getPalette() {
         return palette;
     }
@@ -234,10 +232,7 @@ public class Terminal {
     }
 
     public synchronized TextBuffer getLine(int y) {
-        if (y >= 0 && y < height) {
+        return text[y];
-            return text[y];
-        }
-        return null;
     }
 
     public synchronized void setLine(int y, String text, String textColour, String backgroundColour) {
@@ -248,17 +243,11 @@ public class Terminal {
     }
 
     public synchronized TextBuffer getTextColourLine(int y) {
-        if (y >= 0 && y < height) {
+        return textColour[y];
-            return textColour[y];
-        }
-        return null;
     }
 
     public synchronized TextBuffer getBackgroundColourLine(int y) {
-        if (y >= 0 && y < height) {
+        return backgroundColour[y];
-            return backgroundColour[y];
-        }
-        return null;
     }
 
     public final void setChanged() {

projects/core/src/main/java/dan200/computercraft/core/util/Colour.java

@@ -5,6 +5,8 @@
  */
 package dan200.computercraft.core.util;
 
+import javax.annotation.Nullable;
+
 public enum Colour {
     BLACK(0x111111),
     RED(0xcc4c4c),
@@ -29,6 +31,7 @@ public enum Colour {
         return Colour.VALUES[colour];
     }
 
+    @Nullable
     public static Colour fromHex(int colour) {
         for (var entry : VALUES) {
             if (entry.getHex() == colour) return entry;

projects/core/src/main/java/dan200/computercraft/core/util/IoUtil.java

@@ -17,6 +17,7 @@ public final class IoUtil {
         try {
             if (closeable != null) closeable.close();
         } catch (IOException ignored) {
+            // The whole point here is to suppress these exceptions!
         }
     }
 }

projects/core/src/main/java/dan200/computercraft/core/util/StringUtil.java

@@ -12,8 +12,6 @@ public final class StringUtil {
     }
 
     public static String normaliseLabel(String label) {
-        if (label == null) return null;
-
         var length = Math.min(32, label.length());
         var builder = new StringBuilder(length);
         for (var i = 0; i < length; i++) {

projects/core/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java

@@ -23,7 +23,6 @@ import org.opentest4j.AssertionFailedError;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.io.File;
 import java.io.IOException;
@@ -249,7 +248,6 @@ public class ComputerTestDelegate {
     }
 
     public static class FakeModem implements IPeripheral {
-        @Nonnull
         @Override
         public String getType() {
             return "modem";
@@ -267,7 +265,6 @@ public class ComputerTestDelegate {
     }
 
     public static class FakePeripheralHub implements IPeripheral {
-        @Nonnull
         @Override
         public String getType() {
             return "peripheral_hub";

projects/core/src/test/java/dan200/computercraft/core/apis/ObjectWrapper.java

@@ -8,7 +8,6 @@ package dan200.computercraft.core.apis;
 import dan200.computercraft.api.lua.*;
 import dan200.computercraft.core.asm.LuaMethod;
 
-import javax.annotation.Nonnull;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
@@ -51,7 +50,7 @@ public class ObjectWrapper implements ILuaContext {
     }
 
     @Override
-    public long issueMainThreadTask(@Nonnull ILuaTask task) {
+    public long issueMainThreadTask(ILuaTask task) {
         throw new IllegalStateException("Method should never queue events");
     }
 }

projects/core/src/test/java/dan200/computercraft/core/asm/GeneratorTest.java

@@ -11,7 +11,6 @@ import dan200.computercraft.core.computer.ComputerSide;
 import org.hamcrest.Matcher;
 import org.junit.jupiter.api.Test;
 
-import javax.annotation.Nonnull;
 import java.io.IOException;
 import java.util.Collection;
 import java.util.Map;
@@ -241,7 +240,7 @@ public class GeneratorTest {
 
     private static final ILuaContext CONTEXT = new ILuaContext() {
         @Override
-        public long issueMainThreadTask(@Nonnull ILuaTask task) {
+        public long issueMainThreadTask(ILuaTask task) {
             return 0;
         }
     };

projects/core/src/test/java/dan200/computercraft/core/asm/MethodTest.java

@@ -13,7 +13,6 @@ import dan200.computercraft.core.computer.ComputerBootstrap;
 import dan200.computercraft.core.computer.ComputerSide;
 import org.junit.jupiter.api.Test;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.util.Collections;
 
@@ -99,7 +98,6 @@ public class MethodTest {
             return 123;
         }
 
-        @Nonnull
         @Override
         public String getType() {
             return "main_thread";
@@ -112,21 +110,18 @@ public class MethodTest {
     }
 
     public static class Dynamic implements IDynamicLuaObject, ILuaAPI, IDynamicPeripheral {
-        @Nonnull
         @Override
         public String[] getMethodNames() {
             return new String[]{ "foo" };
         }
 
-        @Nonnull
         @Override
-        public MethodResult callMethod(@Nonnull ILuaContext context, int method, @Nonnull IArguments arguments) {
+        public MethodResult callMethod(ILuaContext context, int method, IArguments arguments) {
             return MethodResult.of(123);
         }
 
-        @Nonnull
         @Override
-        public MethodResult callMethod(@Nonnull IComputerAccess computer, @Nonnull ILuaContext context, int method, @Nonnull IArguments arguments) {
+        public MethodResult callMethod(IComputerAccess computer, ILuaContext context, int method, IArguments arguments) {
             return callMethod(context, method, arguments);
         }
 
@@ -140,7 +135,6 @@ public class MethodTest {
             return new String[]{ "dynamic" };
         }
 
-        @Nonnull
         @Override
         public String getType() {
             return "dynamic";
@@ -179,7 +173,6 @@ public class MethodTest {
             throw new LuaException("!");
         }
 
-        @Nonnull
         @Override
         public String getType() {
             return "throw";
@@ -192,7 +185,6 @@ public class MethodTest {
     }
 
     public static class ManyMethods implements IDynamicLuaObject, ILuaAPI {
-        @Nonnull
         @Override
         public String[] getMethodNames() {
             var methods = new String[40];
@@ -200,9 +192,8 @@ public class MethodTest {
             return methods;
         }
 
-        @Nonnull
         @Override
-        public MethodResult callMethod(@Nonnull ILuaContext context, int method, @Nonnull IArguments arguments) throws LuaException {
+        public MethodResult callMethod(ILuaContext context, int method, IArguments arguments) throws LuaException {
             return MethodResult.of();
         }
 

projects/core/src/test/java/dan200/computercraft/core/terminal/TerminalTest.java

@@ -34,20 +34,6 @@ class TerminalTest {
         assertEquals("fedcba9876543210", terminal.getBackgroundColourLine(1).toString());
     }
 
-    @Test
-    void testGetLineOutOfBounds() {
-        var terminal = new Terminal(16, 9, true);
-
-        assertNull(terminal.getLine(-5));
-        assertNull(terminal.getLine(12));
-
-        assertNull(terminal.getTextColourLine(-5));
-        assertNull(terminal.getTextColourLine(12));
-
-        assertNull(terminal.getBackgroundColourLine(-5));
-        assertNull(terminal.getBackgroundColourLine(12));
-    }
-
     @Test
     void testDefaults() {
         var terminal = new Terminal(16, 9, true);

projects/core/src/testFixtures/java/dan200/computercraft/test/core/ArbitraryByteBuffer.java

@@ -8,7 +8,6 @@ package dan200.computercraft.test.core;
 import net.jqwik.api.*;
 import net.jqwik.api.arbitraries.SizableArbitrary;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import java.math.BigInteger;
 import java.nio.ByteBuffer;
@@ -43,25 +42,21 @@ public final class ArbitraryByteBuffer implements SizableArbitrary<ByteBuffer> {
         return DEFAULT;
     }
 
-    @Nonnull
     @Override
     public SizableArbitrary<ByteBuffer> ofMinSize(int minSize) {
         return new ArbitraryByteBuffer(minSize, maxSize, distribution);
     }
 
-    @Nonnull
     @Override
     public SizableArbitrary<ByteBuffer> ofMaxSize(int maxSize) {
         return new ArbitraryByteBuffer(minSize, maxSize, distribution);
     }
 
-    @Nonnull
     @Override
-    public SizableArbitrary<ByteBuffer> withSizeDistribution(@Nonnull RandomDistribution distribution) {
+    public SizableArbitrary<ByteBuffer> withSizeDistribution(RandomDistribution distribution) {
         return new ArbitraryByteBuffer(minSize, maxSize, distribution);
     }
 
-    @Nonnull
     @Override
     public RandomGenerator<ByteBuffer> generator(int genSize) {
         var min = BigInteger.valueOf(minSize);
@@ -78,7 +73,6 @@ public final class ArbitraryByteBuffer implements SizableArbitrary<ByteBuffer> {
         };
     }
 
-    @Nonnull
     @Override
     public EdgeCases<ByteBuffer> edgeCases(int maxEdgeCases) {
         return EdgeCases.fromSuppliers(Arrays.asList(
@@ -133,13 +127,11 @@ public final class ArbitraryByteBuffer implements SizableArbitrary<ByteBuffer> {
             this.minSize = minSize;
         }
 
-        @Nonnull
         @Override
         public ByteBuffer value() {
             return value;
         }
 
-        @Nonnull
         @Override
         public Stream<Shrinkable<ByteBuffer>> shrink() {
             return StreamSupport.stream(new Spliterators.AbstractSpliterator<Shrinkable<ByteBuffer>>(3, 0) {
@@ -160,7 +152,6 @@ public final class ArbitraryByteBuffer implements SizableArbitrary<ByteBuffer> {
             }, false);
         }
 
-        @Nonnull
         @Override
         public ShrinkingDistance distance() {
             return ShrinkingDistance.of(value.remaining() - minSize);

projects/core/src/testFixtures/java/dan200/computercraft/test/core/apis/BasicApiEnvironment.java

@@ -16,7 +16,6 @@ import dan200.computercraft.core.metrics.Metric;
 import dan200.computercraft.core.terminal.Terminal;
 import dan200.computercraft.test.core.computer.BasicEnvironment;
 
-import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
 public abstract class BasicApiEnvironment implements IAPIEnvironment {
@@ -32,25 +31,21 @@ public abstract class BasicApiEnvironment implements IAPIEnvironment {
         return 0;
     }
 
-    @Nonnull
     @Override
     public ComputerEnvironment getComputerEnvironment() {
         return environment;
     }
 
-    @Nonnull
     @Override
     public GlobalEnvironment getGlobalEnvironment() {
         return environment;
     }
 
-    @Nonnull
     @Override
     public IWorkMonitor getMainThreadMonitor() {
         throw new IllegalStateException("Main thread monitor not available");
     }
 
-    @Nonnull
     @Override
     public Terminal getTerminal() {
         throw new IllegalStateException("Terminal not available");
@@ -128,10 +123,10 @@ public abstract class BasicApiEnvironment implements IAPIEnvironment {
     }
 
     @Override
-    public void observe(@Nonnull Metric.Event summary, long value) {
+    public void observe(Metric.Event summary, long value) {
     }
 
     @Override
-    public void observe(@Nonnull Metric.Counter counter) {
+    public void observe(Metric.Counter counter) {
     }
 }

projects/core/src/testFixtures/java/dan200/computercraft/test/core/computer/BasicEnvironment.java

@@ -16,7 +16,6 @@ import dan200.computercraft.core.metrics.Metric;
 import dan200.computercraft.core.metrics.MetricsObserver;
 import dan200.computercraft.test.core.filesystem.MemoryMount;
 
-import javax.annotation.Nonnull;
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
@@ -60,13 +59,11 @@ public class BasicEnvironment implements ComputerEnvironment, GlobalEnvironment,
         return this;
     }
 
-    @Nonnull
     @Override
     public String getHostString() {
         return "ComputerCraft 1.0 (Test environment)";
     }
 
-    @Nonnull
     @Override
     public String getUserAgent() {
         return "ComputerCraft/1.0";

projects/core/src/testFixtures/java/dan200/computercraft/test/core/filesystem/MemoryMount.java

@@ -5,16 +5,17 @@
  */
 package dan200.computercraft.test.core.filesystem;
 
+import dan200.computercraft.api.filesystem.FileOperationException;
 import dan200.computercraft.api.filesystem.IWritableMount;
 import dan200.computercraft.core.apis.handles.ArrayByteChannel;
 
-import javax.annotation.Nonnull;
 import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.IOException;
 import java.nio.channels.Channels;
 import java.nio.channels.ReadableByteChannel;
 import java.nio.channels.WritableByteChannel;
+import java.nio.charset.StandardCharsets;
 import java.util.*;
 
 /**
@@ -30,7 +31,7 @@ public class MemoryMount implements IWritableMount {
 
 
     @Override
-    public void makeDirectory(@Nonnull String path) {
+    public void makeDirectory(String path) {
         var file = new File(path);
         while (file != null) {
             directories.add(file.getPath());
@@ -39,7 +40,7 @@ public class MemoryMount implements IWritableMount {
     }
 
     @Override
-    public void delete(@Nonnull String path) {
+    public void delete(String path) {
         if (files.containsKey(path)) {
             files.remove(path);
         } else {
@@ -55,9 +56,8 @@ public class MemoryMount implements IWritableMount {
         }
     }
 
-    @Nonnull
     @Override
-    public WritableByteChannel openForWrite(@Nonnull final String path) {
+    public WritableByteChannel openForWrite(final String path) {
         return Channels.newChannel(new ByteArrayOutputStream() {
             @Override
             public void close() throws IOException {
@@ -67,9 +67,8 @@ public class MemoryMount implements IWritableMount {
         });
     }
 
-    @Nonnull
     @Override
-    public WritableByteChannel openForAppend(@Nonnull final String path) throws IOException {
+    public WritableByteChannel openForAppend(final String path) throws IOException {
         var stream = new ByteArrayOutputStream() {
             @Override
             public void close() throws IOException {
@@ -90,35 +89,36 @@ public class MemoryMount implements IWritableMount {
     }
 
     @Override
-    public boolean exists(@Nonnull String path) {
+    public boolean exists(String path) {
         return files.containsKey(path) || directories.contains(path);
     }
 
     @Override
-    public boolean isDirectory(@Nonnull String path) {
+    public boolean isDirectory(String path) {
         return directories.contains(path);
     }
 
     @Override
-    public void list(@Nonnull String path, @Nonnull List<String> files) {
+    public void list(String path, List<String> files) {
         for (var file : this.files.keySet()) {
             if (file.startsWith(path)) files.add(file.substring(path.length() + 1));
         }
     }
 
     @Override
-    public long getSize(@Nonnull String path) {
+    public long getSize(String path) {
         throw new RuntimeException("Not implemented");
     }
 
-    @Nonnull
     @Override
-    public ReadableByteChannel openForRead(@Nonnull String path) {
+    public ReadableByteChannel openForRead(String path) throws FileOperationException {
-        return new ArrayByteChannel(files.get(path));
+        var file = files.get(path);
+        if (file == null) throw new FileOperationException(path, "File not found");
+        return new ArrayByteChannel(file);
     }
 
     public MemoryMount addFile(String file, String contents) {
-        files.put(file, contents.getBytes());
+        files.put(file, contents.getBytes(StandardCharsets.UTF_8));
         return this;
     }
 }

projects/core/src/testFixtures/java/dan200/computercraft/test/core/package-info.java

@@ -0,0 +1,19 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+
+/**
+ * Test helpers for ComputerCraft's core Lua runtime and APIs.
+ */
+@DefaultQualifier(value = NonNull.class, locations = {
+    TypeUseLocation.RETURN,
+    TypeUseLocation.PARAMETER,
+    TypeUseLocation.FIELD,
+})
+package dan200.computercraft.test.core;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;

src/main/java/dan200/computercraft/shared/computer/terminal/TerminalState.java

@@ -5,7 +5,6 @@
  */
 package dan200.computercraft.shared.computer.terminal;
 
-import dan200.computercraft.core.util.IoUtil;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufInputStream;
 import io.netty.buffer.ByteBufOutputStream;
@@ -119,14 +118,10 @@ public class TerminalState {
         if (compressed != null) return compressed;
 
         var compressed = Unpooled.buffer();
-        OutputStream stream = null;
+        try (OutputStream stream = new GZIPOutputStream(new ByteBufOutputStream(compressed))) {
-        try {
-            stream = new GZIPOutputStream(new ByteBufOutputStream(compressed));
             stream.write(buffer.array(), buffer.arrayOffset(), buffer.readableBytes());
         } catch (IOException e) {
             throw new UncheckedIOException(e);
-        } finally {
-            IoUtil.closeQuietly(stream);
         }
 
         return this.compressed = compressed;
@@ -135,9 +130,7 @@ public class TerminalState {
     private static ByteBuf readCompressed(ByteBuf buf, int length, boolean compress) {
         if (compress) {
             var buffer = Unpooled.buffer();
-            InputStream stream = null;
+            try (InputStream stream = new GZIPInputStream(new ByteBufInputStream(buf, length))) {
-            try {
-                stream = new GZIPInputStream(new ByteBufInputStream(buf, length));
                 var swap = new byte[8192];
                 while (true) {
                     var bytes = stream.read(swap);
@@ -146,8 +139,6 @@ public class TerminalState {
                 }
             } catch (IOException e) {
                 throw new UncheckedIOException(e);
-            } finally {
-                IoUtil.closeQuietly(stream);
             }
             return buffer;
         } else {

src/main/java/dan200/computercraft/shared/peripheral/diskdrive/DiskDrivePeripheral.java

@@ -9,9 +9,9 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.api.peripheral.IComputerAccess;
 import dan200.computercraft.api.peripheral.IPeripheral;
+import dan200.computercraft.core.util.StringUtil;
 import dan200.computercraft.shared.MediaProviders;
 import dan200.computercraft.shared.media.items.ItemDisk;
-import dan200.computercraft.core.util.StringUtil;
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
@@ -79,17 +79,16 @@ public class DiskDrivePeripheral implements IPeripheral {
      * If the inserted disk's label can't be changed (for example, a record),
      * an error will be thrown.
      *
-     * @param labelA The new label of the disk, or {@code nil} to clear.
+     * @param label The new label of the disk, or {@code nil} to clear.
      * @throws LuaException If the disk's label can't be changed.
      */
     @LuaFunction(mainThread = true)
-    public final void setDiskLabel(Optional<String> labelA) throws LuaException {
+    public final void setDiskLabel(Optional<String> label) throws LuaException {
-        var label = labelA.orElse(null);
         var stack = diskDrive.getDiskStack();
         var media = MediaProviders.get(stack);
         if (media == null) return;
 
-        if (!media.setLabel(stack, StringUtil.normaliseLabel(label))) {
+        if (!media.setLabel(stack, label.map(StringUtil::normaliseLabel).orElse(null))) {
             throw new LuaException("Disk label cannot be changed");
         }
         diskDrive.setDiskStack(stack);

src/main/java/dan200/computercraft/shared/peripheral/printer/PrinterPeripheral.java

@@ -135,7 +135,7 @@ public class PrinterPeripheral implements IPeripheral {
     @LuaFunction
     public final void setPageTitle(Optional<String> title) throws LuaException {
         getCurrentPage();
-        printer.setPageTitle(StringUtil.normaliseLabel(title.orElse("")));
+        printer.setPageTitle(title.map(StringUtil::normaliseLabel).orElse(null));
     }
 
     /**

src/testMod/kotlin/dan200/computercraft/gametest/core/ManagedComputers.kt

@@ -73,13 +73,15 @@ object ManagedComputers : ILuaMachine.Factory {
             apis.add(api)
 
             if (api is OSAPI) {
-                val newMachine = if (api.computerID != 1) {
+                val id = api.computerID
-                    CobaltLuaMachine(environment)
+                val label = api.computerLabel
-                } else if (api.computerLabel != null) {
+                val newMachine = when {
-                    KotlinMachine(environment, api.computerLabel[0] as String)
+                    id != 1 -> CobaltLuaMachine(environment)
-                } else {
+                    label != null && label[0] != null -> KotlinMachine(environment, label[0] as String)
-                    LOGGER.error("Kotlin Lua machine must have a label")
+                    else -> {
-                    CobaltLuaMachine(environment)
+                        LOGGER.error("Kotlin Lua machine must have a label")
+                        CobaltLuaMachine(environment)
+                    }
                 }
 
                 this.delegate = newMachine
@@ -106,7 +108,8 @@ object ManagedComputers : ILuaMachine.Factory {
         }
     }
 
-    private class KotlinMachine(environment: MachineEnvironment, private val label: String) : KotlinLuaMachine(environment) {
+    private class KotlinMachine(environment: MachineEnvironment, private val label: String) :
+        KotlinLuaMachine(environment) {
         override fun getTask(): (suspend KotlinLuaMachine.() -> Unit)? = computers[label]?.poll()
     }