Use Comparator's comparing(), nullsLast() and reversed() methods.

app/src/main/java/us/shandian/giga/get/Mission.java

@@ -35,6 +35,10 @@ public abstract class Mission implements Serializable {
      */
     public StoredFileHelper storage;
 
+    public long getTimestamp() {
+        return timestamp;
+    }
+
     /**
      * Delete the downloaded file
      *

app/src/main/java/us/shandian/giga/service/DownloadManager.java

@@ -12,6 +12,7 @@ import java.io.File;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.Iterator;
 
 import us.shandian.giga.get.DownloadMission;
@@ -198,7 +199,7 @@ public class DownloadManager {
         }
 
         if (mMissionsPending.size() > 1)
-            Collections.sort(mMissionsPending, (mission1, mission2) -> Long.compare(mission1.timestamp, mission2.timestamp));
+            Collections.sort(mMissionsPending, Comparator.comparingLong(Mission::getTimestamp));
     }
 
     /**