Solved ConcurrentModificationException error

Dear,
I got an ConcurrentModificationException error in my loadChests() method, and I can't get it fixed. Here is my exact error:

Code:

[14:47:52 ERROR]: Error occurred while enabling SurvivalGames v1.0 (Is it up to
date?)
java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(Unknown Source) ~[?:1.
8.0_51]
        at java.util.ArrayList$Itr.next(Unknown Source) ~[?:1.8.0_51]
        at com.TheWGB.SG.arena.Arena.loadChests(Arena.java:426) ~[?:?]
        at com.TheWGB.SG.arena.Arena.<init>(Arena.java:62) ~[?:?]
        at com.TheWGB.SG.Utils.registerArenas(Utils.java:215) ~[?:?]
        at com.TheWGB.SG.Main.onEnable(Main.java:82) ~[?:?]
        at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:321) ~[S
pigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader
.java:335) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManage
r.java:405) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.craftbukkit.v1_8_R1.CraftServer.loadPlugin(CraftServer.jav
a:356) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.craftbukkit.v1_8_R1.CraftServer.enablePlugins(CraftServer.
java:316) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.craftbukkit.v1_8_R1.CraftServer.reload(CraftServer.java:74
6) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.Bukkit.reload(Bukkit.java:534) [Spigot18.jar:git-Spigot-4f
af77a-17a3db7]
        at org.bukkit.command.defaults.ReloadCommand.execute(ReloadCommand.java:
25) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:14
1) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.craftbukkit.v1_8_R1.CraftServer.dispatchCommand(CraftServe
r.java:646) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at org.bukkit.craftbukkit.v1_8_R1.CraftServer.dispatchServerCommand(Craf
tServer.java:632) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at net.minecraft.server.v1_8_R1.DedicatedServer.aM(DedicatedServer.java:
353) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:3
17) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:6
34) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java
:537) [Spigot18.jar:git-Spigot-4faf77a-17a3db7]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_51]

And here is the method:

Code:

    private void loadChests() {
        originalChests = new ArrayList<Chest>();
      
        List<String> chestsFormat = Arenas.get().getStringList(ID+".chests");
      
        for(String s : chestsFormat) {
            String[] parts = s.split(" ");
          
            int x = Integer.parseInt(parts[0]);
            int y = Integer.parseInt(parts[1]);
            int z = Integer.parseInt(parts[2]);
          
            Location loc = new Location(world, x, y, z);
            Block block = loc.getBlock();
            if(block.getType() != Material.CHEST) {
                chestsFormat.remove(s);
                continue;
            }
            Chest chest = (Chest) block.getState();
          
            originalChests.add(chest);
        }
      
        if(chestsFormat != Arenas.get().getStringList(ID+".chests")) {
            Arenas.get().set(ID+".chests", chestsFormat);
          
            Arenas.save();
        }
    }

Thanks!

 

You can't edit a list while iterating over it with a nested for-loop. Use something like this:
for (int i = 0; i < list.size(); i++) {
String str = list.get(i);

}

Or an iterator

 

Firstly, you need to know what a ConcurrentModificationException is.
Oracle provides a documentation for this class: http://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html

So, how do we fix this?

1. The way FisheyLP suggested above (recommended).

2. Iterate through a clone of that List. This way, you'll be modifying chestsFormat whilst the clone is being iterated, preventing a ConcurrentModificationException:

Code:

       for (String s : new ArrayList<>(chestsFormat)) {
            String[] parts = s.split(" ");
         
            int x = Integer.parseInt(parts[0]);
            int y = Integer.parseInt(parts[1]);
            int z = Integer.parseInt(parts[2]);
         
            Location loc = new Location(world, x, y, z);
            Block block = loc.getBlock();
            if(block.getType() != Material.CHEST) {
                chestsFormat.remove(s);
                continue;
            }
            Chest chest = (Chest) block.getState();
         
            originalChests.add(chest);
        }

3. Make another local List that contains all the Strings to remove.

Code:

       List<String> chestFormatsToRemove = new ArrayList<>();
       for (String s : chestFormatsToRemove) {
            String[] parts = s.split(" ");
         
            int x = Integer.parseInt(parts[0]);
            int y = Integer.parseInt(parts[1]);
            int z = Integer.parseInt(parts[2]);
         
            Location loc = new Location(world, x, y, z);
            Block block = loc.getBlock();
            if(block.getType() != Material.CHEST) {
                chestFormatsToRemove.add(s);
                continue;
            }
            Chest chest = (Chest) block.getState();
         
            originalChests.add(chest);
        }
        for (String chestFormatToRemove : chestFormatsToRemove) chestFormats.remove(chestFormatToRemove);

More information: http://www.javacodegeeks.com/2011/05/avoid-concurrentmodificationexception.html

By the way, you should never use "!=" for any Object if you're checking for a difference in values.
Here's why: http://perso.ensta-paristech.fr/~diam/java/online/notes-java/data/expressions/22compareobjects.html
It should be: if (!chestsFormat.equals(Arenas.get().getStringList(ID + ".chests"))) {
A faster way would just to have a boolean (local) called 'hasBeenModified', by default being false. If and when the List is modified, make 'hasBeenModified' true.

 

Use an iterator