Async or Sync tasks?

So, what I'm trying to do is loop through a list then change a block. The problem is I get an Error when I use an Async task and I get an error when I use a Sync task. Heres my code:

Code:

    //@SuppressWarnings("deprecation")
    public void regenerateGravelTimer() {
        this.getServer().getScheduler().scheduleSyncRepeatingTask(this, new Runnable() {
            @Override
            public void run() {
               
                regenerateGravel();
               
            }       
        }, 0L, 300L);
    }
   
    public void regenerateGravel() {
       
        if(gravel.size() == 0) {
            return;
        }

        int a = gravel.size();
        int r = new Random().nextInt(a) + 1;
        forcount = 0;
       
        for(String uuid : gravel) {
                               
            forcount++;
           
            if(forcount == r) {
               
                forcount = 0;
               
                String world = getGravel().getString("Gravel." + uuid + ".World");
                int x = getGravel().getInt("Gravel." + uuid + ".X");
                int y = getGravel().getInt("Gravel." + uuid + ".Y");
                int z = getGravel().getInt("Gravel." + uuid + ".Z");
               
                if(world == null) {
                    getGravel().set("Gravel." + uuid, null);
                    saveGravel();
                    return;
                }
               
                Location loc = new Location(Bukkit.getWorld(world), x, y, z);
                Block b = loc.getBlock();
               
                b.setType(Material.GRAVEL);
               
                getGravel().set("Gravel." + uuid, null);
                saveGravel();
               
                gravel.remove(uuid);
               
            }                   
        }

    }

When I use the Sync task I get a ConcurrentModificationException and when I use and Async task I get a Asynchronous block creation error. What do I do?

 

@bowlerguy66 The reason you are getting a ConcurrentModificationException is because you are removing items from the list while looping through it.
When you do:
gravel.remove(uuid);

If you want to remove the items as you loop, try using an iterator, simply change the beginning of your loop to:
for(String uuid : gravel.iterator()) {
and see if that works. I'm not gonna do it for you, but with that, you should be able to figure it out.

 

@567legodude When I do that I get
"Can only iterate over an array or an instance of java.lang.Iterable"

 

@bowlerguy66 You didn't show us the gravel list in your code (You need to include everything) so I don't know what kind of collection it is.
Is it a List, ArrayList, HashMap?

 

@bowlerguy66

1. Don't use bukkit methods inside async tasks or you'll give @mythbusterma cancer or something.
2. @567legodude's suggestion was a poor one. It would be best to use an iterator but you should use it with a while or a regular for-loop.
   

 

@567legodude This is my code for it: List<String> gravel = new ArrayList<String>();

 

Everything checks out.

The concurrent modification exception comes from the fact you modify when you're iterating. Use an Iterator and your code will work fine.

Also, don't use asynchronous if you don't know what it means.

 

@mythbusterma What?

 

@bowlerguy66

I was responding to Teej.

If you use Bukkit methods inside asynchronous methods, you risk breaking the server in ways you can't even imagine (or even reproduce).

 

@mythbusterma

Hmm. Okay, so if I were to use .setType() in an Async task, it would screw everything up?

 

@bowlerguy66

It could.

 

@teej107 The code example was my bad, I was just trying to get a quick reply. But I still had the right idea.