java.lang.ClassCastException; no stacktrace with TreeSet

Question

I have this method defined:

public static TreeSet<ItemStack> getItems() {
    TreeSet<ItemStack> things = new TreeSet<>();
    things.add(new ItemStack(Material.ACACIA_DOOR));
    return things;
}

I also tried putting multiple things in the TreeSet, it doesn't work either.

Now I have this code part:

TreeSet<ItemStack> things = getItems();

If I run this, nothing happens. If I surround it with try/catch, there seems to be an exception thrown. But if I print the error like that:

} catch (Exception exc) {
    System.out.println("Catched an exception:")
    exc.printStackTrace();
    System.out.println(exc.getMessage());
    System.out.println(exc);
}

There isn't any error / Stacktrace or so coming up. It just says:

[INFO]: Catched an exception:
[WARN]: java.lang.ClassCastException
[INFO]: null
[INFO]: java.lang.ClassCastException

So what is the error, how do I prevent it and why do I get it by casting a TreeSet with ItemSets to a TreeSet with ItemSets?

Does `ItemStack` implement `Comparable`? I'm going to guess it doesn't. — Andy Turner, Dec 02 '16 at 16:50
@AndyTurner I think it doesn't: public class ItemStack extends Object implements Cloneable, ConfigurationSerializable — writzlpfrimpft, Dec 02 '16 at 16:58
Right, it doesn't. Note that `extends Object` is unnecessary also. — Andy Turner, Dec 02 '16 at 16:59
@Kerooker making a list with multiple ItemStacks and looping through them at another place — writzlpfrimpft, Dec 02 '16 at 17:54
@AndyTurner Yeah I know, it is unnecessary. I didn't do that, it's premade by Bukkit (API) — writzlpfrimpft, Dec 02 '16 at 17:57
And why do you want to use a TreeSet, not a normal list, for instance? — LeoColman, Dec 02 '16 at 18:03
You can simply use List items = new ArrayList(); and then items.add(new ItemStack....); — LeoColman, Dec 02 '16 at 18:04
@ScrouthTV Welcome to SO. If you found the answer you were looking for, be sure to accept it with the check. — LeoColman, Dec 02 '16 at 18:18
As for why there is no stack trace, it's possible that [this question](http://stackoverflow.com/q/2411487/3991344) will explain why that happens (but I'm not sure entirely). — Pokechu22, Dec 06 '16 at 15:27

score 1 · Answer 1 · answered Dec 02 '16 at 18:17

What you can do to solve your situation, and thus guaranteeing your intended behaviour is using a normal list instead of a TreeSet to store your items:

ArrayList<ItemStack> items = new ArrayList<ItemStack>();
items.add(new ItemStack(Material.GRASS);
items.add(new ItemStack(Material.DIRT);
//Items is now a list with 2 ItemStacks inside

for (ItemStack is : items) {
    //Do something
}

In that way you fix your issue. Treeset should be used if you want a certain ordain on your Items, and ItemStacks cannot be ordered normally.

score 0 · Answer 2 · answered Dec 02 '16 at 16:53

Read the Javadoc.

Even if you don't know what you're looking for, hit Ctrl-f, and look for ClassCastException in the page. In the documentation of the constructor you're using, TreeSet(), you'll find:

All elements inserted into the set must implement the Comparable interface. ... If the user attempts to add an element to the set that violates this constraint ..., the add call will throw a ClassCastException.

So, either:

make your ItemStack implement Comparable<ItemStack>.
Provide a Comparator<ItemStack> to the constructor of the TreeSet.

score 0 · Answer 3 · answered Dec 02 '16 at 16:56

0

The docs, I think maybe the problem is that the item of TreeSet should implments Comparable or extend 'Comparator'. Hope it can help you out.

answered Dec 02 '16 at 16:56

秃德666

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java.lang.ClassCastException; no stacktrace with TreeSet

3 Answers3