Can't add to an array list

Question

I have the oddest problem, probably with a simple solution.

I have created and initialized a list and then proceeded to create 4 objects of the list's type. In the constructor of these they place themselves in the list. Or at least are supposed to. I always get an out of bound exception and I cant figure out why. I set the list to have a size of 402 (for all possible VK values) but in the console and debug it always says it has size 0, no matter how large or empty I set it too....

public class InputHandler implements KeyListener
{

public static List<Key> keyList = new ArrayList<Key>(KeyEvent.KEY_LAST);

public Key up = new Key(KeyEvent.VK_UP);
public Key down = new Key(KeyEvent.VK_DOWN);
public Key left = new Key(KeyEvent.VK_LEFT);
public Key right = new Key(KeyEvent.VK_RIGHT);


public class Key
    {

    public int keyCode;

    public Key(int defaultCode)
    {
        this.keyCode = defaultCode;
        keyList.add(keyCode,this);
    }

    public Key reMapKey(int newKey)
    {
        keyList.remove(keyCode);
        keyList.set(newKey, this);
        this.keyCode = newKey;
        return this;
    }

}

}

There is more to the code but I attempted to SSCCE it.

The only info of value from the console is this:

Exception in thread "RogueLoveMainThread" java.lang.IndexOutOfBoundsException: Index: 38, Size: 0

Much apologies for my stupidity

If this is related to Swing, use KeyBindings instead. See How to Use Key Bindings. — Eng.Fouad
– Eng.Fouad, Commented May 8, 2013 at 19:40
Possible duplicate of ArrayList initial capacity and IndexOutOfBoundsException — Raedwald
– Raedwald, Commented Jan 26, 2016 at 22:14

Community · Accepted Answer · 2020-06-20 09:12:55Z

5

You've created a new ArrayList with a capacity of 402, but it's still got a size of 0 after the constructor call. From the docs of the constructor call you're using:

public ArrayList(int initialCapacity)

Constructs an empty list with the specified initial capacity.

Parameters:
initialCapacity - the initial capacity of the list

And from the docs of ArrayList itself:

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically.

The capacity is not the size.

So, some options:

Populate the list with null entries until you have got the size you want
Use an array instead of a list (after all, you only need it to be a fixed size, right?)
Use a Map<Integer, Key> instead

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered May 8, 2013 at 19:37

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Benjamin Tilbury Over a year ago

Thank you. I can't use an array as I need to do some things with remapping of the keys, I think I'll just fill it with nulls

skuntsel Over a year ago

Eh... What about the 'options' section at the end of the answer?

Jon Skeet Over a year ago

@BenjaminTilbury: What do you mean by "need to do some things"? If you have other methods which take a list, you can always use Arrays.asList.

Benjamin Tilbury Over a year ago

@skuntsel that wasn't there when I said that :L

Benjamin Tilbury Over a year ago

@JonSkeet Theres some methods that I will add later that will be much easier to do with a list rather than making an array into a list

Matthias Meid · Accepted Answer · 2013-05-08 19:49:59Z

4

keyList.add(keyCode, this) inserts at the position of keyCode. As your list is still empty (size 0) you cannot insert at any position position greater than 0.

You might want to have map codes to keys, do you? There is a Map<K, V> for this:

static Map<Integer, Key> keyMap = new TreeMap<>();

public Key(int defaultCode) {
    keyMap.add(keyCode, this);
}

If you need a key by its code, you can receive it from the Map as follows:

keyMap.get(keyCode);

edited May 8, 2013 at 19:49

answered May 8, 2013 at 19:38

Matthias Meid

12.5k7 gold badges48 silver badges79 bronze badges

Collectives™ on Stack Overflow

Can't add to an array list

2 Answers 2

5 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related