ArrayIndexOutOfBoundsException on a initialized Vector

Question

I have this:

public class DoubleList<Key, Elem> implements ADTDoubleList<Key, Elem> {

    private Vector<Node<Key, Elem>> leftRight = new Vector<Node<Key, Elem>>(2);
    private int[] numLeftNumRight = new int[2];

    public DoubleList() {
        this.leftRight.set(0, null);
        this.leftRight.set(1, null);
        this.numLeftNumRight[0] = 0;
        this.numLeftNumRight[1] = 0;
    }
}

and it throws an ArrayIndexOutOfBoundsException.

I don't know why. Could someone help me?

Answer 1

You can't set an element in a Vector or any other List if that index isn't already occupied. By using new Vector<Node<Key, Elem>>(2) you're ensuring that the vector initially has the capacity for two elements, but it is still empty and so getting or setting using any index won't work.

In other words, the list hasn't grown big enough for that index to be valid yet. Use this instead:

this.leftRight.add(null);  //index 0
this.leftRight.add(null);  //index 1

You could also do:

this.leftRight.add(0, null);
this.leftRight.add(1, null);