Question

I’m not sure if I’m using the correct terms, but I am curious how it’s determined how much to increase the size of a Collection in Java when it gets full? I’ve tried searching but I’m not really coming up with anything useful.

So, if I have something like

List l = new ArrayList(1);
l.add("1");
l.add("2");

How does it determine how much to increase the size of the list? Is it always a set value and if so, what is that value? I'd also be interested in this information for BitSet if it differs.

Thanks and let me know if I should clarify any.

Answer 1

it calls ensureCapacity:

/**
 * Increases the capacity of this <tt>ArrayList</tt> instance, if
 * necessary, to ensure that it can hold at least the number of elements
 * specified by the minimum capacity argument.
 *
 * @param   minCapacity   the desired minimum capacity
 */
public void ensureCapacity(int minCapacity) {
    modCount++;
    int oldCapacity = elementData.length;
    if (minCapacity > oldCapacity) {
        Object oldData[] = elementData;
        int newCapacity = (oldCapacity * 3) / 2 + 1;
        if (newCapacity < minCapacity)
            newCapacity = minCapacity;
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
}

so as you can see the newCapacity is the (oldCapacity * 3) / 2 + 1