Pool

public final class Pool<E>
{
    private Map<Integer, E> indexToData;
    private Map<E, Integer> dataToIndex;
    private int index;

    public Pool()
    {
        this.indexToData = new HashMap<>();
        this.dataToIndex = new HashMap<>();
    }

    public int get(final E e)
    {
        if (this.dataToIndex.containsKey(e))
        {
            return this.dataToIndex.get(e);
        }
        else
        {
            final int index = this.index;
            this.index++;
            this.dataToIndex.put(e, index);
            this.indexToData.put(index, e);
            return index;
        }
    }

    public E getByIndex(final int index)
    {
        return this.indexToData.get(index);
    }

    public int size()
    {
        return this.index;
    }
}