Stack.java

package ch.lihsmi.collections;

import java.util.Arrays;

final public class ArrayStack<T> implements Stack<T> {

    private T[] elements;

    private int position = 0;

    private static final int DEFAULT_CAPACITY = 16;

    @SuppressWarnings("unchecked")
    public ArrayStack() {
        this.elements = (T[]) new Object[DEFAULT_CAPACITY];
    }

    @Override
    public void pop() throws EmptyStackException {
        throwIfEmpty();
        elements[position--] = null;
    }

    @Override
    public T top() throws EmptyStackException {
        throwIfEmpty();
        return elements[position - 1];
    }

    @Override
    public T topAndPop() throws EmptyStackException {
        T element = top();
        pop();
        return element;
    }

    @Override
    public void push(T element) {
        ensureCapacity();
        elements[position++] = element;
    }

    @Override
    public boolean isEmpty() {
        return position == 0;
    }

    private void throwIfEmpty() throws EmptyStackException {
        if (position == 0) {
            throw new EmptyStackException();
        }
    }

    private void ensureCapacity() {
        if (elements.length == position) {
            elements = Arrays.copyOf(elements, 2 * position + 1);
        }
    }

}