public class AStack<E> implements StackInterface<E>{
    private static final int CAPACITY = 40;
    private E[] theStack;
    private int theSize;  
    @Override
    public int size() {
	return theSize;
    }
    @Override
    public boolean isEmpty() {
	return theSize==0;
    }
    @Override
    public E pop() {
	if (theSize==0) return null;
	return theStack[--theSize];
    }
    @Override
    public E top() {
	return theSize==0 ? null : theStack[theSize];
    }

    @Override
    public void push(E element) throws IllegalStateException {
	if (theSize==CAPACITY)
	    throw new IllegalStateException("Stack Full!");
	theStack[theSize++]=element;
    }   
    public AStack() {
	this(CAPACITY);
    }   
    @SuppressWarnings("unchecked")
    public AStack(int capacity) {
	theStack = (E[])new Object[capacity];
    }
}