/** Array-based stack implementation */
class AStack<E> implements Stack<E> {
    private static final int defaultSize = 10;

    private int maxSize; // Maximum size of stack
    private int top; // Index for top Object
    private E[] listArray; // Array holding stack

    /** Constructors */
    AStack() { this(defaultSize); }

    @SuppressWarnings("unchecked") // Generic array allocation
    AStack(int size) {
	maxSize = size;
	top = 0;
	listArray = (E[])new Object[size]; // Create listArray
    }

    /** Reinitialize stack */
    public void clear() { top = 0; }

    /** Push "it" onto stack */
    public void push(E it) {
	assert top != maxSize : "Stack is full";
	listArray[top++] = it;
    }

    /** Remove and pop element */
    public E pop() {
	assert top != 0 : "Stack is empty";
	return listArray[--top];
    }

    /** @return Top element */
    public E topValue() {
	assert top != 0 : "Stack is empty";
	return listArray[top - 1];
    }

    /** @return Stack size */
    public int length() { return top; }
}