/**
 * This class implements a generic Stack using dynamic array.
 */
public class Stack<T> {
    int capacity;
    int size;
    T[] istack;

    /**
     * Creates a new stack of T with capacity cap.
     */
    public Stack(int cap) {
        capacity = cap;
        size = 0;
        istack = (T[]) new Object[cap];
    }

    private static int DEFAULT_CAPACITY = 10;
    public Stack() { this(DEFAULT_CAPACITY); }

    /**
     * Doubles the capacity of the array.
     */
    private void resize() {
        capacity *= 2;
        T[] newStack = (T[]) new Object[capacity];
        for (int i = 0; i < size; i++)
            newStack[i] = istack[i];
        istack = newStack;
    }

    /**
     * Pushes item onto the stack.
     * @param item the item to be inserted into the stack
     */
    public void push(T item) {
        if (isFull())
            resize();
        istack[size++] = item;
    }

    /**
     * Returns the topmost item on the stack without disturbing it.
     * @return the topmost item on the stack
     */
    public T top() {
        if (isEmpty())
            throw new RuntimeException("Trying to access an empty stack.");
        return istack[size-1];
    }

    /**
     * Pops off the topmost item on the stack and returns it.
     * @return the item on top of the stack
     */
    public T pop() {
        if (isEmpty())
            throw new RuntimeException("Trying to pop an empty stack.");
        return istack[--size];
    }

    /**
     * @return true if and only the stack is empty
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * @return true if and only if the stack is full
     */
    public boolean isFull() {
        return size == capacity;
    }

    /**
     * @return a string representation of the stack
     */
    public String toString() {
        if (isEmpty()) return "EMPTY STACK";
        StringBuffer sb = new StringBuffer();
        for (int i = size-1; i >= 0; i--)
            sb.append(" " + istack[i]);
        return sb.toString();
    }

    public static void main(String[] args) {
        Stack is1 = new Stack<Integer>(5);
        Stack is2 = new Stack<Integer>();

        System.out.println("is1 = " + is1);
        System.out.println("is2 = " + is2);
        for (int i = 0; i < 20; i++) {
            is1.push(i);
            is2.push(2*i);
        }
        System.out.println("is1 = " + is1);
        System.out.println("is2 = " + is2);

        System.out.println("top(is1) = " + is1.top());
    }
}