public class Stack<T> { 

    private T[] stack;
    private int capacity;
    private int size;

    public Stack() {
        this(10);
    }

    public Stack(int initialCapacity) {
        stack = (T[]) new Object[initialCapacity];
        capacity = initialCapacity;
        size = 0;
    }

    private void doubleCapacity() {
        T[] newStack = (T[]) new Object[2*capacity];
        for (int i = 0; i < capacity; i++)
            newStack[i] = stack[i];
        capacity = 2*capacity;
        stack = newStack;
    }

    /** Pushes item onto the stack */
    public void push(T item) {
        if (isFull())
            doubleCapacity();   
        stack[size++] = item;
    }

    /** Pops off an item from the stack */
    public T pop() {
        if (isEmpty())
            throw new RuntimeException("Trying to pop an empty stack.");
        return stack[--size];
    }

    /** Returns the item at the top of the stack */
    public T top() {
        if (isEmpty())
            throw new RuntimeException("Trying to access an empty stack.");
        return stack[size-1];
    }

    /** Returns true iff the stack is empty. */
    public boolean isEmpty() {
        return size == 0;
    }

    /** Returns true iff the stack is full. */
    private boolean isFull() {
        return size == capacity;
    }

    /** Returns a string representation of the stack. */
    public String toString() {
        String s = "";
        for (int i = size - 1; i >= 0; i--)
            s += stack[i] + " ";
        return s;
    }

    public static void main(String[] args) { 
        int capacity = 10;

        Stack<Integer> is = new Stack<Integer>(capacity);
        if (is.isEmpty())
            System.out.println("stack is empty");
        else
            System.out.println("stack is not empty");
        for (int i = 0; i < capacity; i++)
            is.push(i);

        is.push(capacity);
        System.out.println("stack before popping: " + is);
        System.out.println(is.pop());
        System.out.println("stack after popping: " + is);
        System.out.println("top of stack is: " + is.top());

        ///////////////////////////////////////////////////
        Stack<String> ss = new Stack<String>(capacity);

        ss.push("Now");
        ss.push("is");
        ss.push("the");
        ss.push("time");

        System.out.println(ss);
    }
}