public class IntegerStack { 

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

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

    private void doubleCapacity() {
        int[] newStack = new int[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(int item) {
        if (isFull())
            doubleCapacity();   
        stack[size++] = item;
    }

    /** Pops off an item from the stack */
    public int 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 int 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;
        IntegerStack is = new IntegerStack(capacity);

        // int n = is.pop();  this should cause an exception
        // int n = is.top();  this should cause an exception

        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());
    }
}