/*************************************************************************
 *  Compilation:  javac DoublingStack.java
 *  Execution:    java DoublingStack
 *  
 *  Stack implementation with an array. Resizes by doubling and halving.
 *
 *************************************************************************/

import java.util.Iterator;
import java.util.NoSuchElementException;

// suppress unchecked warnings in Java 1.5.0_6 and later
@SuppressWarnings("unchecked")

public class DoublingStack<Item> implements Iterable<Item> {
    private Item[] a;         // array of items
    private int N = 0;        // number of elements on stack

    // create an empty stack
    public DoublingStack() {
        a = (Item[]) new Object[2];
    }

    public boolean isEmpty() { return N == 0; }
    public int size()        { return N;      }



    // resize the underlying array holding the elements
    private void resize(int capacity) {
        assert(capacity >= N);
        Item[] temp = (Item[]) new Object[capacity];
        for (int i = 0; i < N; i++)
            temp[i] = a[i];
        a = temp;
    }

    // push a new item onto the stack
    public void push(Item item) {
        if (N == a.length) resize(2*a.length);    // double size of array if necessary
        a[N++] = item;                            // add item
    }

    // delete and return the item most recently added
    public Item pop() {
        if (isEmpty()) { throw new RuntimeException("Stack underflow error"); }
        Item item = a[N-1];
        a[N-1] = null;                              // to avoid loitering
        N--;
        // shrink size of array if necessary
        if (N > 0 && N == a.length/4) resize(a.length/2);
        return item;
    }


    // string representation (inefficient because of string concatenation)
    public String toString() {
        String s = "[ ";
        for (int i = 0; i < N; i++)
            s += a[i] + " ";
        s += "]";
        return s;
    }

    public Iterator<Item> iterator()  { return new ArrayStackIterator();  }

    // an iterator, doesn't implement remove() since it's optional
    private class ArrayStackIterator implements Iterator<Item> {
        private int i = N;
        public boolean hasNext()  { return i > 0;                               }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            return a[--i];
        }
    }



   /***********************************************************************
    * Test routine.
    **********************************************************************/
    public static void main(String[] args) {
        DoublingStack<String> stack = new DoublingStack<String>();
        stack.push("Hello");
        stack.push("World");
        stack.push("how");
        stack.push("are");
        stack.push("you");

        for (String s : stack)
            System.out.println(s);

        System.out.println();

        while (!stack.isEmpty())
            System.out.println(stack.pop());
    }

}