ResizingArrayStack.java

import java.util.Iterator;
import java.util.NoSuchElementException;
public class ResizingArrayStack<Item> implements Iterable<Item>{
    private Item[] values;
    private int count;
    
    public ResizingArrayStack(){
        values = (Item[]) new Object[1]; // initialize the array to hold 1 element
        count = 0;
    }
    
    public int size(){
        return count;
    }
    
    public void push(Item val){
        if (count == values.length) resize(2 * values.length);
        values[count++] = val; // be careful about the index. If you are not sure, draw a picture and see for yourself
    }
    
    public Item pop() throws Exception{
        if (count == 0) throw new Exception("The Stack is already empty");
        Item val = values[--count];
        values[count] = null; // avoid loitering
        if (count > 0 && count == values.length / 4) resize(values.length / 2); // shrink the array after pop
        return val;
    }
    
    public void resize(int size){
        Item[] newValues = (Item[]) new Object[size];
        for (int i = 0 ; i < count; i++){
            newValues[i] = values[i];
        }
        values = newValues;
    }
    
    public Iterator<Item> iterator(){
        return new ReverseArrayIterator();
    }
    
    private class ReverseArrayIterator implements Iterator<Item>{
        private int i = count;
        public boolean hasNext() {i > 0;}
        public void remove() {throw new UnsupportedOperationException();}
        public Item next(){
            if(!hasNext()) throw new NoSuchElementException();
            Item item = values[--i];
            return item;
        }
    }
}

• Check for loitering
• Check for empty stack before halving