Stack.java

public class Stack {
	private ArrayList elementData = new ArrayList();

	private int size = 0;
	
	public void push(Object o){
		elementData.add(o);
		size++;
	}
	
	public Object pop(){
		return elementData.remove(size);
	}
	
	public Object peek(){
		return elementData.get(size);
	}
	public boolean isEmpty(){
		return size <= 0;
	}
	public int size(){
		return size;
	}
}