Stack.java

public class Stack {
	private ArrayList elementData = new ArrayList();
	
	public void push(Object o){
		elementData.add(o);
	}
	
	public Object pop(){
		return elementData.remove(elementData.size() -1);
	}
	
	public Object peek(){
		return elementData.get(elementData.size()-1);
	}
	public boolean isEmpty(){
		return elementData.isEmpty();
	}
	public int size(){
		return elementData.size();
	}
}