Stack.java

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

    public void push(Object o){
        elementData.add(o);
    }

    public Object pop(){
        return elementData.remove(size() - 1);
    }

    public Object peek(){
        return elementData.get(size() - 1);
    }
    
    public boolean isEmpty(){
        return size() == 0;
    }
    
    public int size(){
        return elementData.size();
    }
}