public class Stack{
private Element current;
public void push(Object o){
Element e=current;
current=new Element();
current.previous=e;
current.object=o;
}
public Object pop(){
if(current==null){
return null;
}
Object o=current.object;
current=current.previous;
return o;
}
private class Element{
Element previous;
Object object;
}
}