EngineerConstucter

import java.util.LinkedList;

public class Main2 {

    public static void main(String[] args) {
        StackImp s = new StackImp();
        s.push(1);
        s.push(2);
        s.push(3);
        s.push(4);
        System.out.println(s.count);
        System.out.println(s.Itempeek());
    }
}

/**
 * @inv isEmpty() implies elements.size() == 0
 */
class StackImp implements Stack {

    private final LinkedList elements = new LinkedList();
    int count = 0;

    public void push(Object o) {
        if (o != null) {
            elements.add(o);
            count++;
        } else {
            throw new RuntimeException("");
        }
    }

    @Override
    public Object pop() {
        final Object popped = top();
        elements.removeLast();
        return popped;
    }

    @Override
    public Object top() {
        return elements.getLast();
    }

    @Override
    public boolean isEmpty() {
        return elements.isEmpty();
    }

    @Override
    public int size() //  method update
    {
        return count;
    }

    @Override
    public Object Itempeek() //  method update
    {
        return top();
    }
}

interface Stack {

    /**
     ** @pre o != null
     * @post !isEmpty()
     * @post top() == o
     */
    void push(Object o);

    /**
     * @pre !isEmpty()
     * @post @return == top @pre
     */
    Object pop();

    /**
     * @pre !isEmpty()
     */
    Object top();

    boolean isEmpty();

    int size();

    Object Itempeek();
}