Java - ayuda en codigo Java marca errores

package pila;

import java.util.*;

    interface IPila{

        public boolean llena();

        public boolean vacia();

        public void push(int elem);

        public int pop();

    }

class Pila implements IPila {

    int tope=-1;

    private int [] pila = new int [10];

    final int MAX = 9;

    public boolean llena (){

        return (tope==MAX);

    }

    public void push (int elem){

     if(this.llena()) then

                  //ERROR

     else{

       tope ++;

       pila[tope]=elem;

     }

    }

    public boolean vacia(){

      return(tope==-1);

    }

    public int pop(){

      if(this.vacia())then

                   //ERROR

    else{

    int x=pila[tope];

    tope --;

    return x;

      }

     }

    }

package pila;

import java.util.Vector;

/**

 *

 * @author MarAnn

 */

public class Stack {

    private Vector items;

    public Stack(){

        items = new Vector(10);

    }

    public Object push(Object item){

        item.addElement(item);

        return item;

    }

    public synchronized Object pop(){

        int len = items.size();

        Object obj = null;

        if(len == 0) throw new EmptryStackException();

        obj = items.elementAt(len-1);

        items.removeElementAt(len-1);

        return obj;

    }

    public boolean isEmpty(){

        if(items.size == 0) return true;

        else return false;

    }

}

interface IPila {

	public boolean llena();

	public boolean vacia();

	public void push(int elem);

	public int pop();

}

class Pila implements IPila {

	int tope = -1;

	private int[] pila = new int[10];

	final int MAX = 9;

	public boolean llena() {

		return (tope == MAX);

	}

	public void push(int elem) {

		if (this.llena()) {

			// ERROR

		} else {

			tope++;

			pila[tope] = elem;

		}

	}

	public boolean vacia() {

		return (tope == -1);

	}

	public int pop() {

		if (this.vacia()) {

			// ERROR

			// Aqui debes retornar algo!

			return 0;

		} else {

			int x = pila[tope];

			tope--;

			return x;

		}

	}

}

import java.util.EmptyStackException;

import java.util.Vector;

public class Stack {

	private Vector items;

	public Stack() {

		items = new Vector(10);

	}

	public Object push(Object item) {

		items.addElement(item);

		return item;

	}

	public synchronized Object pop() {

		int len = items.size();

		Object obj = null;

		if (len == 0)

			throw new EmptyStackException();

		obj = items.elementAt(len - 1);

		items.removeElementAt(len - 1);

		return obj;

	}

	public boolean isEmpty() {

		if (items.size() == 0)

			return true;

		else

			return false;

	}

}