private int timer = 60+Greenfoot.getRandomNumber(180);;

if (--timer == 0)
{
    shoot();
    timer = 60+Greenfoot.getRandomNumber(180);
}

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class Alien here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Alien extends Actor
{
  private int timer = 60+Greenfoot.getRandomNumber(180);
  public Alien()
  {
      setRotation(0);
  }
    
  /**
     * Act - do whatever the Alien wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
       Actor bullet = getOneIntersectingObject(AlienBullet.class); //variable qui retourne la classe bullet
    
       if (--timer==0){
         shoot();
         timer = 30+Greenfoot.getRandomNumber(180);
        }
    }    
    
  public void shoot()
   {
     getWorld().addObject(new AlienBullet(), getX(), getY());   
   }
}

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class AlienBullet here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class AlienBullet extends AllBullets
{
    private static final int DEFAULT_DIRECTION = 0;
    private int direction;
    private int timer = 60+Greenfoot.getRandomNumber(180);
    public AlienBullet()
    {
        this(DEFAULT_DIRECTION);
    }   
    
    public AlienBullet(int direction)
    {
        setRotation(direction);
    }   

     public void shoot()
   {
     getWorld().addObject(new AlienBullet(), getX(), getY());   
   }
   
   public void act()
   {
     if (--timer==0){
       shoot();
       timer = 30+Greenfoot.getRandomNumber(180);
      }   
   
    
   }
}