Pretty new to Java can someone help with an addObject() problem?

Hi I'm pretty knew to programming and I have an assignment which I've been working on but I am stuck

I have to create a 'space invaders' style game, and I've got some basics working. I wanted to add a boss enemy, which appears when all other enemies have been destroyed. It kind of works, only the program spawns a seemingly infinite amount of the boss which then creates a 'wipe' effect across the top of the screen.
Here is my code:

Code :

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import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.util.List;
 
 
public class Space extends World
{
 
     /**
     * Constructor for objects of class Space.
     *
     */
     public Space()
     {
          // Create a new world with 200x250 cells with a cell size of 3x3 pixels.
          super(200, 250, 3);
          //add class with starstreaks
          addObject(new EmptyBox(),1,1);
          //add player character
          addObject(new Player(), 100, 228);
 
          //add 4 romulan enemies row 1
          for(int i=0; i<4; i++){
               addObject(new Romulan(), (25 + 50 * i), 15);
          }
 
          //add 3 klingon enemies row 2
          for(int i=0; i<3; i++){
               addObject(new Klingon(), (50 + 50 * i), 40);
          }
 
          //add 4 romulan enemies row 3
          for(int i=0; i<4; i++){
               addObject(new Romulan(), (25 + 50 * i), 65);
          }
 
          //add 3 klingon enemies row 4
          for(int i=0; i<3; i++){
               addObject(new Klingon(), (50 + 50 * i), 90);
          }
 
          //add 3 shields
          for(int i=0; i<3; i++){
               addObject(new Shield(), (30 + 70 * i), 180);
          }
     }
 
     [B]public void act(){
 
          List<Enemy> enemies = getObjects(Enemy.class);
 
          if(enemies.size()==0 ){
               addObject(new Borg(), 100, 30 );
          }
     }[/B]
}

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I believe the part in bold is the problem, but I dont know why. Klingon and Romulan are subclasses of the Enemy class. The Borg class is it's own seperate class, but with the exact same functions as Enemy. I made it seperate as I figured it would keep respawning if it was a subclass of Enemy.


Any ideas?

Its ok, after more than an hour I've got it.
I simply added private int bossValue = 0; at the beginning and then changed act to this:

Code :

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public void act(){
 
List<Enemy> enemies = getObjects(Enemy.class);
 
 
if(enemies.size()==0 && bossValue==0 ){
//for(int i=0; i<1; i++){
 
addObject(new Borg(), 100, 30 );
bossValue =1;
//}
}
}

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