Build it step by step

Every step below leaves a scenario that compiles and runs. If you stop halfway you will have something that works, not something broken. Follow along in Greenfoot rather than reading straight through.

1. Step 1

   On your screen A row of five enemies spawned across the top of the world.

   public void startWave()
   {
       for (int i = 0; i < 5; i++)
       {
           addObject(new Enemy(2), i, 0);
       }
   }

   Five enemies, one loop. Everything here you already have: the for loop from 4.2, addObject from 3.7, and the Enemy constructor from 3.9.

   They land at x values 0, 1, 2, 3 and 4, all bunched at the left, which the next step fixes.

2. Step 2

   On your screen The same five enemies spaced evenly across the world.

   for (int i = 0; i < 5; i++)
   {
       addObject(new Enemy(2), i * 3, 0);
   }

   One change: `i * 3` instead of `i`. Now they sit at 0, 3, 6, 9 and 12.

   Using the counter in a CALCULATION rather than directly is the single most useful loop trick in this course, and it is the whole basis of tile maps in Unit 6.

3. Step 3

   On your screen A world flooded with hundreds of enemies after the spawn loop was put in act.

   // DISASTER. act() runs every frame.
   public void act()
   {
       for (int i = 0; i < 5; i++)
       {
           addObject(new Enemy(2), i * 3, 0);
       }
   }

   This does not freeze. The loop finishes perfectly every time, so 4.1's rule is not broken.

   It spawns five enemies EVERY FRAME. Within a second there are hundreds, and the scenario grinds to a halt for a completely different reason: not an infinite loop, but a finite loop run infinitely often.

   Spawn loops belong in a method you call ONCE, from prepare() or when a wave begins.

4. Step 4

   On your screen A new wave beginning automatically when the previous one is cleared.

   public void act()
   {
       if (getObjects(Enemy.class).size() == 0)
       {
           waveNumber = waveNumber + 1;
           startWave();
       }
   }

   And the whole unit assembled into a working game loop.

   Every frame, the world checks whether the list of enemies is empty (4.5). When it is, the wave counter goes up (3.8, an instance variable) and the spawn method is called (4.7).

   The loop is still called once per wave, not once per frame, because the if guards it. That is the difference between this and the previous step.