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 world containing several enemies with a count displayed.

   List enemies = getObjects(Enemy.class);
   int howMany = enemies.size();

   `getObjects(Enemy.class)` hands back every Enemy currently in the world.

   `List` is the type. Read the angle parentheses as "of": a List of Enemy. It is new punctuation but not a new idea.

   `.size()` tells you how many. Note the parentheses: it is a method, not a field, unlike the array length you meet in Unit 5. That inconsistency is Java's fault, not yours, and it catches everybody.

2. Step 2

   On your screen A wave-complete check triggered when the enemy list is empty.

   if (getObjects(Enemy.class).size() == 0)
   {
       startNextWave();
   }

   A real use, and it goes in the WORLD's act() method. Yes, the world has one too, and it runs every frame just like an actor's.

   This is the natural home for anything about the game as a whole: how many enemies are left, whether the level is finished, what the score is.

3. Step 3

   On your screen A loop over a list using an index.

   List enemies = getObjects(Enemy.class);
   for (int i = 0; i < enemies.size(); i++)
   {
       Enemy e = enemies.get(i);
       e.turn(180);
   }

   Looping over a list with the counting loop from 4.2.

   `enemies.get(i)` fetches the one at position i. Positions start at 0 and the last is size() minus 1, which is why `i < enemies.size()` is exactly right.

   The next lesson shows a shorter way when you do not need the index.

4. Step 4

   On your screen A ConcurrentModificationException stack trace next to the corrected copy-first version.

   // THROWS ConcurrentModificationException
   for (Enemy e : getObjects(Enemy.class))
   {
       removeObject(e);
   }
   
   // SAFE: remove them all at once instead
   removeObjects(getObjects(Enemy.class));

   Here is the trap, and it is worth meeting before you write it by accident.

   Removing actors from the world while looping over the list you got FROM the world is like tearing pages out of a book while reading it. Java notices and throws ConcurrentModificationException.

   The name is alarming and the fix is one line. `removeObjects(...)` takes the whole list and clears it in one go. When you need finer control, collect what you want to remove into a separate list first, then remove them after the loop has finished.