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 constructor with its name matching the class name, both highlighted.

   public class OceanWorld extends World
   {
       public OceanWorld()
       {
           super(20, 15, 30);
       }
   }

   Two things mark this out as a constructor rather than a method.

   Its name is EXACTLY the class name, capital letter and all. It has NO return type. Not void, not int. Nothing at all between public and the name.

   If you write `public void OceanWorld()` you have quietly made an ordinary method that never runs, and Java will not warn you.

2. Step 2

   On your screen The Greenfoot Reset button being pressed and a fresh world appearing.

   When does it run? Once, when the world is created. In Greenfoot that means when the scenario opens and every single time you press Reset.

   You never call it yourself. Creating the object runs it for you, exactly like act().

   This is why hand-placed actors vanish on Reset: the new world runs its constructor, and the constructor knows nothing about actors you dragged in.

3. Step 3

   On your screen A constructor with super first followed by other setup lines.

   public OceanWorld()
   {
       super(20, 15, 30);   // MUST be first
       setPaintOrder(Fish.class);
       prepare();
   }

   super must be the FIRST statement. Java enforces this, and the reason is sensible: the inherited World part of your object has to exist before anything else touches it. There is no world to set a paint order on until super has run.

   Everything else follows. Here the constructor sets a paint order and then calls prepare(), which is the next lesson and where the actors get placed.