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 The Crab class code open, showing an empty act method with a comment inside it.

   public class Crab extends Actor
   {
       public void act()
       {
           // your code goes here
       }
   }

   Every actor class starts with this. It is empty, which is why a new actor sits there doing nothing. Greenfoot is already calling this method for you, sixty-odd times a second while the scenario runs. It just has nothing to do.

   Read the signature with what you learned in 1.5: `void` so nothing comes back, `act` is the name, `()` so it needs nothing from you. YOU never call this method. Greenfoot calls it.

2. Step 2

   On your screen The crab moving steadily across the world after a move call was added to act.

   public void act()
   {
       move(2);
   }

   One line, and the crab walks by itself. Compile, then press Run.

   Look at what that line actually says: "move 2". Not "move 2 over and over". The repetition is not in your code at all. It comes from Greenfoot calling act() again next frame, and the frame after that.

3. Step 3

   On your screen The crab part way across the world, paused mid-journey after a single press of the Act button.

   Press Reset, then press ACT once instead of Run. The crab moves 2 cells and stops. Press it again: 2 more.

   This is the clearest demonstration of what a frame is, and it is worth doing rather than reading. Run is just this button held down.

4. Step 4

   On your screen The crab turning back on itself at the edge of the world.

   public void act()
   {
       move(2);
       if (isAtEdge())
       {
           turn(180);
       }
   }

   Now it patrols. Each frame: move 2, then ask "am I at the edge?", and if so turn around.

   You have not formally met `if` yet, that is Unit 2. Read it as plain English for now: IF the thing in the parentheses is true, do what is in the braces. `isAtEdge()` is a method that hands back true or false, which is exactly the shape of thing an if needs.