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 while loop in an editor with the condition and body labeled, and the counter increment highlighted.

   int count = 0;
   while (count < 5)
   {
       addObject(new Coin(), count, 0);
       count = count + 1;
   }

   Three parts, and all three are required.

   A starting value: `int count = 0;` A condition: `count < 5`, checked BEFORE each pass. A change: `count = count + 1;`, which moves the condition toward false.

   This places five coins, at x values 0, 1, 2, 3 and 4. Note it stops at 4, not 5, because the check happens first and 5 < 5 is false.

2. Step 2

   On your screen The same loop with the increment line deleted and Greenfoot frozen.

   // FREEZES. count never changes, so count < 5 is true forever.
   int count = 0;
   while (count < 5)
   {
       addObject(new Coin(), count, 0);
   }

   Delete one line and the scenario locks up completely.

   count starts at 0 and nothing ever changes it, so `count < 5` is true on every check, forever. Greenfoot never regains control and the window stops responding.

   When a loop hangs, look for the line that should be moving the condition. It is nearly always missing rather than wrong.

3. Step 3

   On your screen A diagram contrasting a finishing loop inside act with a forever loop inside act.

   // FINE. Runs 5 times, then act() returns.
   public void act()
   {
       int i = 0;
       while (i < 5) { i = i + 1; }
       move(1);
   }

   And here is the reconciliation with Unit 1.

   A loop inside act() is completely fine PROVIDED IT FINISHES. This one runs five times, stops, and act() returns normally. Greenfoot draws the next frame and everything is well.

   What 1.6 warned about was `while (true)`, which never returns. The problem was never loops. The problem was act() not finishing.

4. Step 4

   On your screen A loop whose condition is false at the start, with the body never executing.

   int lives = 0;
   while (lives > 0)
   {
       // never runs
       lives = lives - 1;
   }

   A while loop can run ZERO times. The condition is checked before the first pass, and if it is already false the body is skipped entirely.

   That is usually what you want, and it is worth knowing so a loop that "did nothing" is not automatically assumed broken.