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 method with an int return type and a return statement, with both highlighted.

   public int doubled(int n)
   {
       return n * 2;
   }

   `int` before the name is the promise: this method WILL hand back a whole number.

   `return n * 2;` keeps the promise. It works out n * 2 and sends it back to whoever called.

   The type before the name and the value after return have to agree. Promise an int, return an int.

2. Step 2

   On your screen The returned value being stored and used at the call site.

   int result = doubled(5);   // result is 10
   move(doubled(2));          // moves 4

   Two ways to use the answer, exactly as in 1.5. Store it in a variable, or drop the call straight into another expression.

   If you call `doubled(5);` on its own line and do nothing with the result, the method runs and the answer is thrown away. That is legal and almost always a mistake.

3. Step 3

   On your screen A boolean-returning method used directly inside an if condition.

   public boolean isSafe()
   {
       return !isTouching(Spike.class);
   }
   
   if (isSafe())
   {
       move(3);
   }

   A boolean return is the most useful kind in this course, because it slots straight into an if.

   And look at what it did to the calling code: `if (isSafe())` says what it means. The detail of what "safe" means lives in one place, and if the definition changes you change it once.

4. Step 4

   On your screen A method with a return inside an if and no return afterward, with the missing return statement error shown.

   // WRONG. Java: missing return statement
   public int speed()
   {
       if (hasBoost)
       {
           return 8;
       }
       // if hasBoost is false, we fall off the end with no value
   }

   Java checks EVERY possible route through the method, and if any route can reach the end without returning, it refuses to compile.

   Here, when hasBoost is false, nothing is returned. The fix is either an else with its own return, or a plain `return 4;` after the if as the default.

   This strictness is a gift. The alternative is a method that sometimes hands back nothing, and you finding out at the worst moment.