Unit testing - how do I test a function that returns random output?

Question

I have a function which takes in two parameters, and returns one or the other 50% of the time.

The unit test for this should determine that both parameters could be

Answer 1

You test it through separation and/or injection:

// ** WARNING: Untested code

Original

void funcToTest(String a, String b) {
  int i = new Random().nextInt(2);
  return ((i == 0) ? a : b);
}

Separating

void funcToTest(String a, String b) {
  return funcToTest(a, b, (new Random().nextInt(2) == 0));
}
void funcToTest(String a, String b, boolean r) {
  return (r ? a : b);
}

Now test

funcToTest(String a, String b, boolean r).

You're trusting that, if you test the above, then

funcToTest(String a, String b)

will work just work.

Injecting

interface BoolRandomizer {
  bool getBool();
}
BoolRandomizer BOOL_RANDOMIZER = new BoolRandomizer() {
  public bool getBool() { return (new Random().nextInt(2) == 0); }
}
BoolRandomizer ALWAYS_FALSE_RANDOMIZER = new BoolRandomizer() {
  public bool getBool() { return false; }
}
BoolRandomizer ALWAYS_TRUE_RANDOMIZER = new BoolRandomizer() {
  public bool getBool() { return true; }
}

class ClassUnderTest {
    private final BoolRandomizer br;
    ClassUnderTest() { this(BOOL_RANDOMIZER); }
    ClassUnderTest(BoolRandomizer br) { this.br = br; }
    void funcToTest(String a, String b) {
      return (br.getBool() ? a : b);
    }
}

Now test

new ClassUnderTest(ALWAYS_FALSE_RANDOMIZER).funcToTest()

and

new ClassUnderTest(ALWAYS_TRUE_RANDOMIZER).funcToTest()

There are of course more complex/slicker ways to do this, but this is the basic idea. I used these techniques for a deck of cards program to test the shuffle function. Another case: a function that uses current time by calling system API gettime() could be hard to test unless you are willing to run the unit test at very specific times of day :-). Separation and injection ease testing.