TestInvariant - Invariant Testing

In Design by Contract (DbC) you define an invariant for each class. This invariant states the properties of the class that must always be true. A object must always satisfy its invariant (unless it’s in the middle of doing anything.) A failure in the invariant throws an exception. (This checking may be turned off for production use if desired for performance.)

Applying this idea to TDD means that you define a common method to test the invariant in your production classes and test it in your test code.

It’s time for the usual trivial example.

public class Bowler ...
    int overs, runs, wickets;

A simple invariant for a bowler is that these values should all by non-negative. So you could have an invariant defined like this.

public boolean passesInvariant() {
	return (runs >= 0 && overs >= 0 && wickets >= 0);
}

You would then using during testing after the setup and exercise phases of the test.

public void testConcedingRunsAddsToRunsScore() {
    Bowler botham = new Bowler();       // setup - showing my age
    assert botham.passesInvariant();
    botham.concedeRuns(4);              //exercise
    assert botham.passesInvariant();
    assertEquals(4, botham.getRuns());  //verify
}