Testing class invariants
One way of looking at unit tests is in terms of Design by Contract: most tests define postconditions and there can also be value in having tests showing that a class fails-fast on precondition failure. While helping out on a TDD course this week it occurred to me that Junit 4 provides a simple mechanism for checking class invariants in unit tests: it’s particularly useful to be confident invariants hold even after exceptions have been thrown.
For example, a game of Blackjack may allow dealing only once.
@After
public void ensureInvariants()
{
assertTrue(game.canDeal() == !game.isDealt());
}
Of course, it wouldn’t be rocket science to do the same in Junit 3.8:
protected void runTest() throws Throwable
{
super.runTest();
ensureInvariants();
}
public void ensureInvariants()
{
assertTrue(game.canDeal() == !game.isDealt());
}
(PS credit to Malcolm Sparks for suggesting that I blog this.)
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