Have you ever come across the problem where, in order to construct the object you are trying to test, you end up creating massive scaffolding code just to pass to the constructor? Perhaps an example is in order:
class ChessGame { // Some instance variables public ChessGame (Player player1, Player player2, int timeLimit) { this.player1 = player1; this.player2 = player2; this.timeLimit = timeLimit; } public void playGame() {...} } class ChessGameTest { @Test public void testChessGamePlaysForXseconds () throws Exception { Player player1 = createMock(Player.class); Player player2 = createMock(Player.class); Handicap handicap1 = new Handicap(10, Rules.Standard); Handicap handicap2 = new Handicap(4, Rules.Standard); expect(player1.getName()).andReturn("D. Duck"); expect(player2.getName()).andReturn("M. Mouse"); expect(player1.getHandicap()).andReturn(handicap1); expect(player2.getHandicap()).andReturn(handicap2); replayAll(); new ChessGame(player1, player2, 10).playGame(); // Assertions .... } }
The problem here is that we’ve specified dependencies on far too many objects and values! By asking for two Player objects, you’re essentially saying “give me two objects which have names, ages, total scores, rankings, biographies, etc… and which also have very detailed Handicap information”.
Ask yourself this question:
- Without looking inside the class file, what exactly does the ChessGame need in order to execute playGame()?
It’s not at all obvious is it! My guess would be that we only really need player names, and handicap information (why do we need the player bio to play a match??). If this is the case, why do we pass all that extra information to the ChessGame constructor, if it’s never going to need it?
I can hear you answering “But it’s far more convenient to package up that information as it exists naturally – inside the Player object. It is more readable, and keeps the number of constructor parameters down!”
But this is not a valid reason because of the following:
- Classes should not ask for more information than they need – otherwise they become difficult and cumbersome to test
- Classes should make absolutely clear what their dependencies are. This is why passing around “Contexts” is a bad thing
We can make this a lot easier by using a Factory Method as a convenience:
class ChessGame { public ChessGame (String player1Name, String player2Name, int player1Handicap, int player2Handicap, int timeLimit) { this.player1Name = player1Name; this.player2Name = player2Name; //..etc } public static ChessGame withPlayers (Player player1, Player player2, int timeLimit) { return new ChessGame ( player1.getName(), player2.getName(), player1.getHandicap().getAbsolute(), player2.getHandicap().getAbsolute()); } public void playGame() {...} } class ChessGameTest { @Test public void testChessGamePlaysForXseconds () throws Exception { new ChessGame("D. Duck", "M. Mouse", 10, 4, 10); // Assertions .... } }
The second ChessGame class accepts only what it needs – it has fewer dependencies on real objects. However, we now have a nice convenience method on the object which takes our existing objects.
Why stop there? Here are some more Factory Methods which you might prove useful:
public static ChessGame fromExistingGame (ChessGame previousGame) {...} public static ChessGame againstBot (BotFactory botFactory) {...} public static ChessGame fromDataFile (File dataFile) {...}
So in conclusion – if you’re finding yourself setting up tonnes of scaffolding for a test, ask yourself whether the unit-to-be-tested is asking too much of you!
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