I needed an algorithm to traverse a graph of relationships and test whether it was a connected tree or not, so I coded-up a quick (and relatively efficient) search. No matter how many times I code graph searches, I know myself well enough to expect a couple of mistakes, so I went to write some tests after I'd finished with the algorithm. The reason that I'd put off writing the tests was that the signature for my search was the following:
static FancyIface findRoot(Map<FancyIface,FancyIface[]> graph)
FancyIface is an interface with a bazillion methods and where the API doesn't expose any implementations. Rather than mocking instances of FancyIface for testing purposes, it makes sense to genericize the method:
static <T> T findRoot(Map<T,T[]> graph)
and then code the tests with something nice and simple, like integers:
public void testFindRoot() {
Map<Integer,Integer[]> m = new HashMap<Integer,Integer[]>();
m.put(1,new Integer[] {2,3});
m.put(2,new Integer[] {4});
// should return 1, as this is a tree rooted at 1.
assertTrue(Foo.findRoot(m) == 1);
m.put(3,new Integer[] {4});
// should return null because this is no longer a tree.
assertFalse(Foo.findRoot(m) == null);
}
And that's it.
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