How to unit test instance creation?

Question

I have a Carpenter class that does it's work using a Lathe and a Wood object.

class Carpenter
{
    function Work()
    {
        $tool = new Lathe();
        $material = new Wood();
        $tool->Apply($material);
    }
}

Lathe depends on an interface called Material, so I can easily unit test Lathe by giving it a fake Material in my unit test. Wood doesn't depend on anything, so it can also be easily tested.

interface Material {
    // Various methods...
}

interface Tool {
    function Apply(Material $m);
}

class Wood implements Material  {
    // Implementations of Material methods
}

class Lathe {
    function Apply(Material $m) {
        // Do processing
    }
}

However, Carpenter depends on the concrete classes Lathe and Wood because it has to create instances of them. That means that as it currently stands, I cannot unit test the Work() method without inadvertantly bringing Lathe and Wood under test.

How should I change my design to unit test Carpenter?

Answer 1

There's a couple of different directions you can take here:

• Use Constructor Injection and simply inject the tool and the material instances into the carpenter.
• If injecting instances doesn't work for some reason (perhaps because you need to create new instances for every invocation of the Work method), you can inject Abstract Factories instead.
• You can also use the Factory Method approach described by ctford, but that requires you to also create test-specific overrides to be able to unit test, and while that's a completely valid thing to do, it's just more work and in many cases the other alternatives are better and more flexible.