I watched a presentation by Zsolt Fabok a while ago and saw a technique I hadn't thought of before. How do you verify that a parameter is used in a simple way? Feed the method with something that usually doesn't work and verify that something happens.

Send in null and assert that you actually receive a NullPointerException like this: 

@Test(expected = NullPointerException.class)
public void shouldVerifyParameterUsage() {
    Example example = new Example();
    example.foo(null);
}

There are other ways to trace a parameter or verify that it is being used, but this is a trivial way that often works. Similar to printf debugging. It is not pretty, but it is always available and works.

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