Exploratory Testing: Did a Parameter Object Get Changed?

Exploratory testing is a useful way to find out how legacy code works. If you can write tests that exercise the legacy code in all its paths, you can then feel much safer in refactoring the code. If the refactored code still passes the tests, the refactoring is correct and has not created any side effects.

Sometimes parameter objects are modified inside methods. One way to test for this is to create two instances, pass one in to the method, and then compare the one passed in to see if it is equal to the unmodified one. This assumes that you have a correct implementation of equals().

public class LegacyTest {
    private Legacy legacy;
    private PassedIn passedIn;
    private PassedIn untouched;
    
    
    @Before
    public void setUp()
    throws Exception {
        legacy  = new Legacy();
        passedIn = new PassedIn();
        untouched = new PassedIn();
    }
    
    @Test
    public void passedInDidNotChange() {
        assertEquals(untouched, passedIn);
        legacy.method(passedIn);
        assertEquals(untouched, passedIn);
    }
}