Testing those private methods

21 Jan 2008

I’ve been working my way through Working Effectively with Legacy Code and one of their strategies for testing classes has particularly hit home.

Situation

You’ve fixed a bug in a method that lacks any test cases and is not easily incorporated into a test harness.

More often than not this method is private (because we all test our public methods right?)

The obvious and straight-forward approach to testing (in this situation) would be to write a test case against whatever public method ultimately results in the problem code being executed.

Unfortunately, if the public method was easily tested we hopefully wouldn’t be finding ourselves in this situation (because we’d either have had test cases that already executed the problem code – or they’d be easily added).

What you’re often faced with is a combination of two things, local state maintained by the class (pesky instance variables) and state passed in to the method as parameters. With appropriate use of stubs and mock objects, the latter should be workable. However, unless you’re dealing with a stateless class the matter of local state is an annoying yet unavoidable complication.

What follows is one idea that I’ve seen work quite successfully in these types of situations.

Step 1: The Minor Refactor

This initial refactor involves extracting the pertinent portion of business logic (that you want to put under test) and any instance-level state that it requires. Often this is as simple as extracting a private method and passing any required class variables/state in as parameters. What you should be left with is a method that has all of it’s dependencies provided to it, this is important.

private void updateSpinner(List xyzs)
    {           
        // this map should contain 1 and only 1 spinner
        JSpinner spinner = instanceSpinnerMap.values().iterator().next();
        spinner.setValue(xyzs.size());
        spinner.setEnabled(false);
    }

becomes

private void updateSpinner(List xyzs, JSpinner spinner)
    {
        spinner.setValue(xyzs.size());
        spinner.setEnabled(false);
    }

Fairly trivial change and significantly more testable (ignoring the fact that it’s still a private method).

Step 2: A little less private

Unfortunately, private methods aren’t the easiest things in the world to test. You may not like what I’m going to suggest next… but depending on the situation, you may get a lot of mileage out of making that private method package-private instead. If you’re already programming against interfaces and have a decent package layout, you likely won’t run into too many problems.

Step 3: Write test cases

Now that you’re able to instantiate the class and invoke it’s new package-private utility method, there’s no excuse not to write a couple of unit tests against it.

Step 4: Updated Functionality

Obviously there was a method to our madness and the whole reason we under took this exercise was the change the functionality of this class/method.

Depending on your approach to testing (TDD or TSA – test soon after) you should now be able to update the functionality of the method and write new test cases.

Step 5: Run Tests

Run both the previous and new test cases to verify that you haven’t broken previous functionality and that it also supports the new capabilities you just finished adding.

All in all, I’ve seen this work quite successfully. With a little discipline, there’s no reason why we can’t get more of the code we’re writing under test. Of course this isn’t a replacement for interface-level testing nor a reason to throw the private modifier out the window, but rather another idea to keep in the back of your mind as your bug fixing or making minor feature changes.