Taking Exception with Exceptions

Exceptions in Java are thrown when an abnormal condition arises. Exceptions not handled by a method are passed (via the throws keyword) to its parent. More often that not, test suites focus on the normal behavior of a method, and skip over the cases where the unhandled exceptions are thrown. A comprehensive unit testing effort should test each error condition separately. Thus, fewer thrown exceptions means fewer individual tests will be needed.

Consider a method with this signature:

public void foo() throws Exception1, Exception2, Exception3

What if there's a bug in this method that causes Exception1 to be thrown instead of Exception2? In order to fully test this method, our test suite needs to include at least three JUnit tests to confirm that the proper exception is thrown when the corresponding abnormal condition arises.

Even if the correct type of exception is thrown, it may be the case that all three exception types are handled the same way, or it may be that some of the exceptions can be handled by the method itself. If we can remove even one of the thrown exception types, we'll reduce our testing effort by one-third. There is no shortcut for reducing the number of thrown exceptions, of course -- indeed, each distinct exception may be necessary -- but by keeping in mind the number of thrown exceptions as you code, you may be able to streamline your testing effort down the road.

Lose Those Extra Parameters

Most unit testing strategies focus on a method's parameters. By definition, they control the behavior of the method under test, so varying the input values of the method, one can build a strong test suite. But the more parameters that your method under test has, the more tests that will be required to do complete black-box testing. Obviously, most methods take one or more parameters, but if there are clear ways to limit the number of parameters, it will be a clear win for black-box testing scenarios.

Consider this method signature:

public int addOrSubtract(int x, int y, Operator operator)

Each additional parameter increases the number of possible combinations that you'd likely want to test. If you try three possible values for each parameter, that's 27 tests for this method. If there are only two possible operators (addition and subtraction), we could refactor this method into two simpler ones:

public int add(int x, int y)
public int subtract(int x, int y)

Now, even if we try three possible values for each parameter, we still only wind up with nine combinations for each method, or 18 total -- we've reduced our testing effort by one-third. Obviously, there is no cut-and-dried method for reducing the number of parameters, and there may indeed be cases where your method under test does require many parameters, but by keeping the method's signature to as few parameters as possible, you'll save yourself testing effort down the road.

Cyclomatic Complexity and Unit Testing

I follow both Andrew Binstock and Andy Glover from Stelligent and have much respect for them both and would like to add to a blog that was posted.

I like their approach to using metrics to figure out the number of unit tests needed for a method. Actually, metrics should be used to determine the minimal number of unit tests. Cyclomatic complexity (CC) can be used for this, but so can other metrics like Define-Use. For those who don't know what CC is, it is defined as the minimal number of linearly independent paths within a unit of code (like a method).

The blog above provides a correlation between the number of CC paths and the number of unit tests. I think any unit test is better than no unit test, but I believe their correlation only applies if you don't know what the CC paths are.

For full disclosure, I am a co-founder of a product company (www.codign.com). The products are Eclipse plug-ins that design unit tests based on Cyclomatic Complexity AND Define-Use. We have a new release coming out very soon, which takes a very interesting spin on commodity metrics, but I'll blog about that later :)

Having worked for Tom McCabe for many years (he came up with Cyclomatic Complexity), I understand the benefits and pitfalls to this metric. The calculation is very simple: CC equals the number of decisions + 1. The proof is a bit more complicated :) , but that's the gist of it.

So, for example, a method with 2 decisions has 4 total paths and 3 CC paths. Let's take this example:



The questions I hear are, what are the three CC paths? Shouldn't you try to test them all? Can you test them all?

To figure out what the three CC paths are, you have to define your basis path (or path #1). This is very much a judgment call - it can be the longest path, the shortest path, the happy path, and so on. For us at Codign, our Path 1 (aka our Basis Path) is the path that executes all the true branches in a method. From there, or from any basis path, you simply work your way down by flipping decisions.

With the example above, there are four TOTAL paths:

Path 1: TT (both if statements execute 'TRUE')
Path 2: TF (first if statement is true, second is false)
Path 3: FT (first if statement is false, second is true)
Path 4: FF (first if statement is false, second is false)

There are 2 sets of CC paths:
Set 1
Path 1: TT
Path 2: FT
Path 3: TF

Set 2
Path 1: FF
Path 2: TF
Path 3: FT

Look at the code again and you'll notice that Path 1 from Set 1 (TT) cannot be tested (it is unrealizable). But why not test the other two paths? Given this information, is the effort required to test the other remaining basis paths that difficult?

As an aside, ever measure branch coverage? The above example can be tested with two tests (TF, FT) which would produce 100% branch coverage.

My point is this ... don't take CC at face value. I don't agree that, for example, if you have 20 CC paths you should only write 10 unit tests. I believe that, if a CC path is testable, you should test it.

But like I said earlier, not all CC paths are realizable, so is there a better way to identify the minimal number of unit tests? Ones that are realizable and measurable? There is, but I will blog about that later. :)