EclEmma: the free Java code coverage tool for Eclipse

Unit testing has a lot of aspects which are more or less equally important but code coverage is something on what you really have to pay attention. Depending on the complexity of our methods, more combinations are possible on arbitrary test inputs. Considering every permutation in some not-so-obvious cases takes some time and effort, and the chance to make a mistake – which will result in an uncovered piece of code – is even higher.

EclEmma is the right tool to measure the coverage of your code and to make sure that every single line is executed properly. It has plenty of useful features from which I’ll present only the Coverage tool. In this post you will:

  • see how to run your unit tests through EclEmma
  • how to interpret the output of the Coverage tool

I’m using EclEmma with JUnit4 but it will run with JUnit3 too. You will find any neccessary information regarding to the installation on EclEmma site: http://eclemma.org/.

So lets take a look at an example from the source of Qt Jambi. QPair of the com.trolltech.qt package is used to hold two generic values. There are 3 methods to test: public boolean equals(Object o), public String toString() and public QPair clone().

I won’t give any detailed explanations about the test methods since the main goal of this post is to introduce EclEmma. However if you are new in unit testing too you could take a look at one of my old posts Unit testing with Qt Jambi. There are a lot of resources about unit testing but that post is directly related with the currently studied examples.

Let’s take a look at public boolean equals(Object o) which is a method to check the equality of two objects (beware, it could be very tricky!). If you can’t access the resource above, don’t worry the code is reproduced here:

    /**
     * Returns true if this pair is the same as the other pair. If any
     * of the first or second members are null the result is false regardless.
     * @param The other parameter
     * @return True if they are equal.
     */
    @Override
    public boolean equals(Object o) {
        QPair<?, ?> other = o instanceof QPair ? (QPair<?, ?>) o : null;
        if (other == null || first == null || second == null || other.first == null || other.second == null)
            return false;
        return first.equals(other.first) && second.equals(other.second);
    }

Our test case for this particular method is:

	@org.junit.Test
	public void testEquals() {
		assertTrue("qp1", qp1.equals(qp1));
		assertFalse("qp2", qp1.equals(qp2));
		assertFalse("qp3", qp1.equals(qp3));
	}

       @org.junit.Before
       public void setUp() {

               qp1 = new QPair<Integer, Integer>(3, 5);
               qp2 = new QPair<Integer, Integer>(5, 3);
               qp3 = new QPair<Integer, Boolean>(1, null);
               qp4 = new QPair<Integer, Boolean>(null, true);	
       }

While the test case is opened, from the Run menu select Coverage As > JUnit Test. The JUnit test starts and EclEmma is calculating some measurable properties of your code. Now you should see something like this:

As EclEmma finished its job you can see the coverage result of your unit test code:

and the code of the tested class too:

But some of the lines appeared in yellow instead of green, what happened? If you take a closer look to these lines EclEmma will tell you that there are some uncovered branches in your code. In this particular case we have 5, 3 on line 79 and 2 on line 81:

Lets take a look again at our test and study it carefully:

	@org.junit.Test
	public void testEquals() {
		assertTrue("qp1", qp1.equals(qp1));
		assertFalse("qp2", qp1.equals(qp2));
		assertFalse("qp3", qp1.equals(qp3));
	}

       @org.junit.Before
       public void setUp() {

               qp1 = new QPair<Integer, Integer>(3, 5);
               qp2 = new QPair<Integer, Integer>(5, 3);
               qp3 = new QPair<Integer, Boolean>(1, null);
               qp4 = new QPair<Integer, Boolean>(null, true);	
       }

Is there any assertion in our test which is running through this if:

               if (other == null || first == null || second == null || other.first == null || other.second == null)

Lets see: the expression inside if will be true iff at least one of those conditions between || is true. Our only assertion which involves null is this one:

               assertFalse("qp3", qp1.equals(qp3));

which is, obviously, won’t cover all the possible candidates in which the above if will be executed. This assertion covers exactly one case. Remember QPair(T, S), and because qp3 was constructed with (1, null) it covers:

               if (other == null || first == null || second == null || other.first == null || other.second == null)

Thanks EclEmma for pointing out this obvious mistake! After adding more assertions to cover all the possible cases you will see something like this:

All branches are covered properly! \o/

Of course this is just one from EclEmmma’s useful features. In case of big projects it is good to see how your overall coverage is forwarding, you can check that too:

As you can see we have a lots of work to do.

This was a very short but fair introduction about EclEmma’s main feature, using it on regular basis to check your unit tests functionality will save you lots of time.

Thank you for your time, comments are welcome, as always!

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