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Communicating my thoughts on software development

Using Hamcrest and JUnit

Posted by rnaufal on March 15th, 2010

Lately I started using the core Hamcrest matchers bundled with the JUnit framework to create more readable unit tests.

Hamcrest matchers were created to improve the readability of unit testing code. It’s a framework which facilitates the creation of matcher objects to match rules specified in unit tests. Some examples will let it to be clearer:

   1 import static org.hamcrest.CoreMatchers.equalTo;
   2 import static org.hamcrest.CoreMatchers.is;
   3 import static org.junit.Assert.assertThat;
   4 
   5 @Test
   6 public void shouldBeTheSamePerson()
   7 {
   8     Person me = new Person( "Rafael" );
   9     Person theOther = new Person( "Rafael" );
  10     assertThat( me, is( theOther ) );
  11 }
  12 
  13 @Test
  14 public void shouldHaveFixedSizeNumbers()
  15 {
  16     List<Integer> numbers = Arrays.asList( 1, 2, 3, 4, 5 );
  17     assertThat( numbers.size(), is( equalTo( 5 ) ) );
  18 }

The first example checks if one Person object is equal to another using the Object equals method, which was overridden in the Person class. The is syntax defines a matcher which is a shorthand to is(equalTo(value)). The second one uses the is(equalTo(value)) matcher to check the size of an integer list of fixed size numbers. The assertThat method is used in conjunction with the is(equalTo(value)) matcher, which makes the test sentence very human readable.

An interesting thing is the possibility to create a custom matcher, like this one which tests if a given list only has even numbers:

   1 public class AreEvenNumbers extends TypeSafeMatcher<Collection<Integer>> {
   2 
   3     @Override
   4     public boolean matchesSafely(Collection<Integer> numbers) {
   5         for (Integer number : numbers) {
   6             if (number % 2 != 0) {
   7                 return false;
   8             }
   9         }
  10         return true;
  11     }
  12 
  13     @Override
  14     public void describeTo(Description description) {
  15         description.appendText("even numbers");
  16     }
  17 
  18     @Factory
  19     public static <T> Matcher<Collection<Integer>> evenNumbers() {
  20         return new AreEvenNumbers();
  21     }
  22 }

And below are two tests which uses the AreEvenNumbers custom matcher:

   1 import static org.hamcrest.CoreMatchers.is;
   2 import static org.junit.Assert.assertThat;
   3 import static br.com.rafael.hamcrest.AreEvenNumbers.evenNumbers;
   4 
   5 @Test
   6 public void shouldHaveOnlyEvenNumbers()
   7 {
   8     List<Integer> numbers = Arrays.asList( 2, 4, 6, 8, 10 );
   9     assertThat( numbers, is( evenNumbers() ) );
  10 }
  11 
  12 @Test
  13 public void shouldNotHaveOddNumbers()
  14 {
  15     List<Integer> numbers = Arrays.asList( 1, 2, 4, 6, 8, 10 );
  16     assertThat( numbers, not( evenNumbers() ) );
  17 }

These two tests use the static factory method evenNumbers to instantiate the matcher on the test code. Not the use of the not matcher on the shouldNotHaveOddNumbers test to assert that no odd numbers are present on the given list. All tests use the static import feature, which turns the test not clean and not cluttered with the class qualification.

I haven’t experienced the other common matchers on unit testing code, like the Beans, Collections and Number ones. I think they turn the tests more readable, clean and easy to change. And you? Have you ever used Hamcrest matcher? If you have other examples of using it, post them here!

Updated: Static imports were added to the testing code.

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