Monday, April 23, 2012

Improving On assertEquals with JUnit and Hamcrest

In my blog post Are Static Imports Becoming Increasingly Accepted in Java?, I discussed the increasing use of static imports in Java to make code more fluent in certain contexts. Unit testing in Java has been particularly affected by the static import and in this blog post I provide one quick example of using static imports to make more fluent unit tests that use JUnit and Hamcrest.

The next code listing is a simple IntegerArithmetic class that has one method that needs to be unit tested.

IntegerArithmetic.java
package dustin.examples;

/**
 * Simple class supporting integer arithmetic.
 * 
 * @author Dustin
 */
public class IntegerArithmetic
{
   /**
    * Provide the product of the provided integers.
    * 
    * @param integers Integers to be multiplied together for a product.
    * @return Product of the provided integers.
    * @throws ArithmeticException Thrown in my product is too small or too large
    *     to be properly represented by a Java integer.
    */
   public int multiplyIntegers(final int ... integers)
   {
      int returnInt = 1;
      for (final int integer : integers)
      {
         returnInt *= integer;
      }
      return returnInt;
   }
}

A common approach for testing one aspect of the above method is shown next.

   /**
    * Test of multipleIntegers method, of class IntegerArithmetic, using standard
    * JUnit assertEquals.
    */
   @Test
   public void testMultipleIntegersWithDefaultJUnitAssertEquals()
   {
      final int[] integers = {2, 3, 4 , 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
      final int expectedResult = 2 * 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 * 12 *13 * 14 * 15;
      final int result = this.instance.multiplyIntegers(integers);
      assertEquals(expectedResult, result);
   }

In the fairly typical unit test example shown above, JUnit's assertEquals is called in a fluent fashion because of the static import of org.junit.Assert.* (not shown). However, recent versions of JUnit (JUnit 4.4+) have begun including Hamcrest core matchers and this allows for an even more fluent test as depicted in the next code snippet.

   /**
    * Test of multipleIntegers method, of class IntegerArithmetic, using core
    * Hamcrest matchers included with JUnit 4.x.
    */
   @Test
   public void testMultipleIntegersWithJUnitHamcrestIs()
   {
      final int[] integers = {2, 3, 4 , 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
      final int expectedResult = 2 * 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 * 12 *13 * 14 * 15;
      final int result = this.instance.multiplyIntegers(integers);
      assertThat(result, is(expectedResult));
   }

In this example, JUnit's assertThat (also available as part of the static import of org.junit.Assert.* since JUnit 4.4) is used in conjunction with the included Hamcrest core matcher is(). It's certainly a matter of taste, but I prefer this second approach as more readable to me. Asserting that something (the result) is something else (the expected) seems more readable and more fluent than the older approach. It can sometimes be tricky to remember whether to list the expected or actual result first when using assertEquals and the combination of assertThat and is() makes for a little less work when I write and read tests. Even a little less work, especially when multiplied by numerous tests, is welcome.

3 comments:

Colorado Reader said...

I like hamcrest. This style also removes arguments about whether this:

assertEquals("They Match", a, b);

or

assertEquals("They don't match", a, b);

is correct.

Joel Costigliola said...

You can also try Fest Assert 2.0.x, it allows you writing stuff like

assertThat(frodo.getAge()).isEqualTo(33);

Have a look at https://github.com/alexruiz/fest-assert-2.x/wiki if you are interested.

Regards,

Joel

Sandeep Bhandari said...

Thanks for sharing this. I am also a fan of Hamcrest core matchers. In my unit tests, I have used allOf(), any(), describeAs() etc with all assert flavors. But the only problem I face is about describing all these matchers to fellow team members who just seem to know vanilla JUnit.