AssertJ works in a similar way to Hamcrest. A major difference is that AssertJ assertions can be concatenated.

To work with AssertJ, the dependency must be added to Gradle's dependencies:

testCompile 'org.assertj:assertj-core:2.0.0' 

Let's compare JUnit asserts with AssertJ:

Assert.assertEquals(5, friendships.getFriendsList("Joe").size()); 
List<String> friendsOfJoe = 
   Arrays.asList("Audrey", "Peter", "Michael", "Britney", "Paul");
Assert.assertTrue(  friendships.getFriendsList("Joe")
   .containsAll (friendsOfJoe) 
); 

The same two asserts can be concatenated to a single one in AssertJ:

assertThat(friendships.getFriendsList("Joe")) 
  .hasSize(5) 
  .containsOnly("Audrey", "Peter", "Michael", "Britney", "Paul");

This was a nice improvement. There was no need to have two separate asserts, nor was there a need to create a new list with expected values. Moreover, AssertJ is more readable and easier to understand.

The complete source code can be found in the FriendshipsAssertJTest class at https://bitbucket.org/vfarcic/tdd-java-ch02-example-junit.

Now that we have the tests up and running, we might want to see what the code coverage is that is generated by our tests.