Let's examine how this works by testing the packt.sortutil from the sample address book viewer application. The code is available at the 12-build-tools-and-testing/02-testing location. The src folder contains the packt.sortutil module--the module under test.

To test this, we can create a new test module: packt.sortutil.test. A good convention to follow is to name the test modules with the name of the module being tested followed by .test. Here's the module definition for packt.sortutil.test:

    module packt.sortutil.test { 
      requires packt.sortutil; 
    } 

By declaring the dependency on the module, you can access its exported types and test them through code. Here's a sample class in the test module that verifies that the output is accurate:

    package packt.util.test; 
    public class SortUtilTestMain { 
      public static void main(String[] args) { 
        SortUtil sortUtil = new SortUtil(); 
        List out = sortUtil.sortList(Arrays.asList("b", "a", "c")); 
        assert out.size() == 3; 
        assert "a".equals(out.get(0)); 
        assert "b".equals(out.get(1)); 
        assert "c".equals(out.get(2)); 
      } 
    } 

Compiling and running the code with assertions enabled (the -ea argument) tells us that our tests have passed:

$ javac -d out --module-source-path src --module 
  packt.sortutil,packt.sortutil.test 
$ java -ea --module-path out:lib --module 
  packt.sortutil.test/packt.util.test.SortUtilTestMain 

You should not see any output, which indicates all assertions have successfully passed.