How to write unit test by mocking, when you have zero arg:constructors

Question

I was trying to write unit test using jmocks and junit. (My Project uses core java- no frameworks-) I could not write unit test for some of my classes, by mocking external dependencies, when dependencies were initialized in a a no arg-constructor.

As i cannot provide the actual code, trying to explain the scenario by an example

public interface Apple {

String variety();

}

Implementation.

public class MalgovaApple implements Apple {

  @Override
  public String variety() {
         return "Malgova";

  }

}

Class to be tested

public class VarietyChecker {
private Apple apple;

VarietyChecker(){
this.apple = new MalgovaApple();
// instead of new, a factory method is used in actual application
}

public String printAppleVariety(){
    String variety = apple.variety();
    if(variety.length() < 3){
       System.out.println("Donot use Code names- Use complete names");
       return "bad";
        }
       return "good";
}
}

Junit test using jmock

public class VarietyCheckerUnitTest{
Mockery context = new JUnit4Mockery();
@Before
public void setUp() throws Exception {
}

@After
public void tearDown() throws Exception {
}

@Test
public void test_VarietyChecker() throws Exception{

    final Apple mockapple = context.mock(Apple.class);

    VarietyChecker printer = new VarietyChecker();
    context.checking(new Expectations(){{
        oneOf(mockapple).variety();will(returnValue("as"));
    }});
    String varietyNameValid = printer.printAppleVariety();

    assertEquals("bad",varietyNameValid);


} }

This test fails - Mocking does not work the values "as" is not injected, the test class executes with MalgovaApple ...

Now if we add below constructor to VarietyChecker and use it test case - it gives expected output...

public VarietyChecker(Apple apple) {
    super();
    this.apple = apple;
}

and in unit test create test class object like VarietyChecker printer = new VarietyChecker(mockapple);

Exposing a new constructor just for the purpose of testing is not a good idea. After all it is said that you should not alter the code for testing alone, more than that, i am afraid we have already written "some"(amount) code...

Am i missing something in junit or jmock that can make mocking work even incase of no-arg constructors. Or is this a limitation of simple junit and jmocks and should i migrate to something powerful like Jmockit /PowerMock

Answer 1

You should consider two choices.

1. Use a constructor parameter as you describe.

   In this case, you're not "exposing a new constructor just for the purpose of testing". You're making your class more flexible by allowing callers to use a different factory implementation.

2. Don't mock it.

   In this case, you are declaring that it never makes sense to use a different factory. Sometimes this is okay. At that point, the question changes, though. Instead of, "How do I mock this?" your question is now, "What am I gaining from writing this test?" You might not be gaining much of anything, and it might not make much sense to write the test at all.

   If you don't mock it and decide a unit test is still worth it, then you should be asserting on other aspects of the code. Either an end state or some output. In this case, the factory call becomes an implementation detail that's not appropriate for mocking.

   It's important not to fall for a "unit test everything" mentality. That is a recipe for Test-induced Design Damage. Evaluate your tests on a case by case basis, deciding whether they're providing you any real value or not. Not writing a unit test is a valid option and is even appropriate at times, even if it's option you try very hard to avoid.

Only you can make a determination which one makes the most sense in this case. From the the fact that this is a factory object we're talking about, I'd probably lean toward the former.