Mock private method in the same class that is being tested

Question

I have a Java class named, MyClass, that I want to test with JUnit. The public method, methodA, that I want to test calls a private method, methodB, in the same class to determine which conditional path to follow. My goal is to write JUnit tests for the different paths in methodA. Also, methodB calls a service, so I do not want it to actually be executed when I run the JUnit tests.

What is the best way to mock methodB and control its return so that I can test different paths for 'methodA'?

I prefer to use JMockit when writing mocks, so I am specifically interested in any answer that applies to JMockit.

Here is my example class:

public class MyClass  {

    public String methodA(CustomObject object1, CustomObject object2)  {

        if(methodB(object1, object2))  {
            // Do something.
            return "Result";
        }

        // Do something different.
        return "Different Result";

    }

    private boolean methodB(CustomObject custObject1, CustomObject custObject2)  {

        /* For the sake of this example, assume the CustomObject.getSomething()
         * method makes a service call and therefore is placed in this separate
         * method so that later an integration test can be written.
         */
        Something thing1 = cobject1.getSomething();
        Something thing2 = cobject2.getSomething();

        if(thing1 == thing2)  {
            return true;
        }
        return false;
    }

}

This is what I have so far:

public class MyClassTest  {
    MyClass myClass = new MyClass();

    @Test
    public void test_MyClass_methodA_enters_if_condition()  {
        CustomObject object1 = new CustomObject("input1");
        CustomObject object2 = new CustomObject("input2");

        //  How do I mock out methodB here to return true?

        assertEquals(myClass.methodA(object1, object2), "Result");
    }

    @Test
    public void test_MyClass_methodA_skips_if_condition()  {
        CustomObject object1 = new CustomObject("input1");
        CustomObject object2 = new CustomObject("input2");

        //  How do I mock out methodB here to return false?

        assertEquals(myClass.methodA(object1, object2), "Different Result");
    }

}

Thanks!

Answer 1

Do not be tempted to mock private methods, even if you can engaging in trickery to do so using a mocking tool. Private members are implementation details, which you should be free to change. Instead use the non-private API to exercise the class. If this is troublesome, consider moving the troublesome code into a different class, if it is not there already, and use dependency injection to inject a mock implementation of the troublesome code.