Implementing mocking with JUNITs

JUNITs are an easy and effective way to unit test your code. However when writing a JUNIT you need to ensure that the scope of test case remains within that class
only and does not go beyond it.
What this means is that say you have a class A which call a method of Class B from within it. Then ur unit test case should be limited to methods of Class A and
should not test method of Class B otherwise the test case will become an integration test. This can be achieved using a Mock framework such as Mockito and this is
where JUNITs become tricky as well.

I was trying to write JUNITs using Mockito for a simple case as mentioned above. Class code below:

Class to be tested:

public Class A {

  public String parseEvent(String line) {

// some business logic

ClassB bObj = new ClassB();

String randomNum = bObj.getRandomNum().toString();

return line.append(randomNum);

  }

}

Junit:

public class ATest {

   @Test

   public void testParseEvent() {

       String inputLine= <some data>;

       String expectedOutput =<some data>;

       

       ClassB bObject = mock(ClassB.class);

       when(bOject.getRandomNum()).thenReturn(2);

       

       Class A aObject = new ClassA();

       String result = aObject.parseEvent(inputLine);

       Assert.assertArrayEquals(expectedOutput.getBytes(),result.getBytes());

       

   }

}

This looked alright to me. I was trying to mock class B's method so as to return a predetermined number and then compare expected and actual output.

However despite trying above and many other approaches I could see that B's method was actually getting called instead of getting mocked.

After a lot of googling found out that the issue was not with Junit or Mocking but with the way Class A was written. Since it was instantiating object of Class B, 

the mock object got lost and acutal instance got called. After refactoring Class A, Junit worked fine. Refactored code below:

public ClassA {

   private ClassB obj=null;

   public ClassA(ClassB obj) {

this.obj=obj;

   }

  public String parseEvent(String line) {

// some business logic

String randomNum = obj.getRandomNum().toString();

return line.append(randomNum);

  }

}

After refactoring Class A's code, in my JUNIT i passed the mocked object of Class B when creating object of Class A. The mocking worked fine and Junit returned 

expected result. 

Happy Ending :)