Mock method on a new instance

Question

I am trying to create Junit test case using Mockito framework and junit5. I am working on the below code:

    Class ClasstToBeTested {
      FirstClass a = new FirstClass();

      public String methodToBeTested() {
         String str = a.firstMethod();
         return str;
      }
    }

   Class FirstClass {
      SecondClass b = new SecondClass();

      public String firstMethod() {
          String str = b.secondMethod();
          return str;
      }
   }

I have a class structure like above and I need to mock secondMethod.

I tried @spy on FirstClass and mocked SecondClass and secondMethod, but mocking didn't happen. How can I mock in this case?

Note - I am not in a position to change the structure of the class.

Answer 1

You have some options:

1. (Preferred) Use IoC dependency injection to supply the SecondClass instance instead of building it inside FirstClass :

  class FirstClass {
      private final SecondClass b;

      // Injecting the SecondClass instance
      FistClass(SecondClass b) {
          this.b = b;
      }

      public String firstMethod() {
          String str = b.secondMethod();
          return str;
      }
   }

Then you can just inject a mock in your test.

2. Add a SecondClass setter just for tests.

   class FirstClass {
      SecondClass b = new SecondClass();

      // Annotate with a visibility for test annotation if available.
      // Here one can inject a mock too, but can cause problems if used inadvertently.
      void setSecondClassForTests(SecondClass b) {
         this.b = b;
      }

      public String firstMethod() {
          String str = b.secondMethod();
          return str;
      }
   }

Then you call the setter on the test and pass the mock.

3. Use reflection to get the field and set a mock. Something like (In your test function):

final Field declaredField = instanceOfFirstClass.getClass().getDeclaredFields("b");
declaredField.setAccessible(true);
declaredField.set(instanceOfFirstClass, yourMockedInstance);