how to stub a method in a class which is called from another class

public class A {
    .
    .
    .
    public static String methodA() {
        String x = new B().methodX();
        return x + "how are ya?";
    }
}
public class B {
    .
    .
    public String methodX() {
        return "Hello!";
    }
}

@Test
public void testGreeting() {
    final B b = Mockito.spy(new B());
    Mockito.when(b.methodX()).thenReturn("Hi!");

    String greetings = A.methodA();
    // greetings -> "Hello" but i want it to be "Hi!".
}

I’m not able to get my method to return the desired value. Above implementations shows how I have written my test, but it’s not working.

What am i doing wrong here?

Answer

Yet to clarify given answers: your mistake is that you do not actually use that spy/mock anywhere.

In your class A you have a static method where a new B is always instantiated instead of using your spy:

String x = new B().methodX();

Spy does not make class somehow globally spied but creates just one instance that is spied and can be mocked. And yet again: this new B() is not a spy nor mock.

One possible working solution would be to change your design a bit (as already suggested) and let Mockito inject this spied B in an instance of A:

@RunWith(MockitoJUnitRunner.class)
public class TestClass {

    public static class AlternativeA {
        // this B can be injected and replaced with a spy or mock
        private B b = new B();
        // get rid of static to use spied B
        public String methodA() {
            String x = b.methodX();
            return x + "how are ya?";
        }
    }

    // Using annotations to create and inject mocks
    @Spy
    private B b;
    @InjectMocks
    private AlternativeA a;
    
    @Test
    public void testGreeting() {
        Mockito.when(b.methodX()).thenReturn("Hi!");
        String greetings = a.methodA();
    }
}