Mockito stubbed method not being called

Question

I'm new to Mockito and have been having trouble with stubbing. I wanted to test a method but stub one line of that method which is a call to another class's method. Based on the code below, I was expecting Optional.empty() but got Optional.of("str should not be null"). What am I doing wrong?

public class ClassA {
    public String str;

    public Optional<String> method1() {
        if (str == null) { 
            return Optional.of("str should not be null"); 
        }
        return Optional.empty();
    }
}
        
public class ClassB {
    public String thing;
    public Optional<String> method2() {
        ClassA classA = new ClassA();
        Optional<String> str2 = classA.method1();
        if (str2.isPresent()) return str2;
        if (thing == null) return Optional.of("thing should not be null");
        return Optional.empty();
    }
}

public class ClassBTest {
    @Test
    public void method1_test() {
        ClassB classBSpy = spy(new ClassB());
        ClassA classASpy = spy(new ClassA());
        doReturn(Optional.empty()).when(classASpy).method1();

        assertEquals(Optional.empty(), classBSpy.method2());
    }
}
        

Answer 1

To make a stub of ClassA effective in ClaasB, you need a way to insert the Spy/Mock of CLassA into CLassB, i.e. through the constructor:

public class ClassB {
    public String thing;

    private ClassA classA = new ClassA();

    public ClassB() {
        super();
    }


    public ClassB(ClassA classA) {
        super();
        this.classA = classA;
    }

    public Optional<String> method2() {
        Optional<String> str2 = classA.method1();
        if (str2.isPresent()) return str2;
        if (thing == null) return Optional.of("thing should not be null");
        return Optional.empty();
    }
}
 

public class ClassBTest {
    @Test
    public void method1_test() {
        ClassA classASpy = spy(new ClassA());
        ClassB classB = new ClassB(classASpy);
        doReturn(Optional.empty()).when(classASpy).method1();

        assertEquals(Optional.empty(), classB.method2());
    }
}

But the test still fails since Optional.empty().isPresent() is false and thing is awlays null.