Mocking public method inside a private method

Question

I have a class like below:

public class ClassOne {

    public function1(input, output, context) {
        //some implementation

        private function2(List, String, String);
    } 
    
    private void function2(List, String, String){
        Class2 class2 = new Class2();
        String value = class2.method1(string, string);
    }
}

public Class2 {
    public String method2(string, string) {
        //some implementation
        return string;
    }
}

I am writing Unit test for ClassOne using Mockito and PowerMockito and would like to mock the call to class2 and do not want to actually call method body for method2. How can I achieve this?

I tried Mockito.mock and PowerMockito.spy the class and when(class2.method2).thenReturn() and doReturn().when(class2).method2(); but everything calls the method body when I do classOne.function1. I have spied the ClassOne.

Answer 1

It would be really helpful if you would have also provided your non working Unit Tests. On the other hand I'm pretty sure the problem isn't there anyway :)

Your problem is not that Mockito & PowerMockito are not working. The real problem is in the dependency of you classes. Or to be more specific the way your classes handle this dependency.

In General it is not a good idea to instantiate the dependency (Class2) in the place it is needed (ClassOne). As you can see right now it makes testing pretty hard. It would be better to pass the dependency into the class that needs it. This is called Dependency Injection (DI).

In your example you would pass an object of Class2 into the constructor of ClassOne. The Code would look something like that:

public class ClassOne {

    private final Class2 class2;

    public ClassOne(Class2 class2) {
        this.class2 = class2;
    }

    ...
    
    private void function2(List, String, String){
        String value = class2.method1(string, string);
    }
}

As you can see you simply pass an instance of your dependency and use this one instead of creating it on your own.

In your Unit Test you are now able to pass a Mock of Class2 into your Class1 object which will then be used.