How to use mockito when a funtion is using the other function’s return data?

I am learning Junit with mockito. I got a scenario where I need to write the testcase for a function which calling other function and using that function return data.

I have a two functions called courseComplete and courseCount in the class StudentHelper

courseComplete :

public String courseComplete(Student std) {
    int count = courseCount(std); //Calling another function
    if(count >= 2) {
        return "Courses Registered";
    } else {
        return "Courses Not Registered";
    }
}

courseCount :

public int courseCount(Student std) {
    return std.getCourses.size(); // Student class contains a courses list.
}

Now I need to write unit testcases for the courseComplete function. How can I pass the value to the count variable without calling the courseCount method in courseComplete Function? Is it possible with mockito? I tried by the following code :

when(StudentHelper.courseCount(std)).thenReturn(2); // std is the object which I created in the testclass.

But this is not working. Please anyone help me. Thanks.

Answer

You need to create a mock instance for the class under test:-

final StudentHelper mockHelper = mock(StudentHelper.class);

Then you can go ahead and use ArgumentMatchers to mock your behaviour:-

final Student expected = new Student("Dave"); 
when(mockHelper.courseCount(eq(expected)).thenReturn(2);

The above example will only execute when the method is called with your expected value.

Note that you can use matchers such as any(Student.class) to provide more flexibility.