Question

Should I write unit test for all nested methods or if writing one test for caller is enough?

For instance:

void Main()
{
    var x = new A().AFoo();
}

public class A
{
    public int AFoo()
    {        
        // some logic
        var x = new B().BFoo();

        // might have some logic

        return x;
    }
}

public class B
{
    public int BFoo()
    {
        // some logic
        return ???;
    }
}

Is that sufficient to write unit test for Main() method or I need to write tests for Main, A.AFoo(), B.BFoo() methods? How deep should I go?

Thanks in advance.

Answer 1

Unit tests are supposed to work on units, in the case of OOP the units are classes and the methods of the classes. That means that you should write a separate test class for each class under consideration, and at least one testing method for each method provided in the class. What is more, it is important to isolate the classes as much as possible so that a bug in class B does not cause a failure on class A. This is why Inversion of Control (Dependency Injection) is so useful, because if you can inject the instance of class B into the instance of class A, you can change B to be just a Mock object.

One of the reasons we write unit tests is to explain, in code, exactly how the methods of each class are expected to behave under all conditions, including and especially edge cases. It is hard to detail the expected behaviour of class B by writing tests on the main method.

I would recommend reading some material online explaining test driven development and how to mock objects, and perhaps use some of the excellent mocking libraries that exist such as JMock. See this question for more links.