Mock a superclass constructor

java junit guice mockito jmock
Rishi picture Rishi · May 2, 2012 · Viewed 10.2k times · Source

I would like to know if I can mock a super class constructors call and its super() calls.

For example, I have the following classes

class A
{
    A(..)
    {
        super(..)
    }
}   

class B extends A
{
    B(C c)
    {
        super(c)
    }
}

So, I am planning to unit test some methods in class B, but when creating an instance it does call the super class constructors making it tough to write unit tests. So, how can I mock all the super class constructor calls. Also I would like to mock few methods in class A so that it does return few values as I need.

Thanks!!

Answer

Ryan Nelson picture Ryan Nelson · May 4, 2012

Mocking a constructor is a very bad idea. Doing so is circumventing behavior that will happen in production. This is why doing work in the constructor, such as starting threads and invoking external dependencies, is a design flaw.

Can you honestly say that the work performed in the constructor has no effect on the behavior you're trying to test? If the answer is no, you run the risk of writing a test that will pass in a test environment, but fail in production. If the answer is yes, that is a plain case for moving that "work" outside the constructor. Another alternative is to move the behavior you're trying to test to another class (maybe it's own).

This is even more true if you're using a DI framework like Guice (which I assume because you tagged it that way).

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