Creating new constructors to overwrite the existing instance variables for testing purpose

Question

Is it good/bad practice to add more constructors just for test purposes (to mock the DOCs used in my SUT) like this :

public class A {
    private B b = new B();
    private C c = new C();

    public A(){
    }

    /***
     * used for test puproses
     * @param a
     * @param b
     */
    A(A a, B b) {
        // overwrite the existing variables
        this.a = a;
        this.b = b;
    }
}

class B {
}

class C {
}

Note that i force the new constructor visibility to be package private not to pollute the SUT's api and also choose to instantiate the DOCs inside the SUT for client usage convenience.

SUT: System under test (A)

DOC: depended-on component (B and C)

What do you think ? Thanks in advance.

Answer 1

I would do this regardless of testing just to avoid hard coding B and C. In fact I would reject this code if you didn't provide a way to overwrite b and c. Now, rather than being hard coded, they are simply overwritable default values. That's a good thing.

Answer 2

Your code looks good to me (except the typos in the second constructor, see below).

You might consider the following refactoring, which has the following advantages:

1. It doesn't needlessly instantiate classes B and C.
2. It allows for fields b and c to be declared final, if appropriate.

Refactoring:

public class A {
    private final B b;
    private final C c;

    public A() {
        this(new B(), new C());
    }

    public A(B b, C c) {
        this.b = b;
        this.c = c;
    }
} 

Answer 3

You just discovered a code smell, by the virtue of tests being hard to write. I suggest you provide A & B objects via dependency injection, rather than instantiating them yourself. You'll end up with a more flexible architecture, that is also readily testable via mocks.

Answer 4

In C#, I prefer to create such instances with static methods. They also carry a "ForTesting" in their name, so that their purpose becomes clear.

It could look like

internal static A CreateWithDependenciesForTesting(B b1, C c1)
{
    A instance = new A
    {
        b = b1,
        c = c1
    };
    return instance;
}

I am not a Java programmer, so I cannot tell you if (or how) that method is applicable there.