Dependency Injection

I came across this problem when I was writing code for my product. We follow Test Driven Development as it suites our process. Well there are lots of benefits that you can get from Test Driven Development; I won’t dig deeper into that. But I faced lots of problems trying to adhere to this coding practice.

I had implemented this new interface and a private class that implements this interface. This new class was singleton.   Now the methods in this interface are used across many classes in many projects in the production code, all these classes are Unit tested. Since they unit tested, changing the code in these classes make things messier. But I had to take the pain of modifying the production code as well as the Unit test projects. But the tests still kept failing.

All I was trying to do is create a new interface pointer, and then use it across the class. I created this new interface pointer in the constructor of the classes that use this singleton class.

In the test class I was creating the interface pointer in the setup method of the class that use the singleton, but all the test cases were failing when I tried to call the interface’s methods in the test cases. The failed reason was null pointer. Now I couldn’t understand the reason for being null as the interface pointer is created in the setup method and setup method is definitely called every time.

One fine day I found out the actual root cause, the manner in which the interface pointer was getting created and passed was wrong. Since the new class was singleton, the object creation took place only when the production code was run, generally when you run the test cases the production code won’t get executed, hence the interface pointer was turning out to be null.

The solution for this is Dependency Injection. Since the test classes depended on the classes that used the new class and interface pointer was getting created in the constructor, I had to pass this pointer to the test cases. For the test cases to pass, I had to write a separate constructor with an extra parameter i.e. interface pointer. I am INJECTING the interface pointer so that the test cases ‘that DEPEND on it won’t fail.

So now in the test class, in the setup method, every time I created the production class’s object, I passed the interface’s pointer. After taking the pain of changing the code (again!), I finally managed to make the test cases pass.