Dependency Injection Explained

Simplest java program to demonstrate the significance of dependency injection principle using car and car-engine analogy

WTH is it?

Dependency injection is a programming technique that makes a class independent of its dependencies.

Give me an example please

This is our project structure

Let's see an example where there is no dependency injection used.

Look inside WithoutDependencyInjection folder. There is a class called Car. If you see inside Car, there is an object of DieselEngine class. Here Car is the high level class and DieselEngine is the low level class. The high level class is dependent on the low level class. What this means is that whenever the DieselEngine class changes, the high level class Car should be recompiled. If we choose to change the engine to petrol by writing a new PetrolEngine class, then all instances of DieselEngine class inside Car class should be changed to PetrolEngine. Run the Main class and check the java comments for better clarity.

Let's see if we can solve the problem using dependency injection techniques.

Look inside DependencyInjection folder. So the main problem was the tight coupling of lower level classes like DieselEngine with the higher level class Car. Here we remove that coupling by introducing an abstraction called Engine. Engine is an java interface. Now here DieselEngine and PetrolEngine are written as implementations of this interface. And in the Car class we enforce ourself to refer only to this abstact concept called Engine. We have removed the tight coupling of DieselEngine from Car. Now we use the constructor of the Car class to inject (insert) the dependency. Notice the argument in the Car constructor is Engine. This means at runtime we can pass any implementation of Engine interface and it will work fine. This means the Car class is only working with the abstaction called Engine and not its detailed implementation like DieselEngine or PetrolEngine. The specific implementation is passed at the runtime throught the constructor of Car. Check out the code and comments for better clarity.

The beauty is we can now modify DieselEngine or maybe remove it and add a new class like ElectricEngine without touching anything in the Car class. Now think of its application in large systems with thousands of classes. This way of loosely coupling dependencies make it easier for developers to add new features and modifiy existing features without touching the main system.