Programming to an interface instead of an implementation: Why is assigning the concrete implementation of an object at runtime even better?

Question

In Head First Design Patterns, it was mentioned that you should code to an interface instead of an implementation however the last part of the code example got me confused. How is assigning the concrete implementation of an object at runtime a better design?

Does it mean that its better to put the instantation of the objects within a method in the class that uses the supertype? (a method whose purpose is specifically returning an object to a variable of the superclass)

//Programming to an implementation would be:
Dog d = new Dog();
d.bark();

//Programming to an interface/supertype would be:
Animal animal = new Dog();
animal.makeSound();

//Even better is assigning the concrete implementation at runtime: (says the book)
a = getAnimal();
animal.makeSound();

With the latter you could use dependency injection which would allow you to more easily test each class in isolation. You don't see the value in your simple example, but once you start working on complex multi-class projects, the benefits can be huge. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Oct 2, 2013 at 2:45
persistence ignorance: stackoverflow.com/questions/1974735/…, reduced dependency / coupling: stackoverflow.com/questions/4541952/…, and testability: stackoverflow.com/questions/8578104/…. Please note the testability example is using IOC Container injection, not constructor injection. — Fendy
– Fendy, Commented Oct 2, 2013 at 8:09

Akhilesh Singh · Accepted Answer · 2013-10-02 02:55:39Z

1

Even better way is to get the concrete class figured by someone else depending on the environment/context of your code execution. This is called Inversion of control/ dependency injection where the actual class is either configured in some configuration files or coding by convention is used to identify the correct file.

Coming back to why,imagine a project of moderate complexity where you have to talk to a DB. If you hard code the way to access a DB , you will end up asking customers to stick to a particular type/version of the DB. This is how JDBC works. Similarly for more complex scenarios, imagine moving from Db based system file based. Once you get that software should be designed for flexibility, its easy to appreciate the delayed initialisation of concrete classes.

answered Oct 2, 2013 at 2:55

Akhilesh Singh

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3 Comments

DTK Over a year ago

Hello, I had the same question hoping you could please clarify with one concrete example, more specifically how would one implement the function getAnimal in the a = getAnimal(); a.makeSound()? How would one invoke getAnimal (Without an object nor a Static class)? Would this function contain a constructor of a class?

Akhilesh Singh Over a year ago

With this approach, you can completely remove the dependency of Animal from A. As such , your code can live in different repos and completely different teams can work on module A( loosely used instead of project/jar) and module Animal without knowing even knowing Animal will be used at runtime. You can then create third module with class DifferentAnimal and use it to inject instead of Animal.

Akhilesh Singh Over a year ago

wow! Stackoverflow commenting is so bad. Here is code sample using Spring framework. Beauty is that you don't need to worry about getAnimal() method - that is now the job of the Dependency Injection framework. Spring and Guice are two biggest DI frameworks in Java class A { @Autowired private BeingAnimal a; public void someMethod() { a.makeSound(); } } class Animal implements BeingAnimal { public void makeSound() { } }

Ibrahim Najjar · Accepted Answer · 2013-10-02 12:41:39Z

If I were to say why in one word, then it would be: Centralization.

What I mean by this, is that you assign those concrete types at the start of your application somewhere in a single place and provide them to the classes that need them as interfaces, so those classes don't get coupled to the concrete types themselves but rather rely on those abstractions or interfaces.

This is similar to using the Factory pattern, so you don't spread object creation through out the program but rather in a single spot, which is easy to modify in the future.

This also enables you to use Dependency Injection and Inversion of Control which in turn comes with a bunch of good stuff like better testing, maintainability, run-time configuration, etc .

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