This is an answer to a question asked at Java Ranch: "How often should classes be programmed to an interface?" As this question gets asked regularly, I thought I'd "archive" it here...
I basically introduce an interface for five different reasons:
- I want to mock an object for a unit test
- a client only needs to depend on a small subset of the operations of a class (and for some reason, I can't/won't break down the class into smaller ones) (Interface Segregation Principle)
- to invert the dependency between two modules (Dependency Inversion Principle)
- to use polymorphism with classes that aren't related by a common superclass
- I need to publish an API to a third party
In all other cases, I find that with modern tools it's so easy to introduce an interface after the fact - once I feel the need for it -, that introducing it "just in case" creates more disadvantages than advantages.
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Posted by ilja on 23 July 2009 09:34:02 CEST
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Re: When to Introduce an Interface (in Java code)
Totally agree. I find that people get interfacitis and think *everything* needs to be an interface.
Can you point to examples for the 3rd and 4th points as I'm not so familiar with them.
Thanks,
Declan
Comment from Declan Whelan on 24 July 2009 05:49:27 CEST
Dependency Inversion: a plugin architecture is a perfect example. Even though the main application calls the plugins at runtime, at compile time the plugins depend on the main application, by implementing an interface defined by the main application.
The java.io.Closeable interface is a perfect example for allowing polymorphism: I can write a method that can close both an InputStream, an OutputStream, a Reader and a Writer, even though they don't share a common base class. (That wasn't possible before Java 1.5, where the interface was introduced.) Does that help?
Comment from Ilja on 24 July 2009 17:13:46 CEST
You are being awefully ... rational.
Comment from Marko Schulz on 24 July 2009 10:54:15 CEST
:)
Comment from Ilja on 24 July 2009 16:36:37 CEST
regarding"I want to mock an object for a unit test" why not just use EasyMock Class Extention version (see: http://easymock.org/EasyMock2_4_ClassExtension_Documentation.html)which doesn't require an interface.
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