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I've been programming in Java for a few courses in the University and I have the following question:
Is it methodologically accepted that every class should implement an interface? Is it considered bad practice not to do so? Can you describe a situation where it's not a good idea to use interfaces?
Edit: Personally, I like the notion of using Interfaces for everything as a methodology and habit, even if it's not clearly beneficial. Eclipse automatically created a class file with all the methods, so it doesn't waste any time anyway.
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Every class does implement an interface (i.e. contract) insofar as it provides a non-private API. Whether you should choose to represent the interface separately as a Java interface depends on whether the implementation is "a concept that varies".
When discussing these principles in the book, I regularly encourage the reader to add more interfaces to their classes, to make the overall design of the package or application more flexible. However, not every class needs an interface, and not every interface makes sense.
That means all the methods in an interface are declared with an empty body and are public and all fields are public, static, and final by default. A class that implements an interface must implement all the methods declared in the interface.
A class can implement more than one interface at a time. A class can extend only one class, but implement many interfaces. An interface can extend another interface, in a similar way as a class can extend another class.
You don't need to create an interface if you are not going to use it.
Typically you need an interface when:
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