Separation of interface and implementation is central to Java's spirit, and the Java interface construct enables you to achieve this separation in your designs. Two major activities of any software system design are identifying parts (the subsystems within a program or system of programs) and specifying the interfaces between the parts. In designing a Java-based system, you should use Java interfaces to represent abstract interfaces -- the ways in which the parts will interact with each other.
So this is how I ended up thinking about Java's interfaces: as the preferred means of communicating with the parts of your program that represent abstractions that may have several implementations. For example, two parts of a program I describe in my September Design Techniques installment, "The Event Generator Idiom", were
TelephoneListener and Telephone. In this design, I decided that the "telephone listener" represented an abstraction that could have multiple implementations, but thatTelephone did not. Thus, I made Telephone a class that didn't implement any interfaces, and defined TelephoneListener as an interface. Telephone, an event source, passed events to (communicated with) listeners through the TelephoneListenerinterface.
I see interfaces as a fundamental tool for achieving flexibility in the design of Java-based systems. Any class can provide an implementation of an interface. As long as you don't change the interface itself, you can make all kind of changes to the implementing classes, or plug in new classes, without impacting code that depends only on the interface. Thus, if you have a subsystem that represents an abstraction that may have multiple implementations, whether the subsystem is a single object, a group of objects, an entire Java applet or application, you should define Java interfaces through which the rest of the world communicates with that subsystem. When you use interfaces in this way, you decouple the parts of your system from each other and generate code that is more flexible: more easily changed, extended, and customized.
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