Slide 14.7: Interfaces

Interfaces

Like a class, an interface can have methods and variables, but the methods declared in interface are by default abstract (only method signature, no body).

Interfaces specify what a class must do, but not how. It is the blueprint of the class.

An interface is about capabilities like a Player may be an interface and any class implementing Player must be able to (or must implement) move(). So it specifies a set of methods that the class has to implement.

If a class implements an interface and does not provide method bodies for all functions specified in the interface, then class must be declared abstract.

A Java library example is Comparator interface, which imposes a total ordering on some collection of objects. If a class implements this interface, then it can be used to sort a collection.

The syntax for declaring an interface is given on the right:

interface <interface_name> {
  // Declare constant fields.
  // Declare methods that are abstract by default.
}

To declare an interface, use interface keyword. It is used to provide total abstraction. That means all the methods in interface are declared with empty body and are public and all fields are public, static, and final by default. A class that implements interface must implement all the methods declared in the interface. To implement interface use implements keyword.