Further abstraction techniques - Objects First with Java: A Practical Introduction Using BlueJ - page 355

10.6.1 An Actor interface
Code 10.9 shows Actor defined as an interface type.

Code 10.9
The Actor interface

/**
* The interface to be extended by any class wishing
* to participate in the simulation.
*/
public interface Actor
{
/**
* Perform the actor's regular behavior.
* @param newActors A list for receiving newly created actors.
*/
void act(List<Actor> newActors);

/**
* Is the actor still active?
* @return true if still active, false if not.
*/
boolean isActive();

}

Java interfaces have a number of significant features:

Concepts:

A Java interface
is a specification of
a type (in the form
of a type name and
a set of methods)
that does not
define any imple-
mentation for the
methods.

■ The keyword interface is used instead of class in the header of the declaration.

■ All methods in an interface are abstract; no method bodies are permitted. The abstract
keyword is not needed, therefore.

■ Interfaces do not contain any constructors.
■ All method headers in an interface have public visibility, so the public keyword is not needed.

■ Only public constant class fields ( static and final ) are allowed in an interface. The
public , static , and final keywords may be omitted, therefore.

A class can inherit from an interface in a similar way to that for inheriting from a class.
However, Java uses a different keyword— implements —for inheriting interfaces.

A class is said to implement an interface if it includes an implements clause in its class header.
For instance:

public class Fox extends Animal implements Drawable
{
// Body of class omitted.
}

As in this case, if a class both extends a class and implements an interface, then the extends
clause must be written first in the class header.