Packages and Interfaces - Java: A Beginner's Guide - page 382

an interface must implement all of its methods. In an interface, methods are implicitly
public .
Variables declared in an interface are not instance variables. Instead, they are implicitly
public , final , and static and must be initialized. Thus, they are essentially constants. Here
is an example of an interface definition. It specifies the interface to a class that generates a
series of numbers.
This interface is declared public so that it can be implemented by code in any package.
Implementing Interfaces
Once an interface has been defined, one or more classes can implement that interface. To
implement an interface, include the implements clause in a class definition and then create
the methods required by the interface. The general form of a class that includes the imple-
ments clause looks like this:
To implement more than one interface, the interfaces are separated with a comma. Of
course, the extends clause is optional.
The methods that implement an interface must be declared public . Also, the type sig-
nature of the implementing method must match exactly the type signature specified in the
interface definition.
Here is an example that implements the Series interface shown earlier. It creates a class
called ByTwos , which generates a series of numbers, each two greater than the previous
one.