An interface declaration consists of modifiers, the keyword
interface,
the interface name, a comma-separated list of parent interfaces (if any), and the interface body. For example:
public interface GroupedInterface extends Interface1,
Interface2, Interface3 {
// constant declarations
double E = 2.718282; // base of natural logarithms
// method signatures
void doSomething (int i, double x);
int doSomethingElse(String s);
}
The
public access specifier indicates that the interface can be used by any
class in any package. If you do not specify that the interface
is public, your interface will be accessible only to classes
defined in the same package as the interface.
An interface can extend other interfaces,
just as a class can extend or subclass another class. However,
whereas a class can extend only one other class, an interface
can extend any number of interfaces. The interface declaration includes
a comma-separated list of all the interfaces that it extends.
The Interface Body
The interface body contains method declarations for all the
methods included in the interface. A method declaration within
an interface is followed by a semicolon, but no braces, because
an interface does not provide implementations for the methods
declared within it. All methods declared in an interface are implicitly
public, so the public modifier can be omitted.
An interface can contain constant declarations in addition to
method declarations. All constant values defined in an interface
are implicitly public, static, and final. Once again, these
modifiers can be omitted.
