Re: Understanding Implements

[jamesl@xxxxxxxxxxx]

Fellow programmers:

Please forgive the funky formatting; I'm using my ISP's
somewhat flaky web-mail 
interface again.

Interfaces represent a
compromise in the design of the Java language, between 
those who insist that
a true object-oriented language MUST have multiple 
inheritance, and those
who insist that a true object-oriented language MUST 
NEVER have multiple
inheritance.

They provide a form of purely abstract secondary inheritance:
purely abstract 
because in interface cannot contain any implemented methods,
any class 
implementing the interface is required to implement any methods
defined in the 
interface, and any instance of an implementing class is not
only considered an 
instance of the class and all its superclasses, but also
of the interface. It 
is strictly secondary inheritance because an interface
cannot be instantiated 
or subclassed (though it can be
sub-interfaced).

Thus a parameter can be typed to an interface, and a
variable can be both 
checked for whether it's an instance of the interface
and cast to that 
interface, with complete independence of what class the
object happens to be, 
so long as it implements the interface.

--
James
H. H. Lampert
Professional_Dilettante
http://www.hb.quik.com/jamesl