extends vs. implements

[Mike]

What's the difference between extends and implements, and when should I use
each...in real world programming of course.

[Paul Clapham]

Mike <mbjohnson59@hotmail.com> wrote in message
news:3944db77$1@news.devx.com...
>
> What's the difference between extends and implements, and when should I
use
> each...in real world programming of course.

"Extends" causes the class to be a new class based on a superclass. Your
class automatically inherits all of the superclass's methods and you can
override any of them (except the ones defined "final") or none of them
(except the ones defined "abstract"), whatever you need.

"Implements" causes the class to be a new class that implements an
interface. Your class MUST override ALL of the interface's methods, because
the interface only has definitions, not code.

For getting started in Java, you'll only need "extends". I've been
programming Java for about a year now and the only use I've found for
"implements" was in Visual Basic, which doesn't allow you to extend a class.
There are places where "implements" makes sense, but only when you have a
concept that's so abstract that it can be implemented in so many ways that
it's better not to try providing base code.

[Tom Duffy]

Hello Mike:

I recently did a DHTML lesson on Inheritance for the Java class that I teach.
You'll need IE4 or later as we've standardized on IE at the school and I
didn't have time to make it cross-browser. Check it out here:

http://www.bright-moments.com/dynamic

If you are running Netscape, much of the same content is available here:

http://www.bright-moments.com/javasite

Just click on Instruction, then choose the lesson on Inheritance

Some of it is a little abstract. I try to teach my students that interfaces
represent a contract between classes to implement the same set of methods.
When beginning to program in Java, many of the interfaces you'll use are
related to event-handling.

Hope this helps,

Tom Duffy

"Mike" <mbjohnson59@hotmail.com> wrote:
>
>What's the difference between extends and implements, and when should I
use
>each...in real world programming of course.

[Kevin Casault]

"Mike" <mbjohnson59@hotmail.com> wrote:
>
>What's the difference between extends and implements, and when should I
use
>each...in real world programming of course.

simply put, extents is used when creating a subclass and implements is used
when implementing an interface. Since java only allows for single inheritance
of superclasses implementing interfaces is a way around this restriction.