Why to use Interface if methods are not implemented??

Hello,
I am having a problem to clearify as, why to use the interfaces which defines only methods and no implementation??
When a class implements an interface the methods are implemented by the class itself, don't you think that the same functionality can be achieved if the class defined the method itself...
The why to use interfaces, just that the same method name can be used by many classes or some other reasons..

did you google on that? There is lots of information I am sure explaining why you code to an interface defined type rather than a class defined type.
However, fundamentally you are correct, classes define their own type. The idea is that you use an interface because it allows you to have more than one implementation. Plus you can more easily change the structure of your program if you later wish if you did not use the class type directly.
You get better answers if you ask in the Patterns forum below.

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hi all!
i m confused that what is the advantage of using interfaces for classes if it is for security,data hiding pupose then we can use private modifiers,then y is this think built in java and which r the cases in which we must use interfaces and y?
plz give me any simple example to understand ur point
thanx in advance.
sajjad ahmad

Hmm... First, I'm sorry about my English... but i will try.
In C++ we can use multi-inheritance that let the developer "extends" one or more classes.
eg.
class Hello extends A,B,C,D
This case can be happened in C++.
But If both class A and B have a method call "void sayMyName()"
How can we refer which method was called when we call "Hello.sayMyName()"
A.sayMyName() or B.sayMyName()
So java don't allow this thing happen.
Java allow developers to extend "only one class"
but to enable multi-inheritance ability Java allow developer to implement more than one class.
That is "interface" which was implemented.
Now lets talk about it's benefits.
Interface is not for security or data hiding purpose.
But interface is used for "make a template".
When I created one class called Hello.
But I have created another class too.
Now I want to make sure that my classed have a method "void say()"
What can I do ?
1. Make some class that have a method "void say()" and let my classes extend it.
2. Make an interface and implement it.
Choise number one is quite good ... but !!!
If I have to extend some another class that is more important such as "Applet"
Now I can't extend another class.
I need only method "void say()". But I don't care what it does.
So I make an interface that force the implementing class to create a method "void say()"
and I implement it.
Question: Why I have to implement this interface ? If I only define a method "void say()"
The problems is gone ....
Answer: That may be true if I want them to have a method. But how can I refer to them.
Lets see this code.
class A extends Applet {
void say() {System.out.println("My name is A");}
... go on ...
class B extends Applet {
void say() {System.out.println("My name is B")}
... go on ...
class GO {
public void letMeSay(............. o) {
o.say();
What can I fill in the argument of method "letMeSay()"?
Object ? Object doesn't have a method "void say()"
Applet ? With the same reason.
Now lets see another code.
interface Sayable {
abstract void say();
class A extends Applet implements Sayable {
void say() {System.out.println("My name is A");}
... go on ...
class B extends Applet implements Sayable {
void say() {System.out.println("My name is B")}
... go on ...
public void letMeSay(Sayable o) {
o.say();
It's ok right ... I can refer to interface "Sayable" that can "say"
Next benefit of interface ... "define Constants field"
We can define constants in an interface and implements it to everywhere we want to use those constants. When we change a value in this interface ... Constants in another place will be changed too .... that is very useful ....(But this may have change in J2SE 1.5 ... see more info)
"I HOPE THIS REPLY MAY GIVE YOU SOME KNOWLEDGE ... GOOD LUCK WITH PROGRAMMING"

Why to use Interface if methods are not implemented??

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