Why Interface??

     What is the need of interface?? when we r not defining any code in the methods in the interface, what purpose does it serve?? we can as well define the methods in the current class(the class which we r implementing the interface), which saves us from overriding all the other methods of the interface - which is another headache.

     im new to Java Programming. i want to have some detailed information and explanation on Interfaces. i know that Intarfaces r used bcoz java doesn't support multiple inheritance. but still what i couldnt get is when there is no code in interface, what is the need for it??

thanking you
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The interface is a contract between classes implementing the inteface and the client that uses them.

Here's an example:

Let's say that your application communicates with another server via XML transactions.  You could write a class called XMLCommunicator that does this communication, and that works fine.  Now suppose you add another server to your network that communicates with some sort of binary stream.  You have to code another class to do this, say you call it BinaryCommunicator.  This works, but now you have to re-code your main application to instantiate the correct class based on which server you want to talk to.  Putting this logic in your main application is bad from a OO point of view, as well as a maintenance point of view.

A better solution would be to define an interface called Communicator with a single method called send().  Your XMLCommunicator and BinaryCommunicator both implement this interface.  Now you can write a class called CommunicatorManager that has a method called getCommunicator(int nType).  Here's how it looks:

public interface Communicator {
public void send();

public class CommunicatorManager {
public static final int XML = 0;
public static final int BINARY = 1;
public static Communicator getCommunicator (int nType) {
return (nType == XML) ? new XMLCommunicator() : new BinaryCommunicator();

public class MainApp {
public static void main (String[] args) {
Communicator c = CommunicatorManager.getCommunicator(Integer.parseInt(args[0]));


This model abstracts your sending logic from your main application.  To add more Communicators, all you have to do is implement the send() methos and add a few lines of code to CommunicatorManager without having to touch your main application.  This hides how the message is sent from the main app, so it can keep it;s code clean.

The JDBC classes are a good example of interfaces at work.  You can connect to mysql or sqlserver by changing two lines of code somewhere in your application to switch from one database to another, provided you have the JDBC implementations for each database.  These databases behave entirely different, but your code doesn't need to know this because it is hidden behind the interfaces.
It is a promise of code.  Doesn't that just sound horrible.  When I first heard that I was about to promise the book a lighter.  But you have to work with it to really understand it.

Lest say you define an Interface that promises roughly 10 methods.  Basically some getting and setting methods.  Then you implement that interface in different classes.  Class A works with a Database.  Class B works with .txt data.  Class C is a GUI class.  Class D is something else (work with me here, I am streching my imagination as it is..).

So basically class A would look like:  public class A implements myInter{.....}  and all the other ones would be somewhat similar.  (sure maybe some extends and further implements, but you get the idea.)  

Now suppose you are in class C somewhere and you want to use an instance of the interface?  What?  It is an interface we can't do that!  Yes we can.  All that is needed is to return one of the implementing classes as an instance of that interface.  So in effect class C can access class A but call it the inteface name.  And if you happen to be anticipating dynamic events that could work to your advantage.  ie a method in Class T that has the interface as a parameter.  Then classes A,B,or C could be passed in.  The methods available would only be those defined in the Interface, but with good planning those would be anticipated.

How is that for some good ol' rambling?

It also enables you to use "multiple" inheritance, which the java language preventing. You can extends only one class, but you can implements as many interfaces as you need.
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Reading s_lavie's comment, you might wonder why Java designers chose to prohibit multiple inheritance (you cannot inherit from multiple classes as s_lavie said), but allowed multiple interface implementation instead.
Indeed, after all an interface can be seen as "an abstract class with abstract methods only and no attributes"

My guess is that those guys were C++ programmers who got tired of tracking bugs in C++ code using mutiple inheritance.

Let me give you an example :

public class HumanBeing
  public String fullName;

public class Man extends HumanBeing
  public boolean lovesBeerAndSoccer(){return true;}

public class Brit extends HumanBeing
  public boolean lovesRugbyAndJelly(){return true;}

Now what if you want a BritMale class ? Suppose you could write something like :

public class BritMale extends Brit,Man{...}

Wouldn't there be a problem with the "fullName" attribute, both inherited from Man and Brit classes ?
Yes there would be, so C++ introduced the idea of virtual inheritance. Let's stop here on details, the key point is that virtual inheritance is "bad" because it's :
- difficult to learn and code
- error-prone

Rather than trying to find an easy and clean way to do multiple class inheritance, Java designers stated that attributes wouldn't be inherited anyway, and they probably had a lot of pretty good reasons to do so.
Which, I hope, clarifies the interfaces vs. abstract classes debate.

FYI a rumor says that MS C# will bring multiple inheritance back onto the dancefloor in a future version.


Interfaces are also usefull for doing 'call back' functions, something normally found in games programming and various other system activities.

Imagine you have two classes,

Class A, and Class B.

Class A could be a game, and class B could be the keyboard controller for the game, that would scan all the keys, and send a message back to its instatiator to say when a direction or the fire key is pressed.

It's Class A's job to instantiate B such as:

public class A
  B b;

  public A
    b=new B();


Now imagine B does some stuff, for example trapping keyboard events, it looks at the key, and if it is q,a,o,p or space, it's job is to send a message to its parent class, in this case A.

Ok, at this point you ask yourself, How?

How can the B class access anything inside A?
The answer is not directly and not easilly if you dont use interfaces.

The solution using interfaces would be to have an interface containing the name of a call back function, such as 'sendMessage()'

Class A would then implement sendMessage() togeather with an instance of class B that 'knew' the existence of A.

public class A implements Message
  B b;

  public void sendMessage(String s)

  public A
    b=new B(this);


and the B class would then use an interface variable that could hold a reference to the parent class.

public class B
  Message callingObjectRef;

  public B(Message callingObject)

  public void sendAMessage(String a)

Class B remembers the reference to it's parent class, by using the interface name instead of the class name. This means that ANY class can instantiate the B class, and the B class can always call the expected interface method.

An example of the technique can be found found here:

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