Why Interface??

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?

Tom

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.

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.

-S

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)
    {
    System.err.println(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)
    {
    callingObjectRef=callingObject;
    }

  public void sendAMessage(String a)
    {
    callingObjectRef.sendMessage("TESTING!");
    }
  }

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:

http://skyscraper.fortunecity.com/solarcity/440/demo/demo.html

ADMINISTRATION WILL BE CONTACTING YOU SHORTLY.  Moderators Computer101 or Netminder will return to finalize these if they are still open in 14 days.  Please post closing recommendations before that time.

Below are your open questions as of today.  Questions which have been inactive for 21 days or longer are considered to be abandoned and for those, your options are:
1. Accept a Comment As Answer (use the button next to the Expert's name).
2. Close the question if the information was not useful to you, but may help others. You must tell the participants why you wish to do this, and allow for Expert response.  This choice will include a refund to you, and will move this question to our PAQ (Previously Asked Question) database.  If you found information outside this question thread, please add it.
3. Ask Community Support to help split points between participating experts, or just comment here with details and we'll respond with the process.
4. Delete the question (if it has no potential value for others).
   --> Post comments for expert of your intention to delete and why
   --> YOU CANNOT DELETE A QUESTION with comments; special handling by a Moderator is required.

For special handling needs, please post a zero point question in the link below and include the URL (question QID/link) that it regards with details.
https://www.experts-exchange.com/jsp/qList.jsp?ta=commspt
 
Please click this link for Help Desk, Guidelines/Member Agreement and the Question/Answer process.  https://www.experts-exchange.com/jsp/cmtyHelpDesk.jsp

Click you Member Profile to view your question history and please keep them updated. If you are a KnowledgePro user, use the Power Search option to find them.  

Questions which are LOCKED with a Proposed Answer but do not help you, should be rejected with comments added.  When you grade the question less than an A, please comment as to why.  This helps all involved, as well as others who may access this item in the future.  PLEASE DO NOT AWARD POINTS TO ME.

To view your open questions, please click the following link(s) and keep them all current with updates.
https://www.experts-exchange.com/questions/Q.20222663.html
https://www.experts-exchange.com/questions/Q.20274293.html
https://www.experts-exchange.com/questions/Q.20283706.html


To view your locked questions, please click the following link(s) and evaluate the proposed answer.
https://www.experts-exchange.com/questions/Q.20033735.html
https://www.experts-exchange.com/questions/Q.20096949.html
https://www.experts-exchange.com/questions/Q.20113412.html

*****  E X P E R T S    P L E A S E  ******  Leave your closing recommendations if this item remains inactive another seven (7) days.  If you are interested in the cleanup effort, please click this link https://www.experts-exchange.com/jsp/qManageQuestion.jsp?ta=commspt&qid=20274643 
POINTS FOR EXPERTS awaiting comments are listed here -> https://www.experts-exchange.com/commspt/Q.20277028.html
 
Moderators will finalize this question if in @14 days Asker has not responded.  This will be moved to the PAQ (Previously Asked Questions) at zero points, deleted or awarded.
 
Thank you everyone.
 
Moondancer
Moderator @ Experts Exchange

No comment has been added lately, so it's time to clean up this TA.
I will leave a recommendation in the Cleanup topic area that this question is:
Answered by: m_onkey_boy
Please leave any comments here within the next seven days.
 
PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!
 
Venci75
EE Cleanup Volunteer

Per recommendation, force-accepted.

Netminder
EE Admin