Delegation

[Richard Teston]

I understand that when an object recieves a request, the object can either handle it or delegate the request to another object. Does anyone has sample program for this, just a simple program will do. thanks.

[Gaurav Mantro]

Hi Richard.
You can look at the following article to see code example of delegation model.
If this doesn't make things clear then please do repost and I will try to write a longer reply with code example.
http://www.javaworld.com/javaworld/jw-03-2000/jw-03-classload.html

[Richard Teston]

I've read the article but i'm a little bit more confound with the example, sorry for asking but can you give me more simpler example or just explain its concepts and significants, i just like to see the pattern. Like when i asked, "how can i use model view controller in a simple gui which access the database" and someone answered...
"well model is for accesing the database and the view is for your graphical interface and the controller will the one who'll listen to the user's action and will glue together the view and model" and that's it. I put it into practice and then it worked. I'm studying design pattern and delegation is one that i'm focusing now and the decorator design pattern. I hope this was not to much to ask. Thanks for your first reply.

[Karthik Guru]

Consider a iterface that every utility provider needs to implement.

interface Utility{
  public void getTheWorkDone();
}
//for getting the laundry done
class Laundry implements Utility{
 public void getTheWorkDone(){
 //laundry specific stuff
 }
}
//to take care of your tax issues
class TaxPayer implements Utility{
 public void getTheWorkDone(){
 //tax specific stuff 
 }
}

Say then you have another class , someone is using it already and it's does a good job of fethcing you grocery items.

class GetGrocery{
 public void getGrocery(){
  //grocery specific code
 }
}

Note that this class does not implement the Utility interface you defined earlier.
But it does the job! of getting you the grocery items
but does'nt confine to your interface and you expect everyone to stick to your interface.
So you have some options now.
1. Write a class Grocery that implements your Utility and copy the code from getGrocery method of GetGrocery class already available to you and make it work. c'd be messy.
2. Write a class Grocery that implements Utility and *delegate* the job to the GetGrocery instance

class Grocery implements Utility{
  /** hold on to the instance **/
  private GetGrocery instance;
  public Grocery(GetGrocery inst){
   this.instance = inst;
  }
  /** delegate the actual work to the enclosed
  class instance 
  **/
  public void getTheWorkDone(){
    instance.getGrocery();
  }
}

hope it's clear.

[Richard Teston]

thanks karthik! it's a lot more clearer now, thanks for spending your time explaining this things.