	CS 635: Advanced Object-Oriented Design & Programming

Spring Semester, 1998
Chain of Responsibility
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© 1998, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 23-Apr-98

Contents of Doc 25, Chain of Responsibility

CS 635 Doc 25 Chain of Responsibility

References	slide # 1

Chain of Responsibility	slide # 2
...Intent	slide # 2
...Participants	slide # 3
...Motivation	slide # 4
...When to Use	slide # 4
...Implementation Issues	slide # 6

References

Design Patterns: Elements of Resuable Object-Oriented Software, Gamma, Helm, Johnson, Vlissides, Addison-Wesley, 1995, pp 223-232

Doc 25, Chain of Responsibility Slide # 2

Chain of Responsibility

Intent

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

Class Structure

Sample Object Structure

Doc 25, Chain of Responsibility Slide # 3

Participants

Handler

Defines the interface for handling the requests

May implement the successor link

ConcreteHandler

Handles requests it is responsible for

Can access its successor

Handles the request if it can do so, otherwise it forwards the request to its successor

Consequences

Reduced coupling

Added flexibility in assigning responsibilities to objects

Not guaranteed that request will be handled

Doc 25, Chain of Responsibility Slide # 4

Motivation

Context Help System

When to Use

When more than on object may handle a request, and the handler isn't known a priori

When you want to issue a request to one of several objects without specifying the receiver explicitly

When the set of objects that can handle a request should be specified dynamically

Doc 25, Chain of Responsibility Slide # 5

How does this differ from Decorator?

Doc 25, Chain of Responsibility Slide # 6

Implementation Issues

The successor chain

Use existing links in the program

The concrete handlers may already have pointers to their successors, so just use them

Define new links

Give each handler a link to its successor

Representing Requests

Each request can be a hard-coded

abstract class HardCodedHandler
     {
     private HardCodedHandler successor;
     
     public HardCodedHandler( HardCodedHandler aSuccessor)
          { successor = aSuccessor; }
     
     public void handleOpen()
          { successor.handleOpen(); }
     
     public void handleClose()
          { successor.handleClose(); }

     public void handleNew( String fileName)
          { successor.handleClose( fileName ); }
     }

Doc 25, Chain of Responsibility Slide # 7

A single method implements all requests

abstract class SingleHandler { private SingleHandler successor; public SingleHandler( SingleHandler aSuccessor) { successor = aSuccessor; } public void handle( String request) { successor.handle( request ); } } class ConcreteOpenHandler extends SingleHandler { public void handle( String request) { switch ( request ) { case "Open" : do the right thing; case "Close" : more right things; case "New" : even more right things; default: successor.handle( request ); } }

Doc 25, Chain of Responsibility Slide # 8

Single handle method with Request Object for parameters

abstract class SingleHandler
     {
     private SingleHandler successor;
     
     public SingleHandler( SingleHandler aSuccessor)
          {successor = aSuccessor; }
     
     public void handle( Request data)
          { successor.handle( data ); }
     }

class ConcreteOpenHandler extends SingleHandler
     {
     public void handle( Open data)
          { // handle the open here }
     }
     
class Request
     {
     private int size;
     private String name;
     public Request( int mySize, String myName)
          { size = mySize;  name = myName; }
     
     public int size() { return size; }
     public String name() { return name;}
     }

class Open extends Request
     {// add Open specific stuff here}
     
class Close extends Request
     { // add Close specific stuff here}

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