Applied Design Patterns with Java

Behavioral :: Chain of Responsibility (223) {C ch 16}

Examples - UML : Chainer and HelpChain

The first example Java program is called 'Chainer'.

The UML diagram is above, and the list of Java files is below:

The second example Java program is called 'HelpChain'.

The UML diagram is above, and the list of Java files is below:

Example - Java :: Patterns\Behavioral\Chain of Responsibility

Pattern Concept: to allow a number of classes to attempt to handle a request, without any class knowing of the capabilities of the other classes. The core idea is 'Loose Coupling' in the interactions; the classes can only forward the request along a common chain.

Issues of the Chain of Responsibility pattern include:

• The pattern helps keep separate the knowledge of what each object can do, allowing the objects to act independently;
• The objects decide how to handle or forward the request; this does not heed to be hardcoded in the object initiating the request;
• 'Cleverness' doesn't help - it promotes hardcoding, so no one object should be favored in the handling chain;
• New objects and handling capabilities can be easily added to the chain;
• It may take more than one object to handle a request;
• Chain of Responsibility reduces coupling between objects, each object only needs to know how to forward a request, not how all the other objects work;
• The terminal case ('last object in the chain') has to be handled, which can either be general or specific.

A Chain of Responsibility can be implemented in a Tree instead of a Linear List. The advantage of a Tree is that searches are faster; however, the structure itself is more complex than a Linear List.

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