Decoupling is one of the prominent mantras in software engineering.
Chain of responsibility helps to decouple sender of a request and receiver of the request with some trade-offs.
Using Chain of responsibility, a sender sends a request to a chain of objects, where the objects in the chain decide themselves who to serve the request. If an object in the chain decides not to serve the request, it forwards the request to the next object in the chain.
Real life example
It is similar to 'passing the question in a quiz scenario'.
When the quiz master asks a question to a person, if he doesn’t know the answer, he passes the question to next person and so on. When one person answers the question, the passing flow stops. Sometimes, the passing might reach the last person and still, nobody gives the answer.
The classes and objects participating in this pattern are:
Defines an interface for handling the requests and we can implement the successor link.
Each Concrete handler handles requests for which it is responsible and can access its successor.
If a ConcreteHandler can handle the request, it does so; otherwise, it forwards the request to its successor.
It initiates the request to a ConcreteHandler object of the chain.
ATM dispense machine: A chain of responsibility example
This is one of the common examples of Chain of Responsibility pattern. The user enters the amount to dispense it and the machine dispenses amount in terms of defined currency bills such as Rs. 1000, 500, 100. If the user enters an amount that is not multiples of 100, ATM displays the error message.
Chain of Responsibility pattern used to implement this solution and dispenser handler called from higher to lower amount dispenser.
Chain of Responsibility Pattern Examples
ATM dispense machine.
Usage of Chain of Responsibility Pattern in JDK