Chain-of-Responsibility

🔗 Chain of Responsibility

Chain of responsibility is a behavioral design pattern that lets you pass requests along a chain of handlers. Upon receiving a request, each handler decides either to process the request or to pass it to the next handler in the chain.

Applicability

Use Chain of responsibility when :

• more than one object may handle a request, and the handler isn't known a priori. The handler should be ascertainedautomatically.
• you want to issue a request to one ofseveral objectswithout specifyingthe receiver explicitly.
• the set of objects that can handle a request should be specified dynamically.

General structure

• Handler :
  • defines an interface for handling requests.
  • (optional) implements the successor link.
• Concrete Handler :
  • handles requests it is responsible for.
  • can access its successor.
  • if the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor.
• Client : initiates the request to a ConreteHandler object on the chain.

Example

For example, you have three payment methods (A, B and C) setup in your account; each having a different amount in it. A has 100 USD, B has 300 USD and C having 1000 USD and the preference for payments is chosen as A then B then C. You try to purchase something that is worth 210 USD. Using Chain of Responsibility, first of all account A will be checked if it can make the purchase, if yes purchase will be made and the chain will be broken. If not, request will move forward to account B checking for amount if yes chain will be broken otherwise the request will keep forwarding till it finds the suitable handler. Here A, B and C are links of the chain and the whole phenomenon is Chain of Responsibility.

• Handler : Account.
• ConcreteHandlers : Bank, Paypal, Bitcoin.

The code source : source folder

  public static void main(String[] args) throws Exception {

        // Let's prepare a chain like below
        // bank -> paypal -> bitcoin
        //
        // First priority bank
        // => If bank can't pay then paypal
        // => If paypal can't pay then bitcoin
        Bank bank = new Bank(100);
        Paypal paypal = new Paypal(200);
        Bitcoin bitcoin = new Bitcoin(300);

        bank.setNext(paypal);
        paypal.setNext(bitcoin);

        // Let's try to pay using the first priority i.e. bank
        bank.pay(259);
    }

Output :

    Cannot pay 259.0 using Bank Proceeding...
    Cannot pay 259.0 using Paypal Proceeding...
    Paid 259.0 using Bitcoin