Behavioral patterns - Chain of responsibility

Here i will be explaining about Chain of responsibility design pattern.I want to tell readers of this blog that i m not re inventing the wheel.I googled about this pattern.And i have selected examples which are easier to understand.

So lets start with Definition :

  The Chain of Responsibility pattern uses a chain of objects to handle a request, which is typically an event. Objects in the chain forward the request along the chain until one of the objects handles the event. Processing stops after an event is handled.

Example :

Below is example of this pattern in Java. In this example we have different roles, each having a fix purchase power limit and a successor. Every time a user in a role receives a purchase request, when it's more than his limit, he just passes that request to his successor.

The PurchasePower abstract class with the abstract method processRequest.

abstract class PurchasePower {
 
    protected final double base = 500;
    protected PurchasePower successor;
 
    public void setSuccessor(PurchasePower successor) {
        this.successor = successor;
    }
 
    abstract public void processRequest(PurchaseRequest request);
 
}

Four implementations of the abstract class above: Manager, Director, Vice President, President

class ManagerPPower extends PurchasePower {
 
    private final double ALLOWABLE = 10 * base;
 
    public void processRequest(PurchaseRequest request) {
        if(request.getAmount() < ALLOWABLE) {
            System.out.println("Manager will approve $"+ request.getAmount());
        }
        else if(successor != null) {
            successor.processRequest(request);
        }
    }
 
}
 
class DirectorPPower extends PurchasePower {
 
    private final double ALLOWABLE = 20 * base;
 
    public void processRequest(PurchaseRequest request) {
        if(request.getAmount() < ALLOWABLE) {
            System.out.println("Director will approve $"+ request.getAmount());
        }
        else if(successor != null) {
            successor.processRequest(request);
        }
    }
 
}
 
class VicePresidentPPower extends PurchasePower {
 
    private final double ALLOWABLE = 40 * base;
 
    public void processRequest(PurchaseRequest request) {
        if(request.getAmount() < ALLOWABLE) {
            System.out.println("Vice President will approve $" + request.getAmount());
        }
        else if(successor != null) {
            successor.processRequest(request);
        }
    }
}
 
class PresidentPPower extends PurchasePower {
 
    private final double ALLOWABLE = 60 * base;
 
    public void processRequest(PurchaseRequest request) {
        if(request.getAmount() < ALLOWABLE) {
            System.out.println("President will approve $" + request.getAmount());
        }
        else {
            System.out.println( "Your request for $" + request.getAmount() + " needs a board meeting!");
        }
    }
 
}

The PurchaseRequest class with its Getter methods which keeps the request data in this example.

class PurchaseRequest {
 
    private int number;
    private double amount;
    private String purpose;
 
    public PurchaseRequest(int number, double amount, String purpose) {
        this.number = number;
        this.amount = amount;
        this.purpose = purpose;
    }
 
    public double getAmount() {
        return amount;
    }
    public void setAmount(double amt)  {
        amount = amt;
    }
 
    public String getPurpose() {
        return purpose;
    }
    public void setPurpose(String reason) {
        purpose = reason;
    }
 
    public int getNumber(){
        return number;
    }
    public void setNumber(int num) {
        number = num;
    }   
 
}

And here a usage example, the successors are set like this: Manager -> Director -> Vice President -> President

class CheckAuthority {
 
    public static void main(String[] args) {
        ManagerPPower manager = new ManagerPPower();
        DirectorPPower director = new DirectorPPower();
        VicePresidentPPower vp = new VicePresidentPPower();
        PresidentPPower president = new PresidentPPower();
        manager.setSuccessor(director);
        director.setSuccessor(vp);
        vp.setSuccessor(president);
 
        //enter ctrl+c to kill.
        try{
            while (true) {
                System.out.println("Enter the amount to check who should approve your expenditure.");
                System.out.print(">");
                double d = Double.parseDouble(new BufferedReader(new InputStreamReader(System.in)).readLine());
                manager.processRequest(new PurchaseRequest(0, d, "General"));
           }
        }
        catch(Exception e){
            System.exit(1);
        }  
    }
}