The pattern revolves around a chain of processing objects, where each checks whether it can handle a request. If it cannot, it passes the request to the next processor in the sequence.

Key Components

• Base Processor: An abstract definition that includes a reference to the next link in the chain and a method for handling requests.
• Concrete Processor: Specific implementations that evaluate the request. If the criteria are met, they process it; otherwise, they dleegate to the next link.
• Client: Sets up the chain structure and initiates the first request. The client remains decoupled from the specific handling logic.

Example Scenario: Leave Approval

Consider a system where leave requests are processed based on duration:

1. Team Lead: Approves up to 1 day.
2. Project Manager: Approves up to 3 days.
3. Director: Approves up to 7 days.

Adding a new approval level simply requires adding a new processor to the chain without altering existing logic, adhering to the Open/Closed Prniciple.

Base Processor Definition

public abstract class ApprovalNode {
    protected ApprovalNode nextNode;

    public void setSuccessor(ApprovalNode nextNode) {
        this.nextNode = nextNode;
    }

    public abstract void processLeave(int requestedDays);
}

Concrete Processors

Team Lead Handler:

public class TeamLead extends ApprovalNode {
    @Override
    public void processLeave(int requestedDays) {
        if (requestedDays <= 1) {
            System.out.println("Request approved by Team Lead.");
        } else if (nextNode != null) {
            nextNode.processLeave(requestedDays);
        } else {
            System.out.println("No handler available for this duration.");
        }
    }
}

Project Manager Handler:

public class ProjectManager extends ApprovalNode {
    @Override
    public void processLeave(int requestedDays) {
        if (requestedDays > 1 && requestedDays <= 3) {
            System.out.println("Request approved by Project Manager.");
        } else if (nextNode != null) {
            nextNode.processLeave(requestedDays);
        } else {
            System.out.println("No handler available for this duration.");
        }
    }
}

Director Handler:

public class Director extends ApprovalNode {
    @Override
    public void processLeave(int requestedDays) {
        if (requestedDays > 3 && requestedDays <= 7) {
            System.out.println("Request approved by the Director.");
        } else if (nextNode != null) {
            nextNode.processLeave(requestedDays);
        } else {
            System.out.println("No handler available for this duration.");
        }
    }
}

Client Implementation

public class ClientDemo {
    public static void main(String[] args) {
        // Initialize chain components
        ApprovalNode lead = new TeamLead();
        ApprovalNode manager = new ProjectManager();
        ApprovalNode director = new Director();

        // Link the chain
        lead.setSuccessor(manager);
        manager.setSuccessor(director);

        // Submit request
        System.out.println("Submitting request for 5 days:");
        lead.processLeave(5);
    }
}