Delegation in Java with Example


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OOPS Tutorial


1. Overview

In this article, we will learn the important object-oriented concept - Delegation. 

Delegation means hand over the responsibility for a particular task to another class or method.

It is a technique where an object expresses certain behavior to the outside but in reality delegates responsibility for implementing that behaviour to an associated object.

Applicability 

Use the Delegation in order to achieve the following
  • Reduce the coupling of methods to their class
  • Components that behave identically, but realize that this situation can change in the future.
  • If you need to use functionality in another class but you do not want to change that functionality then use delegation instead of inheritance.

2. Implementation with Examples

Example 1: Let's take the Ticket Booking example

Step 1: Create a TravelBooking interface.
interface TravelBooking {
    public void bookTicket();
}
Step 2: Let's create TrainBooking class to book train tickets.
class TrainBooking implements TravelBooking {
    @Override
    public void bookTicket() {
        System.out.println("Train ticket booked");
    }
}
Step 3: Let's create an AirBooking class to book air tickets.
class AirBooking implements TravelBooking {
    @Override
    public void bookTicket() {
        System.out.println("Flight ticket booked");
    }
}
Step 4: TicketBokkingByAgent provides an implementation of TravelBooking. But it delegates actual ticket booking to other classes at runtime using Polymorphism.
class TicketBookingByAgent implements TravelBooking {

    TravelBooking t;

    public TicketBookingByAgent(TravelBooking t) {
        this.t = t;
    }

    // Delegation --- Here ticket booking responsibility 
    // is delegated to other class using polymorphism
    @Override
    public void bookTicket() {
        t.bookTicket();
    }
}
Step 5: This is a test class for the delegation and polymorphism example.
public class DelegationDemonstration {

    public static void main(String[] args) {
     // Here TicketBookingByAgent class is internally 
     // delegating train ticket booking responsibility to other class
         TicketBookingByAgent agent = new TicketBookingByAgent(new TrainBooking());
         agent.bookTicket();

         agent = new TicketBookingByAgent(new AirBooking());
         agent.bookTicket();
    }
}

Example 2: Printers Implementation Example

In this example, the delegates are CanonPrinter, EpsonPrinter, or HpPrinter they all implement Printer. The PrinterController is a delegator class that also implements Printer.
PrinterController is not responsible for the actual desired action but is actually delegated to a helper class either CanonPrinter, EpsonPrinter, or HpPrinter. The consumer does not have or require knowledge of the actual class carrying out the action, only the container on which they are calling.
You can observe here the implementation is loosely coupled.

Step 1: First create a Printer interface that both the Controller and the Delegate classes will implement.
public interface Printer {
     void print(final String message);
}
Step 2: Specialised Implementation of Printer for a CanonPrinter, in this case, the message to be printed is appended to "Canon Printer: ".
public class CanonPrinter implements Printer {
     private static final Logger LOGGER = LoggerFactory.getLogger(CanonPrinter.class);
     @Override
     public void print(String message) {
          LOGGER.info("Canon Printer : {}", message);
     }
}
Step 3: Specialized Implementation of Printer for an Epson Printer, in this case, the message to be printed is appended to "Epson Printer: ".
public class EpsonPrinter implements Printer {

     private static final Logger LOGGER = LoggerFactory.getLogger(EpsonPrinter.class);
     @Override
     public void print(String message) {
       LOGGER.info("Epson Printer : {}", message);
     }
}
Step 4: Specialized Implementation of Printer for an HP Printer, in this case, the message to be printed is appended to "HP Printer: ".
public class HpPrinter implements Printer {

     private static final Logger LOGGER = LoggerFactory.getLogger(HpPrinter.class);
     @Override
     public void print(String message) {
          LOGGER.info("HP Printer : {}", message);
     }
}
Step 5: it's time to implement a Delegator class.
Delegator Class to delegate the implementation of the Printer.
This ensures two things:
  • when the actual implementation of the Printer class changes the delegation will still be operational
  • the actual benefit is observed when there is more than one implementor and they share a delegation control.
public class PrinterController implements Printer {

     private final Printer printer;

     public PrinterController(Printer printer) {
          this.printer = printer;
     }
     @Override
     public void print(String message) {
          printer.print(message);
     }
}
Step 6: Let's test the Delegation using the main method.
public class App {

     public static final String MESSAGE_TO_PRINT = "hello world";

     /**
      * Program entry point
      *
      * @param args command line args
      */
     public static void main(String[] args) {
          PrinterController hpPrinterController = new PrinterController(new HpPrinter());
          PrinterController canonPrinterController = new PrinterController(new CanonPrinter());
          PrinterController epsonPrinterController = new PrinterController(new EpsonPrinter());

          hpPrinterController.print(MESSAGE_TO_PRINT);
          canonPrinterController.print(MESSAGE_TO_PRINT);
          epsonPrinterController.print(MESSAGE_TO_PRINT);
     }
}

GitHub Repository

The source code of this post is available on GitHub: Object-Oriented Design Guide.
Learn complete Java Programming with Examples - Java Tutorial | Learn and Master in Java Programming with Examples

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Comments

  1. class diagram is wrong. +CanonPrinter() in all the different printer class

    ReplyDelete
    Replies
    1. Class diagram is not wrong. The content CononPrinter is typo in all subclasses.You can fix this.

      Delete
  2. This typo has fixed , thanks for informing.

    ReplyDelete

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