Delegation in Java with Example

1. Overview

In this article, we will learn the important object-oriented concept Delegation. Hand over the responsibility for a particular task to another class or method.
In short, Delegation means delegating responsibility to other class, for example, in Printers Management Application, the PrinterController is not responsible for the actual desired action but is actually delegated to a helper class either CanonPrinter, EpsonPrinter or HpPrinter.

2. Intent/Definition

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.

3. Implementation

Example 1: Let's take Ticket Booking example

Step 1: Create 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 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 class 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 which 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 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 Canon Printer, 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 are more than one implementors 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);
  }

}

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The source code of this post is available on GitHub: Object-Oriented Design Guide

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