Quick Review: Delegator Pattern in Java

In the object-oriented world, there are times when our classes need to share some or all of their functionality with others. There are many different ways we can achieve this. Depending on the programming language, we can tell our class to inherit its behavior from one or more other classes. We might define and implement several interfaces. Many dynamic languages allow "including" or "mixing in" behavior to share functionality.

When to Delegate

Delegation is useful when we want a class to "borrow" only a small subset of methods from another. Inheritance would solve the issue, but we might not want to inherit everything from the parent class. Also, some languages, including Java, won't allow us to specify multiple parent classes. By using delegation, we can compose the behavior of our class from any methods of our choosing.

Delegation is also useful when we need our classes to dynamically change their behavior at runtime. Some languages allow opening and redefining methods of any class or object at any point during execution, but this becomes a maintenance nightmare. The delegation pattern gives us a simple and straightforward way to redefine object method behavior at runtime, even in static languages.

To summarize, use delegation when:

• language constraints don't allow multiple inheritance
• class behavior changes frequently at runtime

In general, it is best to use delegation when we want further decoupling between method behavior and class definition.

MessageController (Delegator)

As a quick example, let's create a new class in a file called MessageController.java:

class MessageController {
  Message messageDelegate = new TextMessage();
  String message = "Default Message";

  public void displayMessage() {
    this.messageDelegate.displayMessage(this.message);
  }

  public void setMessageDelegate(Message m) {
    this.messageDelegate = m;
  }

  public void setMessage(String s) {
    this.message = s;
  }

}

This class is responsible for holding a message and displaying a message. Displaying and setting the message is straightforward. The interesting parts have to do with the type of message the instance of this class represents. This is dictated by the type of Message stored in messageDelegate.

TextMessage, PictureMessage, and VoiceMessage (Delegates)

The types of messages our controller can handle are defined as our delegates. The delegator needs a general way to invoke each delegate. So, we'll create a common interface:

Message.java:

interface Message {

  void displayMessage(String s);

}

Now, we can define our delegates:

TextMessage.java:

class TextMessage implements Message {

  public void displayMessage(String s) {
    System.out.println("A Text Message: " + s);
  }

}

PictureMessage.java:

class PictureMessage implements Message {

  public void displayMessage(String s) {
    System.out.println("A Picture Message: " + s);
  }

}

VoiceMessage.java:

class VoiceMessage implements Message {

  public void displayMessage(String s) {
    System.out.println("A Voice Message: " + s);
  }

}

Each delegate implements its own unique way to display a message, but the method name remains consistent because of our interface.

One Controller, Many Behaviors

Now, we can see how the delegation pattern allows us to pick and choose behaviors from any class we want at runtime. Let's drive our MessageController with a simple program:

Main.java:

class Main {
  public static void main(String[] args) {
    MessageController mc = new MessageController();
    mc.displayMessage();
    // A Text Message: Default Message

    mc.setMessageDelegate(new PictureMessage());
    mc.setMessage("Look, a picture!");
    mc.displayMessage();
    // A Picture Message: Look, a picture!

    mc.setMessageDelegate(new VoiceMessage());
    mc.setMessage("Listen to my voice...");
    mc.displayMessage();
    // A Voice Message: Listen to my voice...
  }
}

Simple enough? Please leave any questions or comments below! There are more complex examples in the Cocoa Fundamentals Guide, as delegation is used heavily in the UI for Mac OSX.