学步园

Applicability / Uses

Use the delegation pattern when:

• you need to reduce the coupling of methods to their class
• you have components that behave identically, but realize that this situation can change in the future.

Generally spoken: use delegation as alternative to inheritance. Inheritance is a good strategy, when a close relationship exist in between parent and child object, however, inheritance couples objects very closely. Often, delegation is the more flexible
way to express a relationship between classes.

This pattern is also known as "proxy chains". Several other design patterns use delegation - the State, Strategy and Visitor Patterns depend on it.

Related Patterns

• Decorator: is similar to delegation and uses it heavily to accomplish its task.
• Delegation is very common, it is used by many other pattern like
  Visitor,
  Observer,
  Strategy, and
  Event Listener.

Structure

This example is actually more advanced than just plain delegation, it shows how delegation makes it easy to compose behaviors at run-time.

Sample

First lets create an Interface for our Delegates, since Delegates can be interchanged they all must implement the same interface:

public interface ISoundBehaviour {
        
        public void makeSound();
}

public class MeowSound implements ISoundBehaviour {

        public void makeSound() {
                System.out.println("Meow");
        }
}

public class RoarSound implements ISoundBehaviour {

        public void makeSound() {
                System.out.println("Roar!");
        }
}

Now lets create a cat class which uses the MeowSound per default:

public class Cat {  
  private ISoundBehaviour sound = new MeowSound();  

  public void makeSound() {  
    this.sound.makeSound();  
  }  

  public void setSoundBehaviour(ISoundBehaviour newsound) {  
        this.sound = newsound;  
  }  
}

Finally a Main class to test our delegation pattern:

public class Main {
        public static void main(String args[]) {
                Cat c = new Cat();
                // Delegation
                c.makeSound();          // Output: Meow
                // now to change the sound it makes
                ISoundBehaviour newsound = new RoarSound();
                c.setSoundBehaviour(newsound);
                // Delegation           
                c.makeSound();          // Output: Roar!
        }
}