delegation example regarding java context

Question

What is delegation in Java? Can anyone give me a proper example?

Answer 1

If you're referring to the delegation pattern, wikipedia has a great example, written in java.

I believe the longer example of the page above is the best one:

interface I {
    void f();
    void g();
}

class A implements I {
    public void f() { System.out.println("A: doing f()"); }
    public void g() { System.out.println("A: doing g()"); }
}

class B implements I {
    public void f() { System.out.println("B: doing f()"); }
    public void g() { System.out.println("B: doing g()"); }
}

class C implements I {
    // delegation
    I i = new A();

    public void f() { i.f(); }
    public void g() { i.g(); }

    // normal attributes
    void toA() { i = new A(); }
    void toB() { i = new B(); }
}


public class Main {
    public static void main(String[] args) {
        C c = new C();
        c.f();     // output: A: doing f()
        c.g();     // output: A: doing g()
        c.toB();
        c.f();     // output: B: doing f()
        c.g();     // output: B: doing g()
    }
}

Answer 2

Here is a simple example of how Delegation is used:

interface IDogBehaviour {

    public void doThis();
}

class BarkSound implements IDogBehaviour {

    public void doThis() {
            System.out.println("Bark!");
    }
}

class WagTail implements IDogBehaviour {

    public void doThis() {
            System.out.println("Wag your Tail!");
    }
}

class Dog {

    private IDogBehaviour sound = new BarkSound();

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

    public void setNewBehaviour( IDogBehaviour newDo ){
        this.sound = newDo;
    }
}

class DelegationDemo {

    public static void main( String args[] ){

        Dog d = new Dog();

        //delegation
        d.doThis();

        //change to a new behaviour type - wag tail
        IDogBehaviour wag = new WagTail();
        d.setNewBehaviour( wag );

        //Delegation
        d.doThis();
    }
}

Answer 3

Same example as aioobe but changed the class names to more intuitive ones. Deriving analogy to real world examples.

public static void main(String[] args) {
    Boss boss = new Boss();
    boss.toDeveloper();
    boss.f(); 
    boss.g(); 

    boss.toSrDeveloper();
    boss.f(); 
    boss.g(); 
}

interface I {
    void f();
    void g();
}

class Developer implements I {
    public void f() {
        System.out.println("Developer: f() is too hard for me.");
    }

    public void g() {
        System.out.println("Developer: g() is not in my domain.");
    }
}

class SrDeveloper implements I {
    public void f() {
        System.out.println("Sr. Developer: Okay, I'll see f()");
    }

    public void g() {
        System.out.println("Sr. Developer: I'll do g() too.");
    }
}

class Boss implements I {
    // delegation
    I i;

    public void f() {
        i.f();
    }

    public void g() {
        i.g();
    }

    void toDeveloper() {
        i = new Developer();
    }

    void toSrDeveloper() {
        i = new SrDeveloper();
    }
}

Answer 4

That's delegation - exactly like in the real world:

public interface Worker() {
  public Result work();
}

public class Secretary() implements Worker {

   public Result work() {
     Result myResult = new Result();
     return myResult;
   }    
}

public class Boss() implements Worker {

   private Secretary secretary;

   public Result work() {
     if (secretary == null) {
        // no secretary - nothing get's done
        return null;
     }
     return secretary.work();
   }

   public void setSecretary(Secretary secretary) {
       this.secretary = secretary;
   }
}

(Added Worker interface to get closer to the Delegator pattern)