Design Pattern - Delegation Pattern

What is it?

Chances are if you have used AWT1.1 or Swing, you have already used the delegation pattern, you just didn’t know you were doing it. The delegation pattern can be defined as follows. When you are creating a class that does everything another class does and more, then instead of subclassing the other class, you have to declare it as a data member or property of your class.

Example Problem

Lets say you are writing a class (ClassA) that does what another class (ClassB) does. ClassA also has to do more things (have more methods) than what ClassB does. You might be tempted to simply have ClassA subclass ClassB. Resist this temptation, becuase it is the wrong thing to do. Inheritance is inherently slow, and is a very strong linkage in your design. As a rule you want to create loosely coupled, but strongly coherent systems.

Solution (design)

The correct design involves defining a data member of type ClassB in ClassA. This way, you have eliminated the need for subclassing and reduced the coupling strength. In fact, ClassB might just be an interface, which is even better for loose coupling.

Code

 1: //ClassA

 2: public class ClassA{

 3:     //data

 4:     private ClassB classB;

 5:

 6:     //methods

 7:     public void doThis(){classB.doThis();}

 8:     public void doThat(){…}

 9: }

 1: //ClassB

 2: public class ClassB{

 3:     public void doThis(){…}

 4: }

Why?

The delegation pattern allows your code to be loosely coupled, which is a very important goal of any object oriented system. It is easy to get this pattern confused with the adapter pattern, they are similar, but completely different. Study both patterns carefully and you will see how they are totally different (yet similar).

 Delegation pattern 

http://en.wikipedia.org/wiki/Delegation_pattern

In software engineering, the delegation pattern is a design pattern in object-oriented programming where an object, instead of performing one of its stated tasks, delegates that task to an associated helper object. There is an Inversion of Responsibility in which a helper object, known as a delegate, is given the responsibility to execute a task for the delegator. The delegation pattern is one of the fundamental abstraction patterns that underlie other software patterns such as composition (also referred to as aggregation), mixins and aspects.

Examples

[edit] Java examples

[edit] Simple

In this Java example, the Printer class has a print method. This print method, rather than performing the print itself, delegates to class RealPrinter. To the outside world it appears that the Printer class is doing the print, but the RealPrinter class is the one actually doing the work.
Delegation is simply passing a duty off to someone/something else. Here is a simple example:

 class RealPrinter { // the "delegate"
     void print() { 
       System.out.println("something"); 
     }
 }
 
 class Printer { // the "delegator"
     RealPrinter p = new RealPrinter(); // create the delegate 
     void print() { 
       p.print(); // delegation
     } 
 }
 
 public class Main {
     // to the outside world it looks like Printer actually prints.
     public static void main(String[] args) {
         Printer printer = new Printer();
         printer.print();
     }
 }

[edit] Complex

By using interfaces, delegation can be made more flexible and typesafe. "Flexibility" here means that C need not refer to A or B in any way, as the switching of delegation is abstracted from C. Needless to say, toA and toB don't count as references to A and B. In this example, class C can delegate to either class A or class B. Class C has methods to switch between classes A and B. Including the implements clauses improves type safety, because each class must implement the methods in the interface. The main tradeoff is more code.

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
    public void toA() { i = new A(); }
    public 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()
    }
}