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

Java examples

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("Hello world!"); 
	}
 }

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

Complex

The more complex case is a Decorator Pattern that 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. In this example, class C can delegate to any class that implements I. Class C has a method to switch to another delegator. 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()"); }
}
 
// changing the implementing object in run-time (normally done in compile time)
class C implements I {
	I i = null;
	// delegation
	public C(I i){ setI(i); }
	public void f() { i.f(); }
	public void g() { i.g(); }
 
	// normal attributes
	public void setI(I i) { this.i = i; }
}
 
public class Main {
	public static void main(String[] arguments) {
		C c = new C(new A());
		c.f();	// output: A: doing f()
		c.g();	// output: A: doing g()
		c.setI(new B());
		c.f();	// output: B: doing f()
		c.g();	// output: B: doing g()
	}
}

See also

• Aspect-oriented programming
• Delegation (programming)
• Design pattern
• Facade pattern
• Schizophrenia (object-oriented programming)

External links

• Delegation on Rosetta Code