What is delegation? When would I want to use it?

In OOP, a term delega开发者_StackOverflow社区tion is mentioned. How is this modelled in a class? I searched on yahoo etc but got links to delegates.

Imagine you have the classes Car and Engine:

public class Car {
   private Engine engine = new Engine(); //or inject it externally

   public void start() {
       engine.start();
   }
}

In this example the Car delegates to the underlying Engine. The user of the car cannot directly start the engine (unless he is a mechanic). But he can tell the car to start, and the car in turn tells the engine to start.

You'd want to use it whenever you use object composition and you need to use a method of one of the composing objects. In that case you create a method that delegates to it.

Delegation is like inheritance except instead of class2 having copied functions and variables from class1 class2 just gets class1 to do that stuff for it and class2 focuses on doing and having the extra functions and variables you give it. One obvious advantage of this is it saves space on your computer.