Question

Explain why composition is generally preferred to inheritance for achieving code reuse.

Answer 1

Before talking about the inclination of Composition over Inheritance for accomplishing code reusability, let me give you a brief idea about Composition and Inheritance.

Composition have "HAS-A" relationship. During this an object owns another objects instead of just use that object. It basically means if the main object are going to be destroyed all the interior objects owned by main object should be destroyed also. Example: during this example, class BMW is said to class Car by composition, because BMW has an instance variable that holds a regard to a Car object.

class Car {
//...
}
class BMW {
public Car car1 = new Car();
//...
}

Inheritance have "IS-A" relationship. it's used for establishing the relationship between different classes. Subclass may inherit the properities and methods of its superclass. Superclass have rights to override some methods and properities to switch the default behaviour of parent class. Example: during this simple example, class BMW is associated with class Car by inheritance, because BMW extends Car. during this example, Car is the superclass and BMW is the subclass.

class Car {
    //...
}
class Car extends BMW {
    //...
}

The reasons are as follows:

1. Composition is more flexible than Inheritance. you can change implementation of class at run-time by changing included object, thus changing behavior of it, but you cannot do that with Inheritance, you cannot change behavior of base class at run-time.
2. Composition offers better test-ability of a category than Inheritance. If one class consists of another class, you'll easily create Mock Object representing composed class for sake of testing. Inheritance doesn't provide this luxury, so as to check derived class, you want to need its super class. Since unit testing is one among the foremost important thing to think about code reuse during software development, especially in test driven development, composition wins over inheritance.
3. Composition allows code reuse even from final classes whereas Inheritance cannot extend code reuse final class.
4. If you employ Composition you're flexible enough to exchange implementation of Composed class with better and improved version.