Question

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

Answer 1

Favor composition over inheritance is one of the popular object-oriented design principles, which helps to create flexible and maintainable code in Java and other object-oriented languages. On composition, a class, which desire to use the functionality of an existing class, doesn't inherit, instead, it holds a reference of that class in a member variable, that’s why the name composition. Inheritance and composition relationships are also referred to as IS-A and HAS-A relationships.

• In OOP, the IS-A relationship is completely inheritance. This means, that the child class is a type of parent class. For example, an apple is a fruit. So you will extend fruit to get an apple.
• On the other hand, composition means creating instances that have references to other objects. For example, a room has a table. So you will create a classroom and then in that class create an instance of a type table.

Composition becomes preferable for a number of reasons:

1. Though both Composition and Inheritance allows you to reuse code, one of the disadvantages of Inheritance is that it breaks encapsulation. If subclass is depending on superclass behavior for its operation, it suddenly becomes fragile. When behavior of super class changes, functionality in sub class may get broken, without any change on its part. If you have used Composition in favor of Inheritance you won't face this problem and your class would have been more robust because you are not relying on super class behavior any more.
2. Composition offers better test-ability of a class than Inheritance. If one class is composed of another class, you can easily create Mock Object representing composed class for sake of testing. Inheritance doesn't provide this luxury. In order to test derived class, you must need its super class.
3. Composition allows you to delay the creation of components until they are needed, or to never create them at all if they are not needed. Perhaps your Car is driving in the day, and the lights aren’t needed, so they never even need to be created.
4. It is more natural to build business-domain classes out of various components than trying to find commonality between them and creating a family tree. For example, an accelerator pedal and a steering wheel share very few common traits, yet both are vital components in a car. What they can do and how they can be used to benefit the car is easily defined. Composition also provides a more stable business domain in the long term as it is less prone to the quirks of the family members. In other words, it is better to compose what an object can do (HAS-A) than extend what it is (IS-A).

Prefer composition over inheritance and you’ll have more flexible, extensible, and testable code.