Many software developers need help choosing between inheritance and composition in their programming projects, especially when new to the OOP. They may also get caught up in comparing the two approaches, even though they are fundamentally different. In this post, I will break down the confusion and provide a clear understanding of when to use each principle, so you can confidently create software that meets your needs.
Inheritance and composition are two fundamental concepts in object-oriented programming that can be used to create complex and flexible software systems.
Inheritance is a mechanism by which one class can be derived from another class, inheriting its properties and methods. For example, we have a Person class that defines basic information about a person, such as their name and age. We could then create a subclass called Employee that inherits from the Person class and adds information about the employee’s job title and salary, which you can see a java example in the pictures below. Sometimes, people may refer to this as an Is-A inheritance relationship.
Inheritance can be helpful when we have a well-defined hierarchy of related classes and want to enforce a standard interface across them. It is best used when both classes are in the same logical domain, and the subclass is a proper subtype of the superclass.
On the other hand, composition is a mechanism by which one class can be composed of different classes or objects, delegating some of its responsibilities to them. For example, we could create a Person class with a DateOfBirth class as a component, which handles all the logic related to the person’s date of birth.
Composition can be useful when we want more flexibility and want to avoid the limitations of inheritance. It can also reduce coupling between classes, simplify code, and create complex domain objects from simpler and value objects. Sometimes you may see people refer to this as a Has-A inheritance which needs to be corrected, and the Has-A relationship is more likely to be correct. See a java example in the pictures below.
It is important to remember that inheritance and composition should not be compared or seen as alternatives. Instead, they should be used when each is more appropriate for the specific context and requirements of the application. The goal should be to create maintainable, flexible, and easy-to-understand code.
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