What's the Difference Between Composition and Inheritance?

By Truelogic Software

By Gaby Kotliar - 03/05/2023 07:15 am ET

The choice between composition and inheritance is akin to selecting the right tool for a specific job. It's a decision that carries significant weight, as it can profoundly impact the structure and functionality of your code. In the following article, we'll explore composition and inheritance in more detail, and when to use each technique.

Composition and inheritance are two fundamental concepts in object-oriented programming (OOP) that enable code reuse and create relationships between classes.

Composition is a design technique that allows a class to contain objects of other classes as members or attributes. It is a "has-a" relationship, where the containing object has a reference to the contained object. The contained objects can be instantiated separately and used by multiple classes, making code more modular and reusable.

Inheritance, on the other hand, is a mechanism that allows a new class to be based on an existing class, known as the parent or base class. The new class, known as the derived or child class, inherits all the methods and attributes of the parent class and can add its own methods and attributes. Inheritance creates an "is-a" relationship between classes, where the derived class is a specialized version of the base class.

The key difference between composition and inheritance is that composition creates a "has-a" relationship between classes by containing objects of other classes as members, while inheritance creates an "is-a" relationship between classes by creating a new class based on an existing class. Both techniques have their own advantages and disadvantages, and choosing the right one depends on the specific requirements of the program being developed.

When would you use one over the other, what are some of the tradeoffs?

Choosing between composition and inheritance depends on the specific requirements of the code being developed, and there are tradeoffs associated with each approach.

Composition is often preferred over inheritance when the relationships between classes are more dynamic and complex, or when the objects being composed need to be used by multiple classes.

Composition allows for more flexibility and modularity, and changes made to the composed objects will not affect the containing class. However, the downside of composition is that it can sometimes result in more complex code, as it requires more objects to be created and managed.

Inheritance is often preferred when a class needs to be extended or modified to create a new class that shares many of the same attributes and behaviors of the parent class. Inheritance allows for code reuse and simplifies the implementation of similar classes.

Still, it can also lead to tight coupling between classes and make it harder to change the program's structure. In addition, overuse of inheritance can lead to a deep and complex hierarchy of classes, which can be difficult to understand and maintain.

The tradeoffs of each approach must be carefully considered, to make the right choice for the specific requirements of the program.

Let's take a look at an example of a good composition case

One example of a good composition case is the relationship between a car and its engine. A car is composed of many different components, including an engine, tires, steering wheel, and so on. However, the engine is a crucial component that is responsible for powering the car and enabling it to move.

In this case, the engine is often implemented as a separate class that is composed by the car class. The car class has a reference to the engine object, which is responsible for providing power to the car. This allows for a more modular and flexible design, as the engine can be swapped out or upgraded without affecting the rest of the car.

Additionally, the engine object can be used by other classes as well, such as a mechanic or a diagnostics tool. This makes composition a good choice for this case, as it allows for greater code reuse and modularity.

Here is an example of how this composition relationship could be implemented in Ruby:

Composition relarionship in Python-2

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Composition and inheritance are not mutually exclusive however, in many cases, it's possible to use both techniques together to design a more robust and flexible object-oriented program. They're both powerful tools that can be used for implementation and the best choice for a particular situation will depend on the specific requirements of the program.



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