Composition vs Inheritance

December 4, 2022

2 min read

Composition and inheritance are two fundamental concepts in object-oriented programming. Both concepts are used to model relationships between classes and objects as well as serve as mechanisms for code reuse. Inheritance is used to model “is a” relationships while composition is used to model “has a” relationships. Understanding this and other differences between composition and inheritance is crucial for choosing the right approach when designing and implementing systems.

At a high level, composition is achieved by combining objects or classes that are typically simpler and smaller in size and scope into more complex ones. This is done by creating references to other objects within a class, allowing the class to delegate behavior to the referenced objects. Inheritance is achieved by creating new classes that are derived from existing ones. This results in a class hierarchy in which the derived classes inherit the attributes (properties) and behaviors (methods) of the base class, making them available for reuse or manipulation.

One key difference between composition and inheritance is that composition allows for more flexibility and control. With composition, an object can be composed of multiple parts, each with its own behaviors and attributes. This means that an object can be easily composed of different parts at runtime given they satisfy the same interface, allowing for greater flexibility and customization. Inheritance is a more rigid approach as a class cannot change its inheritance at runtime and, for languages that do not support multiple inheritance, a class can only inherit from a single base class.

Another important difference between composition and inheritance is that composition is typically considered to be more resilient to changes in the system. Since an object's behavior is composed of multiple parts, changes to a single part will not necessarily affect the entire object. In contrast, inheritance can create dependencies between classes in the class hierarchy, where the further up the class hierarchy a change is made the more potential there is to break derived classes. This is referred to as tightly coupled code and makes systems more fragile and harder to maintain.

When it comes to choosing between composition and inheritance, it's important to consider the relationship you’re attempting to model. In general, however, composition is a better choice when you want to create flexible and customizable objects, or when you need to avoid tight coupling or dependencies between classes. Inheritance, on the other hand, can be useful when you want to create a hierarchy of classes that share a common interface.

Both composition and inheritance are powerful tools at your disposal when building and designing object-oriented systems, each with its own unique set of advantages and disadvantages. Understanding the differences between the two will help you design better systems that are both more maintainable and easier to change.