How do you use inheritance and subtyping to organize your objects?

1 Inheritance basics

Inheritance is a mechanism that allows you to define a new class based on an existing class, inheriting its attributes and methods. The existing class is called the superclass or the parent class, and the new class is called the subclass or the child class. For example, you can define a class called Animal that has attributes like name, age, and color, and methods like eat, sleep, and make_sound. Then, you can create subclasses like Dog, Cat, and Bird that inherit from Animal and add their own attributes and methods, such as breed, fur, and fly. Inheritance allows you to reuse and modify the code of the superclass without affecting its original functionality.

2 Subtyping basics

Subtyping is a principle that states that a subclass can be used in any context where its superclass is expected, without causing any errors or inconsistencies. This is also known as the Liskov substitution principle, which is one of the fundamental rules of object oriented design. For example, if you have a function that takes an Animal as a parameter and calls its make_sound method, you can pass any subclass of Animal to that function, such as a Dog, a Cat, or a Bird, and it will work as intended. Subtyping allows you to achieve polymorphism, which is the ability of an object to behave differently depending on its type.

3 How to use inheritance

To use inheritance in your object oriented design, you need to identify the common features and behaviors of your objects and group them into a superclass. Then, you need to create subclasses that inherit from that superclass and add or override the specific features and behaviors of each subclass. You can also use multiple inheritance, which means that a subclass can inherit from more than one superclass, or hierarchical inheritance, which means that a subclass can have its own subclasses. To implement inheritance in your code, you need to use the syntax and keywords of your programming language, such as extends, super, or base.

4 How to use subtyping

To use subtyping in your object oriented design, you need to ensure that your subclasses follow the contract of their superclasses, which means that they respect the signature, the behavior, and the invariants of the inherited methods and attributes. You also need to avoid violating the subtyping rules, such as narrowing the input types, widening the output types, or changing the exceptions of the overridden methods. To check if your subclasses are valid subtypes of their superclasses, you can use tools like static type checkers, unit tests, or design by contract.

5 Benefits of inheritance and subtyping

Incorporating inheritance and subtyping into your object oriented design can bring a number of advantages, such as modularity, abstraction, polymorphism, and code reuse. Modularity enables you to break down your code into smaller and simpler units that are easier to manage. Abstraction allows you to hide the implementation details of your objects and make their interfaces available for other objects to access. Polymorphism permits the creation of objects that can behave differently based on their type, allowing for more generic and flexible coding. Finally, code reuse prevents duplication by inheriting from existing classes and changing only the necessary parts, thus reducing the risk of errors and inconsistencies.

6 Tips and best practices

To use inheritance and subtyping effectively in your object-oriented design, you should adhere to certain tips and best practices. For instance, you should only use inheritance when there is a logical relationship between the classes, and prefer composition over inheritance when possible. Additionally, you should only use subtyping when the subclasses can fulfill the expectations of the superclasses, and prefer interfaces over abstract classes. Moreover, it is important to apply the principle of least privilege, meaning that objects should have the minimum amount of access and information needed to perform their tasks. Finally, you should also use the principle of single responsibility, which means that each class should have one reason to change.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?