18.1. Inheritance and Polymorphism: Inheritance

Inheritance and polymorphism are two fundamental concepts in object-oriented programming (OOP). These concepts are used to create more efficient and reusable programs.

18.1.1 Inheritance

Inheritance is a mechanism that allows one class to acquire the properties and methods of another class. The class that donates the properties and methods is known as the base class or parent class, while the class that receives the properties and methods is known as the derived class or child class.

For example, imagine we have a class 'Animal' with properties like 'name', 'age' and methods like 'eat' and 'sleep'. Now we want to create a new class 'Dog' which has all the properties and methods of the 'Animal' class, but also has some additional properties and methods like 'breed' and 'bark'. Instead of rewriting all the properties and methods of the 'Animal' class into the 'Dog' class, we can simply make the 'Dog' class inherit from the 'Animal' class. This is done via the 'extends' keyword in many programming languages.

Inheritance allows programmers to create classes that build on top of existing classes. This allows programmers to reuse existing code, which makes software development faster and more efficient.

18.1.2 Polymorphism

Polymorphism is a concept that allows objects of different classes to be treated as objects of a common class. This is especially useful when working with inheritance.

For example, suppose we have an 'Animal' class with a 'speak' method. Now we have several derived classes such as 'Dog', 'Cat' and 'Bird', each with its own implementation of the 'speak' method. If we have an array of 'Animal' objects, we can call the 'speak' method on each object without knowing the specific class of the object. This is polymorphism.

Polymorphism allows programmers to write programs that process objects of classes that share the same superclass in ways that depend on the specific type of object being processed. This can make programs more flexible and extensible.

18.1.3 Inheritance and Polymorphism in Practice

In practice, inheritance and polymorphism are used to create programs that are easy to understand, maintain, and extend. They allow programmers to create class structures that reflect the structure of the problem they are trying to solve.

For example, suppose we are creating a program for a zoo. We can have an 'Animal' class with properties and methods common to all animals. Then we can have derived classes for each type of animal, such as 'Lion', 'Elephant' and 'Penguin', each with its own specific properties and methods. This makes the program easy to understand, because the structure of the code reflects the structure of the zoo.

Also, if we want to add a new type of animal to the program, we can simply create a new class for it that inherits from the 'Animal' class. This makes the program easy to extend because we don't need to modify existing code to add new functionality.

Finally, if we want to perform an action on all animals in the zoo, we can use polymorphism to treat all animals as objects of the 'Animal' class. This makes the program easy to maintain because we can make changes in one place and have those changes reflected throughout the program.

In summary, inheritance and polymorphism are powerful tools that programmers can use to create efficient, reusable, and maintainable programs.

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