[JAVA] "Inheritance" that even beginners of object-oriented programming can understand

Introduction

Do you know the three major elements of object-oriented programming?

• 1. Class (encapsulation)
• 2. Inheritance
• 3. Polymorphism

In this article, I will explain one of the three major elements, "inheritance." Polymorphism is explained in the article here.

What is inheritance?

In a nutshell, it's a mechanism for organizing commonalities and differences between objects (classes). This is the so-called "is-a relationship". The is-a relationship means A is a B., which means "A is a type of B" in Japanese. Also, in the concept of mathematics, it is the relationship between "whole set and subset".

(Example)

• Mammals are a type of animal (Fish and amphibians are also a type of animal)
• Strawberries are a member of the Rosaceae family (Sakura, Ume, and Peach are also members of the Rosaceae family)

Supplement

@shiracamus commented that "superclass is the minimum set of common functions".

Superclass and subclass

• What is a superclass?
• It is the whole set.
• In the above example, "animal" and "Rosaceae" correspond to superclasses.
• Defines common properties in the whole set.
• What is a subclass?
• A subset.
• In the example above, "mammals" and "strawberry" correspond to subclasses.
• Defines unique properties in subsets.

Why is inheritance important?

• This is to eliminate waste of the program. In other words, program creation efficiency and maintainability are improved.
• For example, suppose class A, class B, and class C have the same method.
• When you need to modify that method, you have to modify it in three places: class A, class B, and class C.
• The above is useless work! (Man-hours for program modification and test implementation will be required)
• If you define a function (method) that you want to have in a superclass, you can give the same function to all subclasses that inherit the superclass.
• The Object class, which is a superclass of all classes, has a toString method defined for stringification.
• This is an overview that also leads to "polymorphism".

Specific programming example of inheritance

Program overview

• This is a rock-paper-scissors program.
• Rock-paper-scissors players are humans and robots.
• Players are superclasses, and people and robots are subclasses.
• The player behavior (rock-paper-scissors janken method) is standardized in the superclass.
• Both humans and robots can play rock-paper-scissors (you can call the janken method).

Super class

Player.java

public class Player {
    public void janken() {
        //Rock-paper-scissors processing
    }
}

Subclass

Format: subclass name + extends + superclass name

Person.java

public class Person extends Player {
    //abridgement
}

Robot.java

public class Robot extends Player {
    //abridgement
}

Main class

MainClass.java

public class MainClass {

    public static void main(String[] args) {

        Person person = new Person();
        Robot robot = new Robot();
        
        //Rock-paper-scissors
        person.janken();
        robot.janken();
    }
}

Summary

What is inheritance?

• A mechanism for organizing commonalities and differences between classes.
• Super class
• Whole set
• Classes that define commonalities
• Is-a Class corresponding to "B" in relation (A is a B.)
• Subclass
• Subset
• Classes that define the differences
• Is-a Class corresponding to "A" in relation (A is a B.)

Why is inheritance important?

• This is to eliminate waste of the program.
• This is to reduce the cost of modifying and testing the program. This is a very important point of view in practice.

References

• Why make it object-oriented, 2nd edition

If you want to learn more about object-oriented programming, I recommend you to read it. You can also buy it on Amazon.