Polymorphism that even beginners of object-oriented programming can understand

What is polymorphism?

Wikipedia says:

Polymorphism describes the nature of the type system of a programming language, and that each element of the programming language (constants, variables, expressions, objects, functions, methods, etc.) belongs to multiple types. Refers to the property of forgiving. Also called polymorphism, polymorphism, polymorphism, diversity.

It's difficult to say that each element of a programming language belongs to multiple types.

In short, polymorphism is a common main routine.

What is a common main routine?

• The opposite idea to the common subroutine.
• There is one way to call it, but there are multiple ways to call it (method).

Why use polymorphism

• Even if the number of called parties (classes, methods) increases, the calling code does not have to change.
• In other words, you can reduce the cost of modifying and testing programs.

Implementation of polymorphism

• Normally, when assigning a created instance to a variable, the type of the variable must match the created instance.

Door dr = new Door();     // OK:The types on the left and right sides match (Note: Door is a self-made class)
Door dr2 = new String();  // NG:Compile error because the types on the left and right sides do not match

• Polymorphism allows you to substitute even if the left and right sides have different types.
• To be precise, the two classes must be inherited.

ClassA a = new ClassB();     // OK

//ClassA is a superclass and ClassB is a subclass
public class ClassB extends ClassA {
    //abridgement
}

Specific examples of polymorphism

1. Arrangement using polymorphism

Caller (class with main method)

• About processing (1)

The variable players is declared as an "array of Player classes". An instance of another class (a subclass of the Player class) is assigned to the contents of the array.

• About processing (2)

This is polymorphism (common main routine). There is one way to call "players [X] .janken ()", but there are multiple methods to be called (Person's rock-paper-scissors, Robot's rock-paper-scissors).

MainClass.java

public class MainClass {

    public static void main(String[] args) {

        // (1)Generate a player to play rock-paper-scissors
        Player[] players = new Player[3];
        players[0] = new Person();  //Man
        players[1] = new Robot();   //Robot A
        players[2] = new Robot();   //Robot B

        // (2)Let each player play rock-paper-scissors in turn
        players[0].janken();    //Man
        players[1].janken();    //Robot A
        players[2].janken();    //Robot B
    }
}

Callee (superclass)

Player.java

public class Player {

    private int win;

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

    public int getWin() {
        return win;
    }
}

Callee (subclass)

Person.java

public class Person extends Player {
    //abridgement
}

Robot.java

public class Robot extends Player {
    //abridgement
}

In the future, even if the processing of the janken method changes, the caller does not have to change either (1) or (2). Also, even if the program is modified as shown below, the caller code only needs to be changed in (1).

• The number of players has changed (example: 3 to 10)
• Player type changed (eg, in addition to humans and robots, artificial intelligence, monkeys, aliens, etc.)
• Note: Of course, the called party needs to make changes such as creating an artificial intelligence class.

2. Example of using polymorphism as an argument

Suppose there is a specification change in the rock-paper-scissors program. I added the code of "[Add]" to MainClass.java earlier.

• The constructor of the Entry class requires the arguments of the Player class.
• Actually, you can set a subclass of Player class as an argument.
• Again, the caller code is written the same, even if there are more subclasses in the future.

MainClass.java

public class MainClass {

    public static void main(String[] args) {

        // (1)Generate a player to play rock-paper-scissors
        Player[] players = new Player[3];
        players[0] = new Person();  //Man
        players[1] = new Robot();   //Robot A
        players[2] = new Robot();   //Robot B

        //[Addition] Added processing to register for Janken Tournament
        Entry[] entry = new Entry[3];
        entry[0] = new Entry(players[0]);   //Man
        entry[1] = new Entry(players[1]);   //Robot A
        entry[2] = new Entry(players[2]);   //Robot B

        // (2)Let each player play rock-paper-scissors in turn
        players[0].janken();    //Man
        players[1].janken();    //Robot A
        players[2].janken();    //Robot B
    }
}

Entry.java

public class Entry {

    private int win;

    //constructor
    public Entry(Player player) {
        //Get player information (history of wins, etc.) to register for the Janken Tournament
        win = player.getWin();
    }
}

Summary

What is polymorphism?

• Common main routine.
• The calling method can be unified with a single description.
• There are multiple ways to be called.

Why use polymorphism?

• This is because the calling code does not have to change even if what is called changes.
• This is to reduce the cost of modifying and testing the program. This is a very important point of view in practice.

References

• Head First Java 2nd Edition-The basics of Java that you can learn with your head and body
• 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 buy it on Amazon.