Explain the benefits of the State pattern with a movie rating

Implement the work that the user can see so that it can be sorted according to the movie rating system.

A movie rating system is an age-restricting regulation. G: All ages PG12: 12 years old and over R15 +: 15 years and over R18 +: 18 years old and over There are categories such as.

Class diagram

The state pattern is in the red frame, and it is used by the User. image.png The RatingSystem is used as an interface, and each restriction is implemented in G / PG12 / R15. The lack of R18 assumes that you are not dealing with R18 movies.

merit

-The age limit can be limited to the RatingSystem implementation class (G / PG / R15). (Implementation localization) ・ R18 movies will be handled, and even if specifications are added, it can be easily handled. (Easy to change)

Source code

interface RatingSystem

RatingSystem.java


package state.rating;

public interface RatingSystem {
    public boolean checkLimit(int age);
}

class G

G.java


package state.rating;

public class G implements RatingSystem {

    @Override
    public boolean checkLimit(int age) {
        return true;
    }
}

class PG12

PG12.java


package state.rating;

public class PG12 implements RatingSystem {

    @Override
    public boolean checkLimit(int age) {
        if(12 <= age)
            return true;
        return false;
    }
}

class R15

R15.java


package state.rating;

public class R15 implements RatingSystem {

    @Override
    public boolean checkLimit(int age) {
        if(15 <= age)
            return true;
        return false;
    }
}

class Movie

Movie.java


package state.rating;

public class Movie {
    private String _title;
    private RatingSystem _rating;

    public Movie(String title, RatingSystem rating) {
        _title = title;
        _rating = rating;
    }

    public String title() {
        return _title;
    }

    public boolean checkLimit(int age) {
        return _rating.checkLimit(age);
    }
}

class User

User.java


package state.rating;

public class User {
    private String _name;
    private int _age;

    User(String name, int age){
        _name = name;
        _age = age;
    }

    public String name() {
        return _name;
    }

    public int age() {
        return _age;
    }

    public void play(Movie movie) {
        System.out.println(movie.title() + "To play.");
        if(!movie.checkLimit(_age)) {
            System.out.println(" >>>Cannot play due to age limit error.");
            stop(movie);
        }
    }

    public void stop(Movie movie) {
        System.out.println(movie.title() + "To stop.");
    }
}

Main

Main.java


package state.rating;

public class Main {
    public static void main(String... args) {

        Movie torasan = new Movie("It's hard for a man, Torajiro Sunset, small burn", new G());
        Movie titan = new Movie("Attack on Titan ATTACK ON TITAN", new PG12());
        Movie jingi = new Movie("Battle Without Honor", new R15());

        User shinnosuke = new User("Shinnosuke Nohara", 5);
        System.out.println(shinnosuke.name());
        shinnosuke.play(torasan);
        shinnosuke.play(titan);
        shinnosuke.play(jingi);
        System.out.println();

        User taro = new User("Taro", 12);
        System.out.println(taro.name());
        taro.play(torasan);
        taro.play(titan);
        taro.play(jingi);
        System.out.println();

        User hanako = new User("Hanako", 17);
        System.out.println(hanako.name());
        hanako.play(torasan);
        hanako.play(titan);
        hanako.play(jingi);
        System.out.println();
    }
}

Execution result

Depending on the age of each user, movies that are caught in the age limit will be forcibly stopped.

Shinnosuke Nohara It's hard for a man to play Torajiro Sunset Small Burn. Play Attack on Titan ATTACK ON TITAN. \ >>> Cannot play due to age limit error. Stop Attack on Titan ATTACK ON TITAN. Play Battle Without Honor. \ >>> Cannot play due to age limit error. Stop the battle without honor.

Taro It's hard for a man to play Torajiro Sunset Small Burn. Play Attack on Titan ATTACK ON TITAN. Play Battle Without Honor. \ >>> Cannot play due to age limit error. Stop the battle without honor.

Hanako It's hard for a man to play Torajiro Sunset Small Burn. Play Attack on Titan ATTACK ON TITAN. Play Battle Without Honor.

Support for additional requirements for R18 +

In practice, specification changes are common. Here, we assume a situation where "additional requirements for handling R18 movies have occurred".

"I decided to handle R18 movies, so could you fix it?" If you are told, you should be able to change it immediately, right?

If it's a State pattern, add the R18 class and you're done. There is no need to search for and correct the age limit judgment process (if statement) from here and there.

class R18

R18.java


package state.rating;

public class R18 implements RatingSystem {

    @Override
    public boolean checkLimit(int age) {
        if(18 <= age)
            return true;
        return false;
    }
}

Main

Main.java


package state.rating;

public class Main {
    public static void main(String... args) {

        Movie torasan = new Movie("It's hard for a man, Torajiro Sunset, small burn", new G());
        Movie titan = new Movie("Attack on Titan ATTACK ON TITAN", new PG12());
        Movie jingi = new Movie("Battle Without Honor", new R15());
        Movie devil = new Movie("Devil's Poisonous Monster", new R18());   //* Additional notes

        User shinnosuke = new User("Shinnosuke Nohara", 5);
        System.out.println(shinnosuke.name());
        shinnosuke.play(torasan);
        shinnosuke.play(titan);
        shinnosuke.play(jingi);
        shinnosuke.play(devil); //* Additional notes
        System.out.println();

        User taro = new User("Taro", 12);
        System.out.println(taro.name());
        taro.play(torasan);
        taro.play(titan);
        taro.play(jingi);
        taro.play(devil);   //* Additional notes
        System.out.println();

        User hanako = new User("Hanako", 17);
        System.out.println(hanako.name());
        hanako.play(torasan);
        hanako.play(titan);
        hanako.play(jingi);
        hanako.play(devil); //* Additional notes
        System.out.println();

        //* Additional notes
        User isekai = new User("University student reincarnated in another world", 18);
        System.out.println(isekai.name());
        isekai.play(torasan);
        isekai.play(titan);
        isekai.play(jingi);
        isekai.play(devil);
        System.out.println();
    }
}

Execution result

Among the above Users, the one who can see the R18 movie (The Toxic Avenger Monster) is Only 18-year-old "university students reincarnated in another world".

Shinnosuke Nohara It's hard for a man to play Torajiro Sunset Small Burn. Play Attack on Titan ATTACK ON TITAN. \ >>> Cannot play due to age limit error. Stop Attack on Titan ATTACK ON TITAN. Play Battle Without Honor. \ >>> Cannot play due to age limit error. Stop the battle without honor. Play the devil's poisonous monster. \ >>> Cannot play due to age limit error. Stop the devil's poisonous monster.

Taro It's hard for a man to play Torajiro Sunset Small Burn. Play Attack on Titan ATTACK ON TITAN. Play Battle Without Honor. \ >>> Cannot play due to age limit error. Stop the battle without honor. Play the devil's poisonous monster. \ >>> Cannot play due to age limit error. Stop the devil's poisonous monster.

Hanako It's hard for a man to play Torajiro Sunset Small Burn. Play Attack on Titan ATTACK ON TITAN. Play Battle Without Honor. Play the devil's poisonous monster. \ >>> Cannot play due to age limit error. Stop the devil's poisonous monster.

University student reincarnated in another world It's hard for a man to play Torajiro Sunset Small Burn. Play Attack on Titan ATTACK ON TITAN. Play Battle Without Honor. Play the devil's poisonous monster.

Summary

・ Judgment and functions based on the status can be limited to a specific class. (Implementation localization) -It is relatively easy to add or modify specifications. (Easy to change)

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