https://qiita.com/saku_lica/questions/c4edd137f1988d530dbf

RPG.java

import java.util.Random;

public class RPG {
    public static void main(String[] args) {
        Hero hero = new Hero();
        Monster monster = new Dragon();
        Character attacker = hero;
        Character defender = monster;
        while (attacker.isAlive()) {
            attacker.attack(defender);
            defender.showHp();
            Character nextAttacker = defender;
            Character nextDefender = attacker;
            attacker = nextAttacker;
            defender = nextDefender;
        }
        if (monster.isDead()) {
            System.out.println(hero.name + "は" + monster.name + "をたおした");
        } else {
            System.out.println(hero.name + "はちからつきた");
        }
    }
}

interface Attacker {
    public void attack(Defender defender);
}

interface Defender {
    public void defend(int damage);
}

abstract class Character implements Attacker, Defender {
    public final String name;
    protected int hp;
    protected int maxHp;
    protected int mp;
    protected int maxMp;
    protected final static Random random = new Random();

    public Character(String name, int hp, int mp) {
        this.name = name;
        this.hp = hp;
        this.maxHp = hp;
        this.mp = mp;
        this.maxMp = mp;
    }

    public boolean isAlive() {
        return this.hp > 0;
    }

    public boolean isDead() {
        return !isAlive();
    }

    public void showHp() {
        System.out.println(this.name + "の現在HPは" + this.hp + "だ");
    }

    protected int attackPower() {
        return random.nextInt(maxHp + 1);
    }

    public void attack(Defender defender) {
        System.out.println(name + "の攻撃！");
        defender.defend(attackPower());
    }

    public void defend(int damage) {
        if (damage == 0) {
            System.out.println(name + "は攻撃をかわした");
        } else {
            System.out.println(name + "は" + damage + "のダメージを受けた");
            hp = Math.max(0, hp - damage);
        }
    }
}

class Hero extends Character {
    private Sword sword;
    private Shield shield;

    public Hero() {
        this("勇者", 100, 0, new Sword(), new Shield());
    }

    public Hero(String name, int hp, int mp, Sword sword, Shield shield) {
        super(name, hp, mp);
        this.sword = sword;
        this.shield = shield;
    }

    @Override
    protected int attackPower() {
        return sword.power();
    }

    @Override
    public void defend(int damage) {
        super.defend(shield.protect(damage));
    }
}

class Monster extends Character {

    public Monster(String name, int hp, int mp) {
        super(name, hp, mp);
    }

    protected void run() {
        System.out.println(name + "は逃げ出した");
        this.hp = 0;
    }

    @Override
    public void defend(int damage) {
        if (random.nextDouble() < 0.1) {
            run();
        } else {
            super.defend(damage);
        }
    }
}

class Dragon extends Monster {
    public Dragon() {
        super("ドラゴン", 500, 100);
    }

    @Override
    protected int attackPower() {
        if (random.nextDouble() < 0.3) {
            System.out.println("快心の一撃！");
            return maxHp;
        } else {
            return random.nextInt(maxHp / 10);
        }
    }
}

abstract class Equipment {
    public final String name;
    protected final int power;
    protected final static Random random = new Random();

    public Equipment(String name, int power) {
        this.name = name;
        this.power = power;
    }
}

class Sword extends Equipment {

    public Sword() {
        this("刀", 100);
    }

    public Sword(String name, int power) {
        super(name, power);
    }

    public int power() {
        return random.nextInt(power) + 1;
    }
}

class Shield extends Equipment {

    public Shield() {
        this("盾", 10);
    }

    public Shield(String name, int power) {
        super(name, power);
    }

    public int protect(int damage) {
        return Math.max(0, damage - random.nextInt(power));
    }
}