Enemy.java

public class Enemy {
    private int health;
    public Enemy(int health) {
        this.health = health;
    }

    public void hit(int damage) {
        health -= damage;
    }

    public int getHealth() {
        return health;
    }
}

Player.java

public class Player {
    private Weapon weapon;
    private int damageBase;

    public Player(int damageBase, Weapon weapon) {
        this.damageBase = damageBase;
        this.weapon = weapon;
    }

    public void attack(Enemy enemy) {
        int damage = damageBase + weapon.rollDamage();
        enemy.hit(damage);

        System.out.printf("Player hit enemy for %s damage, enemy is at %s health\n", damage, enemy.getHealth());
    }
}

RPG.java

public class RPG {
    public static void main(String... argv) {
        Weapon weapon = new Weapon(5, 10);
        Player player = new Player(5, weapon);
        Enemy enemy = new Enemy(100);

        do {
            player.attack(enemy);
        }
        while(enemy.getHealth() > 0);
    }
}

Weapon.java

public class Weapon {
    private int damageMin;
    private int damageMax;

    public Weapon(int damageMin, int damageMax) {
        this.damageMax = damageMax;
        this.damageMin = damageMin;
    }

    public int rollDamage() {
        int range = damageMax - damageMin;

        return damageMin + (int)(Math.random() * range);
    }
}