public interface Attackable{ public void attacked(); }

public class Monster extends Entity implements Attackable {
    ... //Previous code
    int health = 10;
    public void attacked(Attackable attacker){ this.health -= 10; }
}
public class Room implements Attackable {
    ... //Previous code
    public void attacked(Attackable attacker){ 
      for(int i = 0; i < this.monsters.length; i++)
        this.monsters[i].health -= 1;
    }
}