public class Skill {
    private int level, xp, xpborder;

    public Skill() {
        level = 1;
        xp = 0;
        xpborder = 100;
    }
    
    
    public int getLevel(){
        return level;
    }
    /**     * 
     * @return 
     * true if the added XP changed the level
     */
    public boolean addXP(int amount){
        xp += amount;
        if(xp > xpborder)
        {
            ++level;
            xp -= xpborder;
            xpborder *= 2;
            return true;
        }
        return false;
    }
}

public final class CharacterStats {

    public final String firstName;
    public final String lastName;
    public final Zodiac zodiac;
    public final Date birtdate;
    public final Skill bravery, willpower, agility, intelligence, comprehension;

    public CharacterStats(String firstName, String lastName, Zodiac zodiac, Date birtdate) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.zodiac = zodiac;
        this.birtdate = birtdate;

        bravery = new Skill();
        willpower = new Skill();
        agility = new Skill();
        intelligence = new Skill();
        comprehension = new Skill();
    }
}

public class Character {
    private final CharacterStats stats;
    private Weapon weapon;
    private int life;
    ...

    public Character(...) {
        ...
    }
        
   
   
    public Damage getAttack();
   
    public void defendAttack(Damage dmg);
   
    public void grantWinOver(Character char);
    
    public int getLife()
    {
        return life;
    }
}

public class Game {

    public enum TimeOfDay {

        MORNING, MIDDAY, EVENING, NIGHT;
    }
    private final Date date;
    private final Character player, enemy;

    public Game() {
        date = new Date(21, 12, 2012);
        ...
        
        fight(player, enemy);
    }    
    
    public void fight(Character a, Character b)
    {
        while(a.getLife() > 0 && b.getLife() > 0)
        {
            a.defendAttack(b.getAttack());
            b.defendAttack(a.getAttack());
        }
        
        if(a.getLife() > 0)
            a.grantWinOver(b);
        else if(b.getLife() > 0)
            b.grantWinOver(a);
        
    }

    public TimeOfDay getTimeOfDay() {
        int h = date.getHours();
        if (h >= 4 && h < 10) {
            return TimeOfDay.MORNING;
        } else if (h >= 10 && h < 16) {
            return TimeOfDay.MIDDAY;
        } else if (h >= 16 && h < 22) {
            return TimeOfDay.EVENING;
        } else {
            return TimeOfDay.NIGHT;
        }
    }
}