How to write a probability algorithm that can be maintained easily?

Question

Supposed I want to create a game. At the start of the game, the player will pick a monster.

It's easy to picks the monster fairly.

// get all monsters with equal chance
public Monster getMonsterFair(){
    Monster[] monsters = {new GoldMonster(), new SilverMonster(), new BronzeMonster()};
    int winIndex = random.nextInt(monsters.length);
    return monsters[winIndex];
}

And picks the monster unfairly.

// get monsters with unequal chance
public Monster getMonsterUnFair(){
    double r = Math.random();
    // about 10% to win the gold one
    if (r < 0.1){
        return new GoldMonster();
    }
    // about 30% to winthe silver one
    else if ( r < 0.1 + 0.2){
        return new SilverMonster();
    }
    // about 70% to win the bronze one
    else {
        return new BronzeMonster();
    }   
}

The problem is that, when I add a new monster to the game, I have to edit the if-else. Or I change the chance of winning GoldMonster to 0.2, I have to change all 0.1 into 0.2 .It's ugly, and not easily maintained.

// get monsters with unequal change & special monster
public Monster getMonsterSpecial(){
    double r = Math.random();
    // about 10% to win the gold one
    if (r < 0.1){
        return new GoldMonster();
    }
    // about 30% to win the silver one
    else if ( r < 0.1 + 0.2){
        return new SilverMonster();
    }
    // about 50% to win the special one
    else if ( r < 0.1 + 0.2 + 0.2){
        return new SpecialMonster();
    }
    // about 50% to win the bronze one
    else {
        return new BronzeMonster();
    }
}

How can this probability algorithm can be refactored so that the codes can be maintained easily when new monster is added and the chances of winning monsters are adjusted?

Answer 1

Basically what @Egor Skriptunoff said. This should scale easily. You could use a collection of Class<Monster> if you didn't want to use an enum.

enum Monster {
    GOLD(1),
    SILVER(3),
    BRONZE(6) // pseudo probabilities

    private int weight;
    // constructor etc..
}

public Monster getMonsterSpecial() {
    List<Monster> monsters = new ArrayList<>();

    for(Monster monsterType : Monster.values()) {
        monsters.addAll(Collections.nCopies(monsterType.getWeight(), monsterType)); 
    }

    int winIndex = random.nextInt(monsters.length);
    return monsters.get(winIndex);
}

You could perhaps make the enum Monsters plural, and have it point to a Class<? extends Monster> if you still want to instantiate monster classes. I just tried to make the example clearer.

Answer 2

I would uses a total weight which increases with each monster added.

private final Random rand = new Random();

public Monster getMonsterSpecial() {
    int weight = rand.nextInt(1+2+2+5);
    if ((weight -= 1) < 0) return new GoldMonster();
    if ((weight -= 2) < 0) return new SilverMonster();
    if ((weight -= 2) < 0) return new SpecialMonster();
    // 50% chance of bronze
    return new BronzeMonster();
}