Random weighted selection in Java

Question

I want to choose a random item from a set, but the chance of choosing any item should be proportional to the associated weight

Example inputs:

item                weight
----                ------
sword of misery         10
shield of happy          5
potion of dying          6
triple-edged sword       1

So, if I have 4 possible items, the chance of getting any one item without weights would be 1 in 4.

In this case, a user should be 10 times more likely to get the sword of misery than the triple-edged sword.

How do I make a weighted random selection in Java?

Answer 1

I would use a NavigableMap

public class RandomCollection<E> {
    private final NavigableMap<Double, E> map = new TreeMap<Double, E>();
    private final Random random;
    private double total = 0;

    public RandomCollection() {
        this(new Random());
    }

    public RandomCollection(Random random) {
        this.random = random;
    }

    public RandomCollection<E> add(double weight, E result) {
        if (weight <= 0) return this;
        total += weight;
        map.put(total, result);
        return this;
    }

    public E next() {
        double value = random.nextDouble() * total;
        return map.higherEntry(value).getValue();
    }
}

Say I have a list of animals dog, cat, horse with probabilities as 40%, 35%, 25% respectively

RandomCollection<String> rc = new RandomCollection<>()
                              .add(40, "dog").add(35, "cat").add(25, "horse");

for (int i = 0; i < 10; i++) {
    System.out.println(rc.next());
}