Let’s see how to make weighted random collection.

What is Weighted Random Collection?

The weighted random collection is the collection to get the value randomly by weighted value. Let’s say there are some values like A, B. You want to get these value ramdomly but by applying the specified percentage.

A:30%, B:70%

When you call get(), then this collection will return A as 70% and B 30% occurrences.


You need to add the value with double type number which is represented the perecentage.

    public void add(double weight, E value) {
        if (weight <= 0) throw new IllegalArgumentException("The weight value is greater than 0.");
        total += weight;
        naviMap.put(total, value);

You can get the value corresponding to the percentage randomly.

    public E get() {
        double value = random.nextDouble() * total;
        Map.Entry<Double, E> ceilingEntry = naviMap.ceilingEntry(value);
        if (ceilingEntry != null) return ceilingEntry.getValue();
        else return null;

Since we use a random function, we will get different results each time.

    WeightedRandomCollection<String> collection = new WeightedRandomCollection<>();
    collection.add(5, "This is 50%!!!!!");
    collection.add(3, "This is 30%!!!");
    collection.add(2, "This is 20%!");

            .forEach(i-> System.out.println(collection.get()));

Result is

This is 50!!!!!%
This is 50!!!!!%
This is 50!!!!!%
This is 50!!!!!%
This is 20!%
This is 50!!!!!%
This is 50!!!!!%
This is 30!!!%
This is 50!!!!!%

Full sorce code is here! Github Source Code