How do I create any Collection given the collection name(ArrayList, LinkedList etc) and items in the collection?

Question

I am trying to create a method that accepts the type of collection and the type of items inside the collection.

ArrayList<Integer> ints = getCollection(ArrayList.class, Integer.class);

How do I write the getCollection so that, given any Collection subclass and any Number subclass, the method should create a collection and populate it with random elements of Number or subtype of Number?

Answer 1

If you can, it would be better to pass in a Supplier for the collection, and a Function to convert from Number to the sub-type, such as:

    private static final SecureRandom RND = new SecureRandom();
    private static final int COUNT_MAX = 100;

    public static void main(String[] args) {
        ArrayList<Integer> ints = getCollection(ArrayList::new, Number::intValue);
        System.out.println(ints);
        Set<Double> doubles = getCollection(HashSet::new, Number::doubleValue);
        System.out.println(doubles);
    }

    private static <T extends Collection<U>, U extends Number> T getCollection(
        Supplier<T> supplier, Function<Number, U> fn) {
        T collection = supplier.get();
        int count = RND.nextInt(COUNT_MAX);
        for (int i = 0; i < count; i++)
            collection.add(fn.apply(RND.nextInt()));
        return collection;
    }

This way, you won't need any casting.

Update using streams:

    private static <T extends Collection<U>, U extends Number> T getCollection(
        Supplier<T> supplier, Function<Number, U> fn) {
        int count = RND.nextInt(COUNT_MAX);
        return RND.ints().limit(count).boxed().map(fn)
            .collect(Collectors.toCollection(supplier));
    }