HappyNumber.java

import java.util.HashSet;
import java.util.Set;
import java.util.stream.IntStream;

public class HappyNumber {

    public static void main(String[] args) {
        IntStream.range(0, args.length == 0 ? 1001 : Integer.parseInt(args[0]))
                .parallel()
                .filter((number) -> {
                    Set<Integer> unique = new HashSet<>();
                    while (unique.add(number)) {
                        int value = 0;
                        while (number > 0) {
                            value += Math.pow(number % 10, 2);
                            number /= 10;
                        }
                        number = value;
                    }
                    return number == 1;
                })
                .mapToObj((n) -> "Happy: " + n)
                .forEach(System.out::println);
    }
}