Непонятная домашка

BigIntegerStream.java

package com.drankov;


import java.math.BigInteger;
import java.util.ArrayList;
import java.util.stream.Stream;

public class BigIntegerStream {

    /**
     *
     * @param N fst numb
     * @param M sen numb
     * @param c reminder
     * @return stream of range
     */
    public static Stream<BigInteger> range_with_last_digit(BigInteger N, BigInteger M, int c) {
        ArrayList<BigInteger> bigIntegers = new ArrayList<>(M.add(N.multiply(new BigInteger("-1"))).divide(BigInteger.TEN).intValue());

        new BigInteger("10");
        while (N.divideAndRemainder(BigInteger.TEN)[1].intValue() != c) {
            N = N.add(new BigInteger("1"));
        }

        while (N.compareTo(M) == -1 || N.compareTo(M) == 0) {
            bigIntegers.add(N);
            N = N.add(BigInteger.TEN);
        }

        return bigIntegers.stream();
    }
}

Main.java

package com.drankov;

import java.io.BufferedWriter;
import java.io.IOException;
import java.math.BigInteger;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;
import java.util.stream.Collectors;



public class Main {
    public static void main(String[] args) {

        if (args.length != 3 && args.length != 3)
        {
            System.out.print("Usage: " +
                    "./executable N M C [file_to_output]");
        }

        BigInteger N = new BigInteger(args[0]);
        BigInteger M = new BigInteger(args[1]);
        Integer C = Integer.parseInt(args[2]);

        List<BigInteger> collect = BigIntegerStream.range_with_last_digit(N, M, C)
                // последовательно .sequential()
                .parallel()
                .filter(s -> s.isProbablePrime(10000))
                .collect(Collectors.toList());


        String s = collect.stream()
                .sequential()
                .map(BigInteger::toString)
                .reduce((x, y) -> x + "," + y)
                .get();

        String answer = String.format("%d:<%s>.", collect.size(), s);

        if (args.length == 3)
            System.out.print(answer);
        else if (args.length == 4) {
            Path path = Paths.get(args[3]);
            try (BufferedWriter writer = Files.newBufferedWriter(path, Charset.forName("UTF-8"))) {
                writer.write(answer);
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

    }
}