udp factorial

// Server.java

import org.jetbrains.annotations.Contract;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;

public final class Server {
    @Contract(pure = true)
    public static final int fac(final int number) {
        return number == 0 ? 1 : fac(number - 1) * number;
    }

    public static final void main(final String[] args) throws IOException {
        final var server = new DatagramSocket(new InetSocketAddress("127.0.0.1", 1337));

        while (true) {
            final var readData = new byte[4];
            final var packet = new DatagramPacket(readData, readData.length);
            server.receive(packet);
            final var readBuffer = ByteBuffer.wrap(packet.getData());
            final var number = readBuffer.getInt();

            final var writeBuffer = ByteBuffer.allocate(4);
            writeBuffer.putInt(fac(number));
            final var writeData = writeBuffer.array();

            server.send(
                    new DatagramPacket(
                            writeData,
                            writeData.length,
                            packet.getSocketAddress()
                    )
            );
        }
    }
}

// Client.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;

import static java.lang.System.out;

public final class Client {
    public static final void main(final String[] args) throws IOException {
        final var client = new DatagramSocket();
        out.println("Number:");

        try (final var reader = new BufferedReader(new InputStreamReader(System.in))) {
            final var writeBuffer = ByteBuffer.allocate(4);
            writeBuffer.putInt(Integer.parseInt(reader.readLine()));
            final var writeData = writeBuffer.array();

            client.send(
                    new DatagramPacket(
                            writeData,
                            writeData.length,
                            new InetSocketAddress("127.0.0.1", 1337)
                    )
            );

            final var readData = new byte[4];
            final var packet = new DatagramPacket(readData, readData.length);
            client.receive(packet);
            final var readBuffer = ByteBuffer.wrap(packet.getData());
            out.println(readBuffer.getInt());
        }
    }
}