poll 1

// Server.java

import java.io.*;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;

public final class Server {
    private static final int fib(final int number) {
        if (number < 3)
            return 1;

        var f = 1;
        var s = 1;

        for (int i = 3; i <= number; i++) {
            final var mem = s;
            s += f;
            f = mem;
        }

        return s;
    }

    public static final void main(final String[] args) throws IOException {
        final var server = ServerSocketChannel.open();
        server.socket().bind(new InetSocketAddress("127.0.0.1", 1337));
        server.configureBlocking(false);

        final var selector = Selector.open();
        server.register(selector, SelectionKey.OP_ACCEPT);

        while (true) {
            selector.select();
            final var it = selector.selectedKeys().iterator();

            while (it.hasNext()) {
                final var key = it.next();

                if (key.isAcceptable()) {
                    final var client = server.accept();
                    client.configureBlocking(false);
                    client.register(selector, SelectionKey.OP_READ);
                } else if (key.isReadable()) {
                    final var readBuffer = ByteBuffer.allocate(8);
                    final var client = (SocketChannel) key.channel();

                    if (client.read(readBuffer) < 0) {
                        client.close();
                    } else {
                        readBuffer.flip();
                        final var writeBuffer = ByteBuffer.allocate(8);
                        writeBuffer.putInt(fib(readBuffer.getInt()));
                        writeBuffer.flip();
                        client.write(writeBuffer);
                    }
                }

                it.remove();
            }
        }
    }
}

// Client.java

import java.io.*;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SocketChannel;
import static java.lang.System.out;

public final class Client {
    public static final void main(final String[] args) throws IOException {
        final var client = SocketChannel.open();
        client.connect(new InetSocketAddress("127.0.0.1", 1337));

        try (final var reader = new BufferedReader(new InputStreamReader(System.in))) {
            out.println("Fibonacci number:");
            final var fibNum = Integer.parseInt(reader.readLine());

            final var writeBuffer = ByteBuffer.allocate(8);
            writeBuffer.putInt(fibNum);
            writeBuffer.flip();
            client.write(writeBuffer);

            final var readBuffer = ByteBuffer.allocate(8);
            client.read(readBuffer);
            readBuffer.flip();
            final var fibAns = readBuffer.getInt();

            out.println(fibAns);
            client.close();
        }
    }
}