Java 17 - NIO

public static final int OP_READ = 1 << 0;
public static final int OP_WRITE = 1 << 2;
public static final int OP_CONNECT = 1 << 3;
public static final int OP_ACCEPT = 1 << 4;

import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;

class NIOServer {
    private static String readDataFromSocketChannel(SocketChannel socketChannel) throws IOException {
        ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
        StringBuilder data = new StringBuilder();
        while(true) {
            byteBuffer.clear();
            int n = socketChannel.read(byteBuffer);
            if(n == -1) {
                break;
            }
            byteBuffer.flip();
            int limit = byteBuffer.limit();
            char[] currentData = new char[limit];
            for(int i = 0; i < limit; i++) {
                currentData[i] = (char) byteBuffer.get(i);
            }
            data.append(currentData);
        }
        return data.toString();
    }

    public static void main(String[] args) throws IOException {
        Selector selector = Selector.open();

        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.configureBlocking(false);
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

        ServerSocket serverSocket = serverSocketChannel.socket();
        InetSocketAddress inetSocketAddress = new InetSocketAddress("127.0.0.1", 8888);
        serverSocket.bind(inetSocketAddress);

        while(true) {
            selector.select();
            Set<SelectionKey> keys = selector.selectedKeys();
            Iterator<SelectionKey> keyIterator = keys.iterator();
            while(keyIterator.hasNext()) {
                SelectionKey selectionKey = keyIterator.next();
                if(selectionKey.isAcceptable()) {
                    ServerSocketChannel serverSocketChannel1 = (ServerSocketChannel) selectionKey.channel();
                    SocketChannel socketChannel = serverSocketChannel.accept();
                    socketChannel.configureBlocking(false);
                    socketChannel.register(selector, SelectionKey.OP_READ);
                } else if(selectionKey.isReadable()) {
                    SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                    System.out.println(readDataFromSocketChannel(socketChannel));
                    socketChannel.close();
                }
                keyIterator.remove();
            }
        }
    }
}

class NIOClient {
    public static void main(String[] args) throws IOException {
        Socket socket = new Socket("127.0.0.1", 8888);
        OutputStream outputStream = socket.getOutputStream();
        String string = "Hello World!";
        outputStream.write(string.getBytes());
        outputStream.close();
    }
}