public void client1() throws Exception {
    Socket localhost = new Socket("localhost", 8899);
    OutputStream outputStream = localhost.getOutputStream();
    outputStream.write("哈囉你好嗎".getBytes(StandardCharsets.UTF_8));
    // 關鍵，這邊write完不shutdown，後面read方法會被阻塞
    localhost.shutdownOutput();
    // 接收回應，這邊複習轉換流
    InputStream is = localhost.getInputStream();
    InputStreamReader isr = new InputStreamReader(is);
    char[] cbuf = new char[10];
    int len;
    while ((len = isr.read(cbuf)) != -1) {
        String str = new String(cbuf, 0, len);
        System.out.print(str);
    }
    // 關閉
    isr.close();
    is.close();
    outputStream.close();
    localhost.close();
}

public void server1() throws Exception {
    ServerSocket serverSocket = new ServerSocket(8899);
    System.out.println("server啟動，正在監聽port:" + serverSocket.getLocalPort());
    Socket s = serverSocket.accept();
    System.out.println("收到請求，來自:" + s.getInetAddress());
    InputStream inputStream = s.getInputStream();
    // 由於要處理中文字，用好朋友ByteArrayOutputStream來裝
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    byte[] buf = new byte[10];
    int len;
    while ((len = inputStream.read(buf)) != -1) {
        baos.write(buf, 0, len);
    }
    System.out.println(baos.toString());
    // 返回訊號
    OutputStream os = s.getOutputStream();
    os.write("server收到訊息了，client你退下吧".getBytes(StandardCharsets.UTF_8));
    // 關閉
    os.close();
    inputStream.close();
    s.close();
    serverSocket.close();
}

public class UDP {
    @Test
    public void sender() throws Exception {
        // UDP的socket不管內容跟要發到哪
        DatagramSocket socket = new DatagramSocket();
        byte[] data = "麥當勞歡樂送".getBytes(StandardCharsets.UTF_8);
        // 具體的Packet才指明內容跟要發給誰
        DatagramPacket datagramPacket = new DatagramPacket(data, 0, data.length, InetAddress.getLocalHost(), 9090);
        // 參數是發送的data、起始與終點、來源(本機)與port
        socket.send(datagramPacket);
        System.out.println("反正送出去了，不管有沒有人接，我先溜了");
        socket.close();

    }

    @Test
    public void receiver() throws Exception {
        // UDP接收端可以在socket先聲明端口
        DatagramSocket socket = new DatagramSocket(9090);
        // 準備接收的容器
        byte[] buf = new byte[100];
        // socket已經說過地址了，這邊就只負責讀內容
        DatagramPacket packet = new DatagramPacket(buf, 0, buf.length);
        socket.receive(packet);
        // 印出接收的內容
        System.out.println(new String(packet.getData(), 0, packet.getLength()));
        socket.close();
    }
}

URL url = new URL("http://192.168.1.100:8080/helloworld.txt"); // 只是表明網址
URLConnection urlConnection = url.openConnection(); // 試著連線看看，可能返回錯誤
urlConnection.connect(); // 真正表示"連上"這個動作
InputStream inputStream = urlConnection.getInputStream(); // 拿到流
...