A simple example with Socket

import java.io.*;
import java.net.Socket;
public class TCPClient {
    public static void main(String args[]) throws Exception{
        Socket socket = new Socket("127.0.0.1",65000);
        OutputStream os = socket.getOutputStream();
        InputStream is = socket.getInputStream();
        os.write(new String("xxx").getBytes());
        int ch = 0;
        byte [] buff = new byte[1024];
        ch  = is.read(buff);
        String content = new String(buff,0,ch);
        System.out.println(content);
        is.close();
        os.close();
        socket.close();

    }
}

import java.net.ServerSocket;
import java.net.Socket;
import java.util.SortedSet;

public class TCPServer {
    public static void main(String args[]) throws Exception{
        //create and bind port
        ServerSocket ss = new ServerSocket(65000);
        //dead loop awaiting client sending request
        while(true){
            Socket socket = ss.accept();
            //only start after accept the require from client
            new LengthCalculator(socket).start();

        }

    }

}

import java.io.*;

import java.net.Socket;

public class LengthCalculator extends Thread{
    private Socket socket;

    public LengthCalculator(Socket socket){
        this.socket = socket;
    }

    public void run(){
        try{
            //output stream
            OutputStream os = socket.getOutputStream();
            //input stream
            InputStream is = socket.getInputStream();
            //ch for reading string length
            int ch=0;
            //buffer for input as isread only accpet byte so we have to made buff byte
            byte[] buff = new byte[1024];

            //isread will return the buff's length
            ch = is.read(buff);

            String content = new String(buff,0,ch);
            System.out.println(content);
            //also os stream only accept array of bytes

            os.write(String.valueOf(content.length()).getBytes());
            //close I/O stream
            is.close();
            os.close();
            socket.close();


        }catch(Exception e){
            System.out.println(e.toString());
        }
    }


}