feat(tcp): implement simple single-client ReverseEcho TCP server and client

What
- Added a TCP server (ReverseEcho) bound to port 2000 that accepts a single client connection.
- Reads lines of text from client, reverses them with StringBuilder.reverse(), and sends the reversed string back.
- Terminates when the reversed message equals "dne".
- Added a standalone Client class that:
  - Connects to the server at given IP and port 2000.
  - Reads user input from stdin.
  - Sends each line to the server and prints back reversed response until "dne" is echoed.

Why
- Demonstrates the fundamentals of socket programming in Java:
  - Creating a ServerSocket and accepting a connection.
  - Using blocking I/O (BufferedReader / PrintStream) for text-based line protocol.
  - Applying simple transformation logic on server (reverse string).
- Useful for beginners to learn how client-server communication works before scaling to multi-threaded or multi-client setups.
- Builds a foundation for understanding protocols, request/response handling, and TCP’s connection-oriented nature.

How
- Server:
  - Create `ServerSocket ss = new ServerSocket(2000)` and block on `ss.accept()` for client.
  - Wrap input with `BufferedReader` for line-based reading, output with `PrintStream` for convenience.
  - Loop: readLine() from client, reverse via `StringBuilder`, send back via println().
  - Exit loop when reversed string equals "dne".
- Client:
  - Connect to server IP and port using `new Socket("device-ip", 2000)`.
  - Wrap keyboard input (System.in) in BufferedReader.
  - Wrap socket I/O in BufferedReader/PrintStream for line protocol.
  - Loop: read line from keyboard, send with ps.println(), read server’s reversed reply, print to stdout.
  - Exit loop when reply equals "dne".

Key considerations / notes
- Host IP:
  - Replace `"Add Your Device IP Address"` with `"localhost"` when testing locally.
- Protocol:
  - Both client and server must use newline-terminated strings because of readLine/println design.
- Termination:
  - Triggered when the reversed message equals "dne".
  - This means client must type "end" to get "dne" after reversing, or directly type "dne".
- Limitations:
  - Server supports only one client at a time and blocks until connection closes.
  - No error handling for client disconnects (readLine() may return null).
  - No multi-threading; new client cannot connect until server restarts.
- Encoding:
  - Uses platform default charset in InputStreamReader/PrintStream; for robustness specify UTF-8 explicitly.

How to run / test
1. Start server:
   - Run `ReverseEcho.main()`.
   - Console shows no logs, but server is bound on port 2000.
2. Start client:
   - Run `Client.main()` with correct host IP (or localhost).
   - Client will block waiting for keyboard input.
3. Interaction:
   - Type a line, press Enter.
   - Server reverses and sends it back; client prints reversed line.
   - Enter "dne" (or any line whose reverse equals "dne") to terminate session.

Real-life applications
- Educational exercises for networking fundamentals.
- Simple text transformers (palindrome checkers, encoders, mock chat bots).
- Introductory labs for teaching TCP/IP and socket programming concepts.

Future improvements
- Add logging statements in server for better visibility.
- Support multiple clients with multi-threaded handler model (one thread per client).
- Gracefully handle client disconnects (check null from readLine).
- Close sockets and streams in finally block or try-with-resources.
- Add configuration for host/port instead of hardcoding.
- Specify UTF-8 charset explicitly for consistent behavior across platforms.
- Enhance protocol with structured commands (not just reversed text).

Files added / modified
- ReverseEcho.java — server implementation (main class).
- Client.java — simple TCP client for testing ReverseEcho server.

Notes
- This is a minimal blocking TCP example: best suited for learning.
- For production use, prefer java.nio (non-blocking) or multi-threaded server with proper resource handling.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section27NetworkProgramming/ReverseEchoServerSingleClient/src/ReverseEcho.java

@@ -2,40 +2,37 @@
 import java.io.*;
 
 public class ReverseEcho {
-    public static void main(String[] args) throws Exception
-    {
+    public static void main(String[] args) throws Exception {
         ServerSocket ss = new ServerSocket(2000);
         Socket stk = ss.accept();
 
-        //Reading complete line from client.
+        //Reading complete line from a client.
         BufferedReader br = new BufferedReader(new InputStreamReader(stk.getInputStream()));
+
         //Print output to the client.
         PrintStream ps = new PrintStream(stk.getOutputStream());
 
         String msg;
-        //Reading a msg from a client and reverse it back and send to client.
-        StringBuilder sb;
-        //References deceleration.
-        do
-        {
+        //Reading a msg from a client and reverse it back and send to a client.
+
+        StringBuilder sb; //References deceleration.
+        do {
             msg = br.readLine();
-            //reversing a msg from client using method present inside the StringBuilder.
+            //reversing a msg from a client using method present inside the StringBuilder.
             /*StringBuilder sb = new StringBuilder(msg);*/
             sb=new StringBuilder(msg);
             sb.reverse();
             msg=sb.toString();
 
-            //write back to the output stream of socket.
+            //write back to the output stream of the socket.
             ps.println(msg);
-        }while(!msg.equals("dne"));
-        //Server part is ready.
+        } while(!msg.equals("dne"));
+        //The server part is ready.
     }
 }
 
-class Client
-{
-    public static void main(String[] args)throws Exception
-    {
+class Client {
+    public static void main(String[] args)throws Exception {
         Socket stk = new Socket("Add Your Device IP Address",2000);
 
         //Read from the keyboard.
@@ -47,15 +44,15 @@ public static void main(String[] args)throws Exception
         PrintStream ps = new PrintStream(stk.getOutputStream());
 
         String msg;
-        //StringBuilder sb; no need here because we are not reading it from client side.
-        do
-        {
+        //StringBuilder sb; no need here because we are not reading it from the client side.
+
+        do {
             msg= keyb.readLine();
-            //After reading from the keyboard it send it to the server.
+            //After reading from the keyboard it sends it to the server.
 
             ps.println(msg);
             msg=br.readLine();
             System.out.println("From Server: "+msg);
-        }while (!msg.equals("dne"));
+        } while (!msg.equals("dne"));
     }
-}
+}