feat: Demonstrate daemon threads in Java

WHAT the code does:
- Defines DemonThread extending Thread:
  - run() loops infinitely, printing "Hello World!".
- In main():
  - Creates DemonThread instance D.
  - Marks D as daemon using setDaemon(true).
  - Starts D with start(), moving it into runnable state.
  - Prints "Main done" and then main thread terminates.

WHY this matters:
- Shows how daemon threads differ from user threads.
- JVM automatically terminates daemon threads when all user threads finish.
- Demonstrates background task behavior (like garbage collector, monitoring, or housekeeping).
- Reinforces lifecycle concepts: even infinite loop threads can be stopped once only daemons remain.

HOW it works:
1. D is created and configured as a daemon thread.
2. D.start() schedules D to run concurrently.
3. main() prints "Main done" and ends.
4. JVM sees only daemon threads remain → shuts down process, killing daemon thread.
5. Output: "Hello World!" may print a few times, but stops once main exits.

Tips and gotchas:
- Must call setDaemon(true) before start(); calling it after throws IllegalThreadStateException.
- Daemon threads are not guaranteed to finish tasks, so avoid them for critical work.
- Best used for background services that support user threads but don’t need to live beyond them.
- Don’t rely on daemon thread output consistency; it may terminate abruptly.

Use-cases / analogies:
- Daemon threads = janitors/housekeepers who clean up as long as the office is open (user threads active).
- Common in loggers, background resource monitoring, or async cache refreshers.
- JVM’s garbage collector itself runs as a daemon.

Short key: java daemon-threads background-task lifecycle jvm-termination.

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

Author: Someshdiwan

Section19MultiThreading/MT/src/DemonThread.java

@@ -1,16 +1,15 @@
-//Demon Threads are working in BackGround is called Demon Threads.
 public class DemonThread extends Thread {
-    public void run(){
-        while(true)
-        {
+    public void run() {
+        while(true) {
             System.out.println("Hello World!");
         }
     }
-
     public static void main(String[] args) {
         DemonThread D = new DemonThread();
         D.setDaemon(true);
         D.start(); //Runnable ne start keya and runnable state mai Dala.
         System.out.println("Main done");
     }
 }
+
+//Demon Threads are working in Background is called Demon Threads.