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ThreadGroup in Java

Theory

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ThreadGroup in Java

ThreadGroup is part of Java concurrency. A thread is a path of work inside a process. Create with Thread or Runnable, then call start() — not run(). run() only runs on the same thread.

For ThreadGroup: if two threads share data, synchronize or the result can be wrong. sleep pauses. join waits for another thread to finish. daemon threads die when all user threads finish.

ThreadGroup deadlock: two threads wait for each other’s lock forever. Avoid by locking in the same order. Don’t guess thread order — it can change every run.

Let's take this on the board with one tiny Main class — no extra files. ThreadGroup in Java — start() schedules the thread; main also prints — order may vary. Start from main, go line by line, and stop at each print. That output is the proof for ThreadGroup.

Exam tip

start() vs run() is a classic interview question.

Example

// Thread demo
public class Main {
  public static void main(String[] args) {
    Thread t = new Thread(() -> System.out.println("Hi from thread"));
    t.start();
    System.out.println("Hi from main");
  }
}

ThreadGroup in Java — start() schedules the thread; main also prints — order may vary.

Short notes

DefThreadGroup — thread = one path of work. start() schedules it. run() does not.
RuleThreadGroup — shared data needs synchronization.
RememberThreadGroup — sleep = pause. join = wait for other thread.
TrapThreadGroup — calling run() instead of start(). Deadlock = locks in wrong order.

// Thread demo public class Main { public static void main(String[] args) { Thread t = new Thread(() -> System.out.println("Hi from thread")); t.start(); System.out.println("Hi from main"); } }

ThreadGroup in Java

Example

Short notes

Questions

ThreadGroup in Java

Example

Short notes

Questions