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Joining Threads in Java

Theory

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Joining Threads in Java

Joining Threads 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 Joining Threads: 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.

Joining Threads 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. Joining Threads 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 Joining Threads.

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");
  }
}

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

Short notes

DefJoining Threads — thread = one path of work. start() schedules it. run() does not.
RuleJoining Threads — shared data needs synchronization.
RememberJoining Threads — sleep = pause. join = wait for other thread.
TrapJoining Threads — 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"); } }

Joining Threads in Java

Example

Short notes

Questions

Joining Threads in Java

Example

Short notes

Questions