Thread Lifecycle & join()

TL;DR ⚡

Abstract

• Thread states: NEW → RUNNABLE → (RUNNING) → BLOCKED / WAITING / TIMED_WAITING → TERMINATED.
• join() = “wait for that thread to finish before I continue.” It is NOT fork/join, and it does not return/combine results.

The 6 thread states (java.lang.Thread.State)

1. NEW — created (new Thread()), not started.
2. RUNNABLE — start() called; eligible to run (running or ready-to-run — the OS scheduler decides).
3. BLOCKED — waiting to acquire a synchronized lock another thread holds.
4. WAITING — waiting indefinitely for another thread (wait(), join() with no timeout).
5. TIMED_WAITING — waiting for a set time (sleep(ms), join(ms), wait(ms)).
6. TERMINATED — run() finished (or threw).

Note: there’s no separate “RUNNING” enum — actually-executing is part of RUNNABLE.

What is join()?

threadA.join() called from the main thread means: “main pauses here until threadA finishes.” It’s how you wait for a worker to complete before using its side effects.

Thread worker = new Thread(task);
worker.start();
worker.join();      // main WAITS here until worker is done
System.out.println("worker finished, continue");

Common confusions (yours!)

• Is join() part of fork/join? No. Fork/Join is a separate framework (ForkJoinPool, work-stealing, for divide-and-conquer parallelism). Thread.join() is just “wait for this thread.” Different things that share the word “join.”
• Does join() combine results from two threads? No. join() returns void — it only waits. To get a result back you use Future/CompletableFuture (see CompletableFuture). To combine two async results, that’s CompletableFuture.thenCombine. join() only synchronizes timing, not data.

Mental model

join() = “I’ll wait at the door until you come out.” It does not carry anything out of the room — that’s what Futures are for.

Test Yourself

Name the thread states.::NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED.
What does join() do?::Makes the calling thread wait until the target thread finishes. Returns void — no result.
Is join() the same as Fork/Join?::No — Fork/Join is a separate divide-and-conquer framework (ForkJoinPool). join() just waits for a thread.
How do you get a RESULT from a thread (not just wait)?::Use Future / CompletableFuture; join() only synchronizes timing, it returns nothing.