Four Programs: Write Down Which Ones Finish

Four short programs. For each, decide whether the process exits, and what is printed. All four before the reveal.

java

// 1
Thread t = new Thread(() -> System.out.println("work"));
t.run();
System.out.println("main done");

// 2
Thread t = new Thread(() -> { while (true) { } });
t.start();
System.out.println("main done");

// 3
Thread t = new Thread(() -> { while (true) { } });
t.setDaemon(true);
t.start();
System.out.println("main done");

// 4
Thread t = new Thread(() -> System.out.println("work"));
t.start();
t.start();

What each of the four does

Each program, what it prints, and whether the process exits
	Prints	Exits?
1	`work` then `main done`, both on the main thread	Yes
2	`main done`, and then nothing	No — the non-daemon loop keeps it alive
3	`main done`	Yes — the daemon is killed where it stands
4	`work`, then a stack trace	Yes, with `IllegalThreadStateException`

The first is the one to keep. run() gave the right output on the wrong thread, in the wrong order, and nothing complained — which is exactly why it is so hard to spot in a real program.

The fourth is the state machine showing through. A terminated thread cannot go back, so a Thread object is one job rather than a reusable worker.