Amit Gupta's Blog

Lets start with some example

method(){
	Statement 1
	Statement 2
}

If Statement 1 is a thread statement then Statement 2 can be executed any time. It doesn’t wait Statement 1 to be completed.
If Statement 1 is simple statement then Statement 2 can’t be executed until Statement 1 is completed.
If Statement 1 is a thread statement calling join() then Statement 2 can’t be executed until Statement 1 is completed.
If Statement 1 is a thread statement calling join(n) then Statement 2 can’t be executed until Statement 1 is completed or time n is over.

public class JoinDemo extends Object {
	public static Thread createThread(String name, long napTime) {
		final long sleepTime = napTime;

		Runnable r = new Runnable() {
			public void run() {
				try {
					print("in run() - entering");
					Thread.sleep(sleepTime);
				} catch ( InterruptedException x ) {
					print("interrupted!");
				} finally {
					print("in run() - leaving");
				}
			}
		};

		Thread t = new Thread(r, name);
		t.start();

		return t;
	}

	private static void print(String msg) {
		String name = Thread.currentThread().getName();
		System.out.println(name + ": " + msg);
	}

	public static void main(String[] args) throws Exception{
		Thread[] t = new Thread[2];
		t[0] = createThread("thread A", 2000);
		//t[0].join();
		t[1] = createThread("thread B", 1000);

	}
}

Explanation:

When createThread() get called it creates a thread. Thread.sleep() interrupts this thread. So another createThread() call takes place.

t[0] = createThread("thread A", 2000); //statement 1
t[1] = createThread("thread B", 1000); //statement 3

Output:
thread A: in run() – entering
thread B: in run() – entering
thread B: in run() – leaving
thread A: in run() – leaving

t[0] = createThread("thread A", 2000); //statement 1
t[0].join(); //statement 2
t[1] = createThread("thread B", 1000); //statement 3

Output
thread A: in run() – entering
thread A: in run() – leaving
thread B: in run() – entering
thread B: in run() – leaving

In above condition, first t[0] will be completed then statement 3 will be called.