You need to know whether something finished, or whether it finished in a certain length of time.

Start that “something” in its own thread and call its join( ) method with or without a timeout value.

The join( ) method of the target thread is used to suspend the current thread until the target thread is finished (returns from its run method). This method is overloaded; a version with no arguments will wait forever for the thread to terminate, while a version with arguments will wait up to the specified time. For a simple example, I’ll create (and start!) a simple thread that just reads from the console terminal, and the main thread will simply wait for it. When I run the program, it looks like this:

darwinsys.com$ java Join
Starting
Joining
Reading
hello from standard input # waits indefinitely for me to type this line
Thread Finished.
Main Finished.
darwinsys.com$

Example 24-7 is the code for the join( ) demo.

public class Join {
    public static void main(String[] args) {
        Thread t = new Thread(  ) {
            public void run(  ) {
                System.out.println("Reading");
                try {
                    System.in.read(  );
                } catch (java.io.IOException ex) {
                    System.err.println(ex);
                }
                System.out.println("Thread Finished.");
            }
        };
        System.out.println("Starting");
        t.start(  );
        System.out.println("Joining");
        try {
            t.join(  );
        } catch (InterruptedException ex) {
            // should not happen:
            System.out.println("Who dares interrupt my sleep?");
        }
        System.out.println("Main Finished.");
    }
}

As you can see, it uses an inner class Runnable (see Section 24.2) in Thread t to be runnable.