/*
 * "hello world" program with synchronization
 *
 * threads perform I/O with delays, introducing need to ensure one-at-a-time
 *   access to System.out.
 *
 * command-line argument specifies number of threads.
 */
public class HelloSynch {

    private static Object lockObj = new Object();

    /* print message, pausing in the middle -- totally artificial, just
     * something to show a need for one-at-a-time access to System.out 
     */
    public static void helloWithDelay() {
        /* "synchronized" here ensures one-at-a-time execution */
        /* comment it out to see what happens without it */
        synchronized (lockObj) {
            System.out.print("hello again from thread ");
            try {
                Thread.currentThread().sleep(10);
            }
            catch (InterruptedException e) {
                System.err.println("this should not happen");
            }
            System.out.println(Thread.currentThread().getName());
        }
     }

    /* main method */
    public static void main(String[] args) {

        /* process command-line argument */

        if (args.length <= 0) {
            System.err.println("usage:  HelloSynch numThreads");
            System.exit(1);
        }
        int numThreads = 0;
        try {
            numThreads = Integer.parseInt(args[0]);
        }
        catch (NumberFormatException e) {
            System.err.println("usage:  HelloSynch numThreads");
            System.exit(1);
        }

        /* create threads */

        Thread[] threads = new Thread[numThreads];

        for (int i = 0; i < numThreads; ++i) {
            threads[i] = new Thread(new Runnable() {
                public void run() {
                    /* "synchronized" here ensures one-at-a-time execution */
                    /* comment it out to see what happens without it */
                    synchronized(lockObj) {
                        System.out.println("hello, world, from thread " + 
                            Thread.currentThread().getName());
                    }
                    helloWithDelay();
                }
            });
        }

        /* start them up */

        System.out.println("starting threads");

        for (int i = 0; i < numThreads; ++i) {
            threads[i].start();
        }

        /* wait for them to finish */

        for (int i = 0; i < numThreads; ++i) {
            try {
                threads[i].join();
            }
            catch (InterruptedException e) {
                System.err.println("this should not happen");
            }
        }

        System.out.println("threads all done");

    }
}