/*
 * "hello world" program, version 2
 *
 * creates threads using static inner class.  this allows creating each
 *   thread with thread-specific information (myID here).
 *
 * number of threads is taken from environment variable NUM_THREADS.
 */
public class Hello2a {
    
    private static final String NUM_THREADS_VAR = "NUM_THREADS";

    public static void main(String[] args) {

        /* look up environment variable */

        String numThreadsEnvVar = System.getenv(NUM_THREADS_VAR);

        if (numThreadsEnvVar == null) {
            System.err.println("environment variable NUM_THREADS must be set");
            System.exit(1);
        }

        int numThreads = 0;
        try {
            numThreads = Integer.parseInt(numThreadsEnvVar);
        }
        catch (NumberFormatException e) {
            System.err.println("invalid value for NUM_THREADS");
            System.exit(1);
        }

        /* create threads */

        Thread[] threads = new Thread[numThreads];

        for (int i = 0; i < numThreads; ++i) {
            threads[i] = new Thread(new Inner(i));
        }

        /* 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");
    }

    /* inner class containing code for each thread to execute */

    private static class Inner implements Runnable {

        private int myID;

        public Inner(int myID) {
            this.myID = myID;
        }

        public void run() {
            System.out.println("hello from " + myID);
        }

    }
}