//
// Very simple program demonstrating the use of threads.
//
// Command-line argument is P (number of threads).
//
// Synchronization via class (static) method.
//

// ---- Class containing main program --------------------------------

public class ThreadsHello2 {

    public static void main(String[] args) {

	if (args.length < 1) {
	    System.err.println("Usage:  java ThreadsHello2 numThreads");
	    System.exit(1);
	}

	int P = Integer.parseInt(args[0]);

	Thread [] threads = new Thread[P];

	// Create threads.
	for (int i = 0; i < P; ++i)
	    threads[i] = new Thread(new Hello2(i));

	// Start them up.
	for (int i = 0; i < P; ++i)
	    threads[i].start();

	// Wait for them to finish.
	// ("join" is declared as possibly throwing an exception,
	//   so to keep the compiler happy we must deal with it.)
	try {
	    for (int i = 0; i < P; ++i)
		threads[i].join();
	} catch (InterruptedException e) {
	    // should never happen
	}

	System.out.println("That's all, folks!");
    }
}

// ---- Class to contain data and code for each thread ---------------

class Hello2 implements Runnable {

    private int myID;

    // Constructor.
    public Hello2(int myID) {
	this.myID = myID;
    }

    // Defines behavior of thread using this object --
    //   invoked by "start()" on thread.
    public void run() {
	printMsg(myID);
    }

    // Prints "hello" message.
    // "synchronized" keyword for class method means only one
    //   thread at a time can execute this method.
    private static synchronized void printMsg(int ID) {
	System.out.print("hello, world, ");
  	// pointless pause, included to make the effects of 
  	//   synchronization (or lack thereof) more obvious
	pause(1);
	System.out.println("from thread " + ID);
    }

    // Sleeps for n milliseconds.
    private static void pause(int n) {
	try {
	    Thread.sleep(n);
	}
	catch (InterruptedException e) { 
	     // should never happen
	}
   }
}