Herb Schildt s Java Prog ramming Cookbook in Java

Printer DataMatrix in Java Herb Schildt s Java Prog ramming Cookbook

// A simple implementation of a "reminder" class // An object of this class starts a daemon thread // that waits until the specified time It then // displays a message class Reminder implements Runnable { // Time and date to display reminder message Calendar reminderTime; // Message to display String message; // message to display // Use this constructor to display a message after // a specified number of seconds have elapsed // This value is then added to the current time // to compute the desired reminder time // // In practice, you might want to change the // delay to minutes rather than seconds, but // seconds make testing easier Reminder(String msg, int delay) { message = msg; // Get the current time and date reminderTime = CalendargetInstance(); // Add the delay to the time and date reminderTimeadd(CalendarSECOND, delay); Systemoutprintf("Reminder set for %tD %1$tr\n", reminderTime); // Create the reminder thread Thread dThrd = new Thread(this); // Set to daemon dThrdsetDaemon(true); // Start execution dThrdstart(); } // Notify at the specified time and date Reminder(String msg, Calendar cal) { message = msg; // Use the specified time and date as the // reminder time reminderTime = cal; Systemoutprintf("Reminder set for %tD %1$tr\n", reminderTime); // Create the reminder Thread dThrd = new Thread(this);

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// Set to daemon dThrdsetDaemon(true); // Start execution dThrdstart(); } // Run the reminder public void run() { try { for(;;) { // Get the current time and date Calendar curTime = CalendargetInstance(); // See if it's time for the reminder if(curTimecompareTo(reminderTime) >= 0) { Systemoutprintln("\n" + message + "\n"); break; // let the thread end } Threadsleep(1000); } } catch(InterruptedException exc) { Systemoutprintln("Reminder interrupted"); } } } class ReminderDemo { public static void main(String args[]) { // Get a reminder 2 seconds from now Reminder mt = new Reminder("Call Harry", 2); // Get a reminder on April 5, 2007 at 2:30 pm Reminder mt2 = new Reminder("Meet with Bill", new GregorianCalendar(2007, 3, 5, 14, 30)); // Keep the main thread alive for 20 seconds for(int i=0; i < 20; i++) { try { Threadsleep(1000); } catch(InterruptedException exc) { Systemoutprintln("Main thread interrupted"); } Systemoutprint(""); } Systemoutprintln("\nMain thread ending"); } }

The output is shown here:

Reminder set for 04/05/07 02:29:56 PM Reminder set for 04/05/07 02:30:00 PM Call Harry Meet with Bill Main thread ending

Here are a few things that you might want to try To prove the benefit of using a daemon thread to execute the reminder, try commenting out the call to setDaemon( ) in both of Reminder s constructors as shown here:

// dThrdsetDaemon(true);

Because the thread is no longer marked as daemon, it remains a user thread and the application will not end until the reminder thread has finished Thus, the application hangs until the future time is reached For ease of testing and experimentation, the delay period used by the first Reminder constructor is assumed to be seconds However, for real-world use, this delay is probably better specified in terms of minutes You might want to try this change One optimization that might be applicable in some cases is to simply have the run( ) method sleep for a period of time equal to the difference between the time at which the Reminder was started and the specified future time With this approach, there is no need to keep checking the time However, this approach will not work in situations in which the reminder is to be displayed based on the system time, which might change for example, if the user changes time zones or if daylight saving time comes into effect Finally, you can try having the reminder display a pop-up window that contains the message rather than using a console-based approach This is easily done using Swing s JFrame and JLabel classes JFrame (which is a top-level Swing container) creates the standard window and JLabel displays the message To try this, substitute the following version of run( ) in the preceding program:

// Display the reminder in a pop-up window public void run() { try { for(;;) { // Get the current time and date Calendar curTime = CalendargetInstance(); // See if it's time for the reminder if(curTimecompareTo(reminderTime) >= 0) { // Create GUI on the event-dispatching thread // as recommended by Sun

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