Chapter 4 - Advanced Topics
2. Using Threads
/*
Threads are needed when you want to perfom actions
over and over again.
We want to show the current time, and update that
every second.
*/
import java.awt.*;
import java.applet.*;
// We need Date to get the current time.
import java.util.Date;
// Implement Runnable, this will make it possible
for threads
// to call the run method when activated.
public class ThreadExample
extends Applet implements Runnable
{
// Define your thread.
Thread clockThread;
// This textfield will show the time.
TextField clockField;
// Date will give us the current hours, minutes and seconds
Date date;
// This variable will remain true for as long
// we want the thread to run.
boolean running = true;
public void init()
{
// a standard layout to place just one textfield
setLayout(new BorderLayout());
clockField = new TextField();
add(clockField,"Center");
// Create the thread.
clockThread= new Thread(this);
// and let it start running
clockThread.start();
}
// Very important. You do not want your thread to keep running when
// the applet is deactivated (eg. user left page)
public void destroy()
{
// will cause thread to stop looping
running = false;
// destroy it.
clockThread = null;
}
// The method that will
be called when you have a thread.
// You always need this
when you implement Runnable (use a thread)
public void run()
{
// loop until told to stop
while (running)
{
// Construct the current date.
date = new Date();
// Get the hours, minutes and hours
String time = date.getHours()+":"+date.getMinutes()+":"+date.getSeconds();
// Put that result in the textfield
clockField.setText(time);
//Now the reason for threads
try
{
// Wait 500milliseconds before continuing
clockThread.sleep(500);
}
catch (InterruptedException e)
{
System.out.println(e);
}
// he has wait and will now restart his actions.
}
}
}
/*
Be cautious when using threads, make sure to
end them sometime and not to let them run too
fast. When threads are used to time on-screen
drawing, things could slow down
heavily.
*/
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