Applet

An applet is a program written in the Java programming language that can be included in an HTML page, much in the same way an image is included in a page. When you use a Java technology-enabled browser to view a page that contains an applet, the it’s code is transferred to your system and executed by the browser’s Java Virtual Machine (JVM).

Consider the SImple HelloWorld Application

import java.applet.*; //Here we need to import this package for creating an applet.

import java.awt.*; // this is needed for implementing AWT applicaion

// the start of an applet – HelloWorld will be the executable class
// Extends applet means that you will build the code on the standard Applet class
public class HelloWorld extends Applet
{

// The method that will be automatically called when the applet is started
public void init()
{
// It is required but does not need anything. you can also omitted this method.
}

// This method gets called when the applet is terminated
// That’s when the user goes to another page or exits the browser.
public void stop()
{
// no actions needed here now. this is also optional.
}

// The standard method that you have to use to paint things on screen
// This overrides the empty Applet method, you can’t called it “display” for example.

public void paint(Graphics g) //this method is needed to display the applet
{
//method to draw text on screen
// String first, then x and y coordinate.
g.drawString(“Hello World!”,20,20);

}

}

/*
Applet will paint special shapes and use colors and fonts
Only new methods are explained
*/

import java.awt.*;
import java.applet.*;

public class DrawExample extends Applet
{

// Specify variables that will be needed everywhere, anytime here
// The font variable
Font bigFont;

// The colors you will use
Color redColor;
Color weirdColor;
Color bgColor;

public void init()
{
// Here we will define the varibles further
// Will use Arial as type, 16 as size and bold as style
// Italic and Plain are also available
bigFont = new Font(“Arial”,Font.BOLD,16);

// Standard colors can be named like this
redColor = Color.red;

// lesser known colors can be made with R(ed)G(reen)B(lue).
weirdColor = new Color(60,60,122);

bgColor = Color.blue;

// this will set the backgroundcolor of the applet
setBackground(bgColor);

}

// now lets draw things on screen
public void paint(Graphics g)
{
// tell g to use your font
g.setFont(bigFont);
g.drawString(“Shapes and Colors”,80,20);

// Now we tell g to change the color
g.setColor(redColor);

// This will draw a rectangle (xco,yco,xwidth,height);
g.drawRect(100,100,100,100);

// This will fill a rectangle
g.fillRect(110,110,80,80);

// change colors again

g.setColor(weirdColor);

// a circle (int x, int y, int width, int height,int startAngle, int arcAngle);
// ovals are also possible this way.

g.fillArc(120,120,60,60,0,360);

g.setColor(Color.yellow);

// Draw a line (int x1, int y1, int x2, int y2)

g.drawLine(140,140,160,160);

// reset the color to the standard color for the next time the applets paints
// an applet is repainted when a part was’nt visible anymore
// happens most often because of browser minimizing or scrolling.

g.setColor(Color.black);

}

}

/*
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.
}
}
}

 

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