Getting Started with J2ME - Creating the “Hello, World” Application
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This section shows the “Hello, World” application. The MIDlet will display the message Hello World! on the screen and remove it (or later put it back) when you click the Toggle Msg button. Clicking the Exit button will terminate the MIDlet. The application consists of two classes: the MIDlet subclass called Hello and the Canvas subclass called HelloCanvas. How it works is sufficiently simple that I’ll leave the explanations of the various steps in the comments. Chapter 2 includes in-depth discussion of how a MIDlet works. Listing 1-2 shows the code for Hello.java.
Listing 1-2.Hello.java
package net.frog_parrot.hello;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
/**
* This is the main class of the "Hello, World" demo.
*
* @author Carol Hamer
*/
public class Hello extends MIDlet implements CommandListener {
/**
* The canvas is the region of the screen that has been allotted
* to the game.
*/
HelloCanvas myCanvas;
/**
* The Command objects appear as buttons in this example.
*/
private Command exitCommand = new Command("Exit", Command.EXIT, 99);
/**
* The Command objects appear as buttons in this example.
*/
private Command toggleCommand = new Command("Toggle Msg", Command.SCREEN, 1);
/**
* Initialize the canvas and the commands.
*/
public Hello() {
myCanvas = new HelloCanvas();
myCanvas.addCommand(exitCommand);
myCanvas.addCommand(toggleCommand);
// you set one command listener to listen to all
// of the commands on the canvas:
myCanvas.setCommandListener(this);
}
//---------------------------------------------------------// implementation of MIDlet
/**
* Start the application.
*/
public void startApp() throws MIDletStateChangeException {
// display my canvas on the screen:
Display.getDisplay(this).setCurrent(myCanvas);
myCanvas.repaint();
}
/**
* If the MIDlet was using resources, it should release
* them in this method.
*/
public void destroyApp(boolean unconditional)
throws MIDletStateChangeException {
}
/**
* This method is called to notify the MIDlet to enter a paused
* state. The MIDlet should use this opportunity to release
* shared resources.
*/
public void pauseApp() {
}
//--------------------------------------------------------- // implementation of CommandListener
/*
* Respond to acommand issued on the Canvas.
* (either reset or exit).
*/
public void commandAction(Command c, Displayable s) {
if(c == toggleCommand) {
myCanvas.toggleHello();
} else if(c == exitCommand) {
try {
destroyApp(false);
notifyDestroyed();
} catch (MIDletStateChangeException ex){
}
}
}
}
Listing 1-3 shows the code for HelloCanvas.java.
Listing 1-3.HelloCanvas.java
package net.frog_parrot.hello;
import javax.microedition.lcdui.*;
/**
* This class represents the region of the screen that has been allotted
* to the game.
*
* @author Carol Hamer
*/
public class HelloCanvas extends Canvas {
//------------------------------------------------------- // fields
/**
* whether the screen should currently display the
* "Hello World" message.
*/
boolean mySayHello = true;
//---------------------------------------------------------
// initialization and game state changes
/**
* toggle the hello message.
*/
void toggleHello() {
mySayHello = !mySayHello;
repaint();
}
//---------------------------------------------------------
// graphics methods
/**
* clear the screen and display the "Hello World" message if appropriate.
*/
public void paint(Graphics g) {
// get the dimensions of the screen:
int width = getWidth ();
int height = getHeight();
// clear the screen (paint it white):
g.setColor(0xffffff);
// The first two args give the coordinates of the top
// left corner of the rectangle. (0,0) corresponds
// to the top-left corner of the screen.
g.fillRect(0, 0, width, height);
// display the "Hello World" message if appropriate:.
if(mySayHello) {
Font font = g.getFont();
int fontHeight = font.getHeight();
int fontWidth = font.stringWidth("Hello World!");
// set the text color to red:
g.setColor(255, 0, 0);
g.setFont(font);
// write the string in the center of the screen
g.drawString("Hello World!", (width - fontWidth)/2,
(height - fontHeight)/2,
g.TOP|g.LEFT);
}
}
}
The “Hello, World” application is simple enough to run with MIDP 1.0 as well as with MIDP 2.0. Figure 1-2 shows what the “Hello, World” application looks like when running on the Wireless Toolkit 1.0.4’s DefaultGrayPhone emulator.
Figure 1-2. The “Hello, World” application
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 This article is excerpted from J2ME Games with MIDP2 by Carol Hamer (Apress, 2004; ISBN 1590593820). Check it out at your favorite bookstore today. Buy this book now.
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