4.3.1  Hello，World

经过那个漫长的（但确实是有趣的）介绍，我们最后终于可以真正开始编写代码了。

首先在j2me home中建立一个projects目录来保存自己的项目。在projects目录中建立一个hello的子目录。如果愿意也可以采用别的名字，本书的其他例子也是这样。

像大多数开发语言那样，我们以Hello，world的MIDlet为例子。第一个MIDlet的代码就是这样的。创建一个名叫hello.java的文件来保存代码，也可以在光盘的source code for Chapter 4中找到这些代码。

import javax.microedition.midlet.MIDletStateChangeException;

import javax.microedition.lcdui.*;

/**

  * A simple example that demonstrates the basic structure of a MIDlet

  * by creating a Form object containing the string "Hello, Micro World!"

  *

  * @author Martin J. Wells

  */

public class Hello extends javax.microedition.midlet.MIDlet

       implements CommandListener

{

   protected Form form;

   protected Command quit;

   /**

    * Constructor for the MIDlet which instantiates the Form object and

    * then adds a text message. It then sets up a command listener so it

    * will get called back when the user hits the quit command. Note that this

    * Form is not activated (displayed) until the startApp method is called.

    */

   public Hello()

   {

      // create a form and add our components

      form = new Form("My Midlet");

      form.append("Hello, Micro World!");

      // create a way to quit

      form.setCommandListener(this);

      quit = new Command("Quit", Command.SCREEN, 1);

      form.addCommand(quit);

   }

   /**

    * Called by the Application Manager when the MIDlet is starting or resuming

    * after being paused. In this example it acquires the current Display object

    * and uses it to set the Form object created in the MIDlet constructor as

    * the active Screen to display.

    * @throws MIDletStateChangeException

    */

   protected void startApp() throws MIDletStateChangeException

   {

      // display our form

      Display.getDisplay(this).setCurrent(form);

   }

   /**

    * Called by the MID's Application Manager to pause the MIDlet. A good

    * example of this is when the user receives an incoming phone call while

    * playing your game. When they're done the Application Manager will call

    * startApp to resume. For this example we don't need to do anything.

    */

   protected void pauseApp()

   {

   }

   /**

    * Called by the MID's Application Manager when the MIDlet is about to

    * be destroyed (removed from memory). You should take this as an opportunity

    * to clear up any resources and save the game. For this example we don't need

    * to do anything.

    * @param unconditional if false you have the option of throwing a

    * MIDletStateChangeException to abort the destruction process.

    * @throws MIDletStateChangeException

    */

   protected void destroyApp(boolean unconditional)

            throws MIDletStateChangeException

   {

   }

   /**

    * The CommandListener interface method called when the user executes

    * a Command, in this case it can only be the quit command we created in the

    * constructor and added to the Form.

    * @param command

    * @param displayable

    */

   public void commandAction(Command command, Displayable displayable)

   {

      // check for our quit command and act accordingly

      try

      {

         if (command == quit)

         {

            destroyApp(true);

            // tell the Application Manager we're exiting

            notifyDestroyed();

         }

      }

      // we catch this even though there's no chance it will be thrown

      // since we called destroyApp with unconditional set to true.

      catch (MIDletStateChangeException me)

      { }

   }

}

不要过于关注这个例子中的太多细节，因为这个例子并不是最精简的。其中第一个MIDlet包含较少的组件，后文将仔细研究这些组件。