Implementation

For information about the design and functionality of the MIDlet, see section Design.

To create the MIDlet:

1. Create the UIDReader.java class file.

2. Import the required classes and packages.

   // Classes for contactless communication
   import javax.microedition.contactless.ContactlessException;
   import javax.microedition.contactless.DiscoveryManager;
   import javax.microedition.contactless.TargetListener;
   import javax.microedition.contactless.TargetProperties;
   import javax.microedition.contactless.TargetType;
   
   // Classes for UI
   import javax.microedition.lcdui.Alert;
   import javax.microedition.lcdui.AlertType;
   import javax.microedition.lcdui.Command;
   import javax.microedition.lcdui.CommandListener;
   import javax.microedition.lcdui.Display;
   import javax.microedition.lcdui.Displayable;
   import javax.microedition.lcdui.Form;
   
   import javax.microedition.midlet.*;

3. Set UIDReader to extend MIDlet and to implement TargetListener and CommandListener. Create the required variables.

   public class UIDReader
           extends MIDlet
           implements TargetListener, CommandListener {
   
       // Global variables for the UI
       private Command exitCommand;
       private Form form;

4. Create the UIDReader class constructor.

       public UIDReader() {

5. Set up the MIDlet UI.

           // Create the UI
           exitCommand = new Command("Exit", Command.EXIT, 1);
           form = new Form("UIDReader");
   
           form.addCommand(exitCommand);
           form.setCommandListener(this);
           form.append("Touch tag to read ID");

6. Register the MIDlet to receive notifications when NDEF targets are detected.

           // Determine the contactless target types supported by the device
           TargetType[] supportedTargetTypes = DiscoveryManager.getSupportedTargetTypes();
   
           // Check if the NDEF_TAG type is supported by the device,
           // and, if it is, register TargetListener for it
           for (int i=0; i<supportedTargetTypes.length; i++) {
               if (supportedTargetTypes[i].equals(TargetType.NDEF_TAG)) {
                   // Register TargetListener (this) for NDEF targets
                   try {
                       DiscoveryManager dm = DiscoveryManager.getInstance();
                       dm.addTargetListener(this, TargetType.NDEF_TAG);
                   } catch (SecurityException se) {
                       displayAlert("Unable to register TargetListener: " + se.toString(), AlertType.ERROR);
                   } catch (ContactlessException ce) {
                       displayAlert("Unable to register TargetListener: " + ce.toString(), AlertType.ERROR);
                   } catch (IllegalStateException ise) {
                       displayAlert("Unable to register TargetListener: " + ise.toString(), AlertType.ERROR);
                   } catch (NullPointerException npe) {
                       displayAlert("Unable to register TargetListener: " + npe.toString(), AlertType.ERROR);
                   }
                   // Break the loop, since a matching target type has been found
                   break;
               }
           }
   
       }

7. Create the mandatory MIDlet lifecycle methods. The MIDlet uses the startApp method to display the UI.

       public void startApp() {
           Display.getDisplay(this).setCurrent(form);
       }
   
       public void pauseApp() {
       }
   
       public void destroyApp(boolean unconditional) {
       }

8. The targetDetected method of the TargetListener interface is called every time the device detects one or more NDEF targets. The MIDlet uses the targetProperties array returned by the method to retrieve the UID of the first found target.

       /**
        * Implements the callback function of TargetListener
        * @param targetProperties: Array of targets found by the device
        */
       public void targetDetected(TargetProperties[] targetProperties) {
   
           // In case no targets are found, exit the method
           if (targetProperties.length == 0) {
               return;
           }
   
           // Read and display the UID of the first target found
           TargetProperties tmp = targetProperties[0];
           displayAlert("UID read: " + tmp.getUid(), AlertType.INFO);
       }

9. Implement command and message handling for the MIDlet UI. The MIDlet uses the commandAction method to handle the exit command, which involves unregistering TargetListener and closing the MIDlet. The MIDlet uses the displayAlert method to display error and other messages through the MIDlet UI.

       /**
        * Implements the callback function of CommandListener
        * @param command: Command given
        * @param displayable: Associated Displayable object
        */
       public void commandAction(Command command, Displayable displayable) {
           if (command == exitCommand) {
               DiscoveryManager dm = DiscoveryManager.getInstance();
               dm.removeTargetListener(this, TargetType.NDEF_TAG);
               destroyApp(false);
               notifyDestroyed();
           }
       }
   
       private void displayAlert(String error, AlertType type) {
           Alert err = new Alert(form.getTitle(), error, null, type);
           Display.getDisplay(this).setCurrent(err, form);
       }
   }