Categories: Java ME | MIDP 2.0 | Code Examples | UI

This page was last modified 10:16, 12 May 2008.

From Forum Nokia Wiki

Overview

This code snippet demonstrates how to navigate between three different Screen instances. Screen is the superclass of all high-level user interface classes, such as Form and List.

In practice, the navigation is done by setting the current Displayable to the specific Screen:

private void switchCurrentScreen(Displayable displayable) {
    Display.getDisplay(this).setCurrent(displayable);
}

Source file

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.lcdui.StringItem;
import javax.microedition.midlet.MIDlet;

public class NavigationMIDlet extends MIDlet implements CommandListener {
    private Form firstScreen;
    private Form secondScreen;
    private Form thirdScreen;
 
    // Navigation commands
    private static final Command NEXT_COMMAND =
        new Command("Next", Command.SCREEN, 1);
    private static final Command PREV_COMMAND =
        new Command("Previous", Command.BACK, 1);
    private static final Command EXIT_COMMAND =
        new Command("Exit", Command.EXIT, 1);;
 
    /**
     * Constructor. Constructs the object and initializes displayables.
     */
    public NavigationMIDlet() {
        // Initialize the screens
        initFirstScreen();
        initSecondScreen();
        initThirdScreen();
    }
 
    private void initFirstScreen() {
        firstScreen = new Form("First example screen");
        StringItem item = new StringItem("Description",
            "This is the first example screen.");
        firstScreen.append(item);
        firstScreen.addCommand(NEXT_COMMAND);
        firstScreen.addCommand(EXIT_COMMAND);
        firstScreen.setCommandListener(this);
    }
    
    private void initSecondScreen() {
        secondScreen = new Form("Second example screen");
        StringItem item = new StringItem("Description",
            "This is the second example screen.");
        secondScreen.append(item);
        secondScreen.addCommand(NEXT_COMMAND);
        secondScreen.addCommand(PREV_COMMAND);
        secondScreen.setCommandListener(this);
    }
    
    private void initThirdScreen() {
        thirdScreen = new Form("Third example screen");
        StringItem item = new StringItem("Description",
            "This is the third example screen.");
        thirdScreen.append(item);
        thirdScreen.addCommand(PREV_COMMAND);
        thirdScreen.setCommandListener(this);
    }
    
    private void switchCurrentScreen(Displayable displayable) {
        Display.getDisplay(this).setCurrent(displayable);
    }
    
    /**
     * From MIDlet.
     * Called when the MIDlet is started.
     */
    public void startApp() {
        // The initial display is the first form
        switchCurrentScreen(firstScreen);
    }
 
    // Other inherited methods omitted for brevity
    // ...
    
    /**
     * From CommandListener.
     * Called by the system to indicate that a command has been invoked on a
     * particular displayable.
     * @param command the command that was invoked
     * @param displayable the displayable where the command was invoked
     */
    public void commandAction(Command command, Displayable displayable) {
        if (command == EXIT_COMMAND) {
            // Exit the MIDlet
            destroyApp(true);
            notifyDestroyed();
        } else if (command == NEXT_COMMAND) {
            if (displayable == firstScreen) {
                switchCurrentScreen(secondScreen);
            } else if (displayable == secondScreen) {
                switchCurrentScreen(thirdScreen);
            }
        } else if (command == PREV_COMMAND) {
            if (displayable == secondScreen) {
                switchCurrentScreen(firstScreen);
            } else if (displayable == thirdScreen) {
                switchCurrentScreen(secondScreen);
            }
        }
    }
}

It is also possible to display an Alert before switching the screen. In addition, the switching can be done only after the user has dismissed the alert. AlertType class defines the type of the alert and whether user interaction is required before the screen is switched (AlertType.CONFIRMATION). To implement showing an informative alert (AlertType.INFO; doesn't require user interaction) before switching the screen, add the following code to the example above:

import javax.microedition.lcdui.Alert;
import javax.microedition.lcdui.AlertType;

private void switchCurrentScreen(Alert alert, Displayable displayable) {
    Display.getDisplay(this).setCurrent(alert, displayable);
}

Alert alert = new Alert("Screen changing",
    "Switching current screen in a moment...", null, AlertType.INFO);
switchCurrentScreen(alert, firstScreen);

Postconditions

The MIDlet demonstrates navigation between three different screens.