Basics of J2ME

1
Introduction to programming in J2ME
2
Basics of J2ME

• Objectives
• Understand the AMS states and lifecycle
• Understand a simple Hello World program
• Appreciate different command objects
• Appreciate different command types
• Appreciate J2ME user-interface API class
  hierarchy

2
2
3
Mobile Information Device Profile (MIDP)

• MIDP device contains a program called the
  application management software (AMS) which
  downloads the MIDlet suite from the server, opens
  the MIDlet suite, then launches the
  user-specified MIDlet on the MIDP device
• LifeCycle of a midlet

constructor method called
paused state
startApp method called
pauseApp method called
active state
destroyApp method called
4
A simple program

• import javax.microedition.midlet.
• import javax.microedition.lcdui.
• public class HelloWorld extends MIDlet
• private Form form
• private Display display
• public HelloWorld()
• super()
• public void startApp()
• form new Form("A simple MIDlet")
• String msg "Hello, World"
• form.append(msg)
• display Display.getDisplay(this)
• display.setCurrent(form)

5
The same program

• import javax.microedition.midlet.
• import javax.microedition.lcdui.
• public class HelloWorld extends MIDlet
• private Form form
• private Display display
• public HelloWorld()
• form new Form("A simple MIDlet")
• String msg "Hello, World"
• form.append(msg)
• display Display.getDisplay(this)
• public void startApp()

6
Command Objects

• Soft-Buttons and keys
• Create a command object
• private Command cmHelp
• cmHelp new Command("Help", Command.HELP, 1)
• Add the command object to a form
• fmMain.addCommand(cmHelp)
• Add a listener to the form
• fmMain.setCommandListener(this)
• Perform an action
• public void commandAction(Command c,
  Displayable s)
• if (c cmHelp)
• display.setCurrent(tbHelp)

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Command types

• BACK
• CANCEL
• EXIT
• HELP
• ITEM
• A request to map the Command to an item on the
  screen. E.g. when using a List component, you can
  mimic the functionality of a context-sensitive
  menu by mapping Commands to the various entries
  In the list
• OK
• SCREEN
• For commands in which it is unlikely there will
  be a specific key mapping available. For example
  you might have Commands to initiate uploading and
  downloading of data. The labels Upload and
  Download will not have direct key mappings on
  the a device
• STOP

8

• import javax.microedition.midlet.
• import javax.microedition.lcdui.
• public class CommandExample extends MIDlet
  implements CommandListener
• private Display display // Reference to
  Display object for this MIDlet
• private Form form // Main Form
• private TextBox texthelp // Textbox to
  display help message
• private Command exit // Exit the MIDlet
• private Command help // Ask for Help
• private Command back // Go "back" to the
  main form
• public CommandExample()
• display Display.getDisplay(this)
• help new Command("Help", Command.HELP, 1)
• back new Command("Back", Command.BACK, 1)
• exit new Command("Exit", Command.EXIT, 1)

9

• import javax.microedition.midlet.
• import javax.microedition.lcdui.
• public class ManyCommands extends MIDlet
  implements CommandListener
• private Display display // Reference to
  Display object for this MIDlet
• private Form form // The main Form
• private TextBox feedback // Textbox to
  show different messages
• private Command exit // Exit the MIDlet
• private Command back // Go "back" to the
  main form
• private Command displayMessage1 //
  message1
• private Command displayMessage2 //
  message2
• public ManyCommands()
• display Display.getDisplay(this)
• exit new Command("Exit", Command.EXIT, 1)
• back new Command("Back", Command.BACK, 1)
• displayMessage1 new Command("Message1",
  Command.SCREEN, 2)

10
J2ME user-interface API class hierarchy
javax.microedition.lcdui.Displayable
(High Level)
(Low Level)
Canvas
Screen
Alert
Form
TextBox
List
Item
ChoiceGroup
DateField
ImageItem
StringItem
TextField
Guage