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Java 2 Platform, Micro Edition
(J2ME)
Masha Gutkhen
Keren Gazit
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Contents
What’s J2ME?
 J2ME core concepts



Configurations (CLDC, CDC)
Profiles (MIDP)

MIDlet, MIDlet Suite

Basic classes API

Installation and using the Toolkit

Demos!!!
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Introduction


Personalized and intelligent information appliances
are necessities in our life today.
Such appliances can be:






cell phones
two-way pagers
smart cards
personal organizers
palmtops
These appliances tend to be special-purpose,
limited-resource, network-connected devices.
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Environment requirements

We need an environment which is adapted for
constrained devices - devices that have
limitations on what they can do when
compared to standard desktop or server
computers.
 The constraints are:




extremely limited memory
small screen sizes
alternative input methods
slow processors
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So – what’s the solution?

Because of the vast
need, Sun has
decided to develop a
special edition of
Java - J2ME (Java
2 Micro Edition).
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Java Editions
Different devices have different
requirements and different expectations
of Java.
 One platform (solution) cannot address
all the market segments (web server,
video games etc.)
 Users/developers want flexibility. They
want to choose what they want to use
and what they do not.

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Java Editions
The Java 2 Platform is split into three
editions.
 Each edition provides a complete
environment for running Java-based
applications, including the Java virtual
machine (VM) and runtime classes.
 The three editions target different kinds
of applications running on different kinds
of devices.

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Java Editions
Java 2 Platform
Java2
Standard Edition
(J2SE™)
Java2
Enterprise Edition
(J2EE™)
Java2
Micro Edition
(J2ME™)
Standard desktop &
workstation applications
Heavy duty server
systems
Small & memory
constrained devices
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Java Editions

Each edition defines different sets of class
libraries.
 There are thousands of core J2SE runtime
classes, taking up to 10-20 megabytes
of space.
J2EE
 J2ME-based devices have
J2SE
fewer classes.
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PersonalJava and EmbeddedJava
J2ME is not the first attempt at adapting
Java for constrained environments.
 PersonalJava

Uses the basic Java 1.1 runtime classes
with a few features from Java 2.
 Implementation still requires a couple of
megabytes of memory and a fast processor
to run.

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PersonalJava and EmbeddedJava

EmbeddedJava
Makes every behavior of the JVM and the
runtime classes optional - the implementor
can choose exactly which classes and
methods are required.
 The limitation: "write once, run anywhere".

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J2ME Core Concepts

Configuration


Minimum platform
required for a
group of devices
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Libraries
Profile


J2ME
Profile
Addresses specific
needs of a certain
device family
Java Language
Java Virtual Machine
Optional Packages
Host Operating System
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J2ME Core Concepts
J2ME is based on 3 core concepts:
 Configurations
 Profiles
 Optional packages
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Configurations

A configuration is a complete Java
runtime environment, consisting of:
Java virtual machine (VM) to execute Java
bytecode
 Native code to interface to the underlying
system
 Set of core Java runtime classes


To use a configuration, a device must
meet certain minimum requirements.
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Configurations
The set of core classes is normally quite
small and must be enhanced with
additional classes supplied by J2ME
profiles or by configuration implementor.
 Configurations do not define any user
interface classes.

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Configurations
Configuration
CLDC
CDC
Connected Limited
Device Configuration
Connected Device
Configuration
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CLDC vs. CDC

CLDC
 For very constrained
devices
 160 - 512 KB of total
memory
 16-bit or 32-bit
processor
 Low power
consumption and
often operating with
battery power
 Connectivity with
limited bandwidth

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CDC
 2 MB or more
memory for Java
platform
 32-bit processor
 High bandwidth
network connection,
most often using
TCP/IP
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CLDC vs. CDC - VM

Features missing in
the CLDC VM:







Floating point types
Object finalization
JNI or reflection
Thread groups or
daemon threads
User Class loaders
The CDC supports a
complete, fullfeatured Java 2
virtual machine
Change in classfile
verification  preverification
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The KVM and CVM

KVM - Java virtual machines for the CLDC
 CVM - Java virtual machines for the CDC
 Written specifically to work in the constrained
environment of a handheld or embedded
device and to be easily ported to different
platforms.
 CLDC and CDC specifications do not require
the use of the KVM or the CVM.
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CLDC vs. CDC – J2SE Subset

The CLDC includes
classes from:




java.lang
java.io
java.util

The CDC includes
17 packages
 Includes more
classes even in the
shared packages
Only selected
classes from each
package are
included
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CLDC vs. CDC – J2SE Subset
J2SE
CDC
J2ME
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Handling I/O
J2SE includes many classes for
performing input and output.
 There are a large number of I/O classes
and they tend to encapsulate I/O models
that are not necessarily found on all
devices.
 For example, some handheld devices do
not have file systems. Socket support is
not universal, either.

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Handling I/O in CLDC

The CLDC has define a new set of APIs for I/O
called the Generic Connection Framework.
 The GCF, part of the new javax.microedition.io
package, defines interfaces for the different
kinds of I/O that are possible.
 The CLDC does not actually define any I/O
implementations these are left to the profiles
and/or the device vendor to define.
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GCF - example
import java.io.*;
import javax.microedition.io.*;
StreamConnection conn = null;
InputStream is = null;
String url = "socket://somewhere.com:8909";
try {
conn = (StreamConnection) Connector.open( url );
is = conn.openInputStream();
.... // etc. etc.
}…
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Handling I/O in CDC
Since the CDC is a superset of the
CLDC, it includes the GCF.
 CDC also requires GCF support for two
specific connection types: files and
datagrams.
 The reason: CDC includes the relevant
classes from java.io and java.net
packages.

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J2ME Core Concepts
J2ME is based on 3 core concepts:
 Configurations
 Profiles
 Optional packages
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Profiles

Adds domain-specific classes to a
configuration:


To fill in missing functionality
To support specific uses of a device

Most profiles define user interface classes for
building interactive applications.
 To use a profile, the device must meet the
minimum requirements of the underlying
configuration and of the profile.
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Profiles
Profile
MIDP
Mobile
Information
Device
Profile
PDAP
Personal
Digital
Assistant
Profile
FP
Foundation
Profile
J2ME
PBP
PP
Personal
Basis Profile
Personal
Profile
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MIDP – MID Profile
MIDP is targeted at a class of devices
known as mobile information devices
(MIDs).
 Minimal characteristics of MIDs:

Enough memory to run MIDP applications
 Display of at least 96 X 56 pixels, either
monochrome or color
 A keypad, keyboard, or touch screen
 Two-way wireless networking capability

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MIDP - Specification
 There
are two versions of the MIDP:
 MIDP
1.0 - released in September
2000. Many devices currently on the
market support it.
 MIDP 2.0 - currently in proposed final
draft form. No devices yet support it.
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MIDP - Specification

The MIDP adds APIs to the basic APIs defined
by the CLDC. The new features include:




Support for application lifecycle management
similar to the way applets are defined in J2SE.
Persistent storage of data.
HTTP-based network connectivity based on the
CLDC's GCF.
Simple user interface support, with enough
flexibility to build games or business applications.
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MIDP - Specification

The MIDP specification is silent about a
number of things:
No standard way to interface to the device's
phonebook, in order to initiate voice calls.
 How MIDP applications are loaded onto a
device and how they are activated or
deactivated.

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MIDP Applications restrictions

Memory is a particularly scarce resource.

The early Motorola J2ME-enabled phones limited
the size of an application to 50K. Some Nokia
phones limit them to even less, about 30K.

MIDP 1.0 applications cannot share classes.
 Placing part of the application in a web or
application server (as a servlet, typically) that
the MIDP application calls is almost a
requirement for anything serious.
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J2ME Core Concepts
J2ME is based on 3 core concepts:
 Configurations
 Profiles
 Optional packages
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Optional Packages
Set of APIs in support of additional,
common behaviors.
 Have specific dependencies on a
particular configuration and/or one or
more profiles.
 Examples of optional packages :

RMI Optional Package
 Bluetooth Optional Package
 JDBC Optional Package

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What it all means

"J2ME application" is an ambiguous term.
 Configuration, profile and optional packages
should be chosen.
 CDC-based profiles make development
simpler due to J2SE-like APIs, but don’t suit
the low-end devices.
 CLDC-based profiles makes the development
task harder, especially when trying to shrink
the size of the application to run on many of
the small devices.
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MIDlets – The heart of J2ME…
MIDP does not support the running of
applications that use a static main
method as their entry point, nor calling
the System.exit method in order to
terminate.
 Instead, we use a MIDlet, which is a MID
Profile application.

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MIDlets – The heart of J2ME…

Every application must extend
javax.microedition.midlet.MIDlet class
to allow the application management
software to:
control the MIDlet
 be able to retrieve properties from the
application descriptor
 notify and request state changes

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MIDlets – The heart of J2ME…
The extending class is the main class of
the application.
 The MIDlet class defines abstract
methods that the main class implements
(for example: startApp(), destroyApp(),
notifyDestroyed()).

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MIDlet Suite

One or more MIDlets are packaged
together into a MIDlet suite, composed
of:
JAR (Java archive) file
 JAD (Java Application Descriptor) file


All the user-defined classes and
resources required by the suite's MIDlets
must be in the JAR file.
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MIDlet Suite
The JAR file must also include a
manifest that describe the MIDlets in the
suite.
 The application descriptor (JAD)
contains similar information, and is used
by devices to obtain information about a
MIDlet suite without having to download
and install the MIDlet suite first.

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Creating a MIDlet
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Creating a MIDlet
Importing MIDP
specific
packages
import javax.microedition.lcdui.*;
import javax.microedition.midlet.*;
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Creating a MIDlet
Extends MIDlet
Implements
CommandListener
public class HelloMIDlet
extends MIDlet
implements CommandListener {
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Creating a MIDlet
Creating the
form, adding
the
Commands
public HelloMIDlet() {
mMainForm = new Form("HelloMIDlet");
mMainForm.append(new StringItem(null,
"Hello, Seminar Software Design!"));
mMainForm.addCommand(new Command("Exit",
Command.EXIT, 0));
mMainForm.setCommandListener(this);
}
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Sample MIDP classes API
MIDlet
 Form
 Command

CommandListener
 ItemCommandListener


Item
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Sample classes API
 MIDlet
– Base class
Form
 Command

CommandListener
 ItemCommandListener


Item
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MIDlet class API



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protected abstract void startApp()
protected abstract void pauseApp()
protected abstract void
destroyApp(boolean unconditional)
public final String
getAppProperty(String key)
public final void notifyDestroyed()
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Sample classes API

MIDlet
 Form

Command
CommandListener
 ItemCommandListener


Item
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Form class

A Form is a Screen that contains an arbitrary
mixture of items: images, read-only text fields,
editable text fields, editable date fields,
gauges, choice groups, and custom items.
 In general, any subclass of the Item class may
be contained within a form.
 The implementation handles layout, traversal,
and scrolling.
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Form class API
 Item
management
 public
int append(Item item)
 public Item get(int itemNum)
 Layout

public void
setItemStateListener(ItemState
Listener iListener)
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Sample classes API
MIDlet
 Form

 Command
 CommandListener
 ItemCommadListener

Item
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Command class

The Command class represents the semantic
meaning of an action. Command objects are
presented in the user interface.
 The action itself is implemented in a
CommandListener object.
 The CommandListener is associated with a
Displayable or an Item.
 Once the Command is called – the
CommandListener is invoked, and the action is
performed.
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Command class

Command label
 public String getLabel()
Command type
 public int getCommandType()
 Command priority
 public int getPriority()

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CommandListener class API
This object is a listener for Commands
that are attached to a Displayable.
 public void
commandAction(Command c,
Displayable d)

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ItemCommandListener class API
When a command (attached to an Item)
is invoked, the application is notified by
having the commandAction() method
called on the ItemCommandListener that
had been set on the Item.
 public void
commandAction(Command c,
Item item)

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Sample classes API
MIDlet
 Form
 Command

CommandListener
 ItemCommandListener

 Item
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Item class
A superclass for components that can be
added to a Form.
 All Item objects have a label field
 Choose the Item’s layout, size, and
appearance
 Attach Commands

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Item class API
public void
setDefaultCommand(Command cmd)
 public void
setItemCommandListener(ItemCom
mandListener listener)
 public void
notifyStateChanged()
 public int getPreferredWidth()

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Getting Started…
 1st
step: Download sun’s J2ME
Wireless Toolkit from:
http://java.sun.com/products/j2mewtoolkit/d
ownload-2_1.html
 2nd
step: Make sure you have J2SE
SDK installed
 3rd step: Install the J2ME Toolkit.
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After the installation….

Shortcuts are available from the start menu.

New directories
created
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Using KToolbar
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Creating a new Application
Press “New Project”.
 Enter the project’s name and the MIDlet’s
class name.
 New directories will automatically be created.

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Where to place your files?
J2ME/apps/{proj}
J2ME/apps/{proj}/res
source, resource, and binary
files
JAR, JAD, unpacked manifest
files.
external class libraries, (JAR or
ZIP) for a specific project
resource files
J2ME/apps/{proj}/src
source files
J2ME/apps/lib
external class libraries, (JAR or
ZIP) for all KToolbar projects.
J2ME/apps/{proj}/bin
J2ME/apps/{proj}/lib
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And then what?





Choose the target platform
JTWI
- conforms
to Java Technology for the Wireless
Write
your
code
Industry (JSR-185).
Save
MIDP1.0 - includes MIDP 1.0 and CLDC 1.0
Custom - user defined settings, you can select project
Build
profile, configurations and various APIs.
(Compile + Preverify)
Run
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Distribution to actual devices
Create a package
 Place your code somewhere on the net.
 Update .jad file
 Download the application to your mobile
 Start playing… 

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Some other issues
OTA provisioning
 Using Servlets
 Working with EclipseME
 Web services

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The END!

You can download all the demos
JARs from:
www.cs.huji.ac.il/~kerengaz/j2me/

Have Fun!!!
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