high level display screens in j2me supplier

CIOs and IT managers are always struggling to improve the availability of corporate data to those who need it to run the business. In the case of salespeople and mobile executives, that"s a tall order. Ideally, you would use the very gear that these people already carry — such as PDAs or cell phones — to act as messaging systems and data terminals that can fetch customer information or place an order.

Unfortunately, the vast differences among these devices — Pocket PCs running Windows CE, PDAs running Palm OS or Linux, cell phones running the Symbian OS — pose significant problems for developers. Even the cell phones from a single vendor such as Motorola (the company I work for) can vary widely in processor type, memory amount, and LCD screen dimensions. Worse, new handsets sporting new features, such as built-in cameras and Bluetooth networking, are released every six months to nine months.

For IT managers whose chief concern is that applications running on device A today also run on device B tomorrow, the best choice among development platforms is J2ME, a slimmed-down version of Java tailored for use on embedded and mobile devices. Most handset vendors implement their own Java VM, and third-party VMs provide Java support in Palm and Pocket PC devices. For a broad range of devices, past, present, and future, J2ME provides a high degree of security and application portability — but not without drawbacks.

As were all editions of Java technology, J2ME was built to provide controlled access to program resources. The run-time architecture uses a “sandbox” mechanism that contains the executing program in a separate, protected memory space. This prevents code malfunctions from damaging other critical components of the run-time environment. Also, the program doesn’t have unfettered access to any low-level hardware or resources outside of the sandbox. Instead, all device operations are conducted via J2ME API calls.

There are good reasons to isolate a program from many of the phone’s functions. Unlike a desktop PC in which a program crash might clobber a word processing session, a cell phone is essentially a two-way radio. The Federal Communications Commission frowns on the improper use of radio transmitters regardless of the culprit — a cell phone’s owner or a malfunctioning program. And you wouldn’t want a Trojan program “phoning home” and transmitting personal information to a cracker. Because over-the-air provisioning allows users to download new applications, this makes the Java security sandbox more important than ever.

However, this protection scheme has its downside. Stated simply, if an API isn’t defined for a specific hardware feature, a developer can’t use that feature. For example, the Sony Ericsson T610 handset comes with an army of features: Bluetooth networking, infrared and serial connectivity, and a built-in digital camera. However, the current APIs in the T610’s J2ME implementation don’t utilize these features. Instead, you must use C++ and the Symbian OS (which lies under the J2ME abstraction layers) to access them.

Most vendors sidestep this issue by providing special Java APIs designed to access proprietary features. Unfortunately, each vendor devises its own unique set of APIs to use similar services. These incompatible APIs defeat J2ME’s prime directive to provide an abstract platform with a consistent interface that accesses all available features across all devices. Using such vendor-specific APIs also ties the application to a particular mobile device, which reduces an IT manager’s options to deploy the application as far as possible across the enterprise.

J2ME limits support for vendor-specific hardware features to accommodate variations among devices. J2ME tackles hardware variations in two ways. First, J2ME defines an abstraction layer known as a configuration, which describes the minimum hardware required to implement Java on an embedded device. The J2ME configuration that addresses resource-constrained devices such as mobile phones and low-end PDAs is the CLDC (Connected Limited Device Configuration).

Second, J2ME defines a second abstraction layer, termed a profile, that describes the device’s hardware features and defines the APIs that access them. Put another way, profiles extend a configuration to address a device’s specific hardware characteristics.

J2ME currently defines one profile for CLDC devices: the MIDP (Mobile Information Device Profile). In addition to stipulating the basic hardware requirements, the MIDP implements the APIs used to access the hardware.

It’s important to understand that a J2ME device can have only one configuration that describes the core hardware. However, multiple profiles are allowed, including the MIDP and vendor profiles that support device-specific features. A vendor can therefore quickly adapt J2ME for a new device by writing profiles for it.

Besides interfaces, the MIDP describes the characteristics of a J2ME application, known as a midlet. Midlets are taxonomically similar to Java applets, the major difference being that a midlet can download data but not additional code through the connection. The end result is that a midlet has the potential to run unaltered on any J2ME-enabled device. Furthermore, because J2ME profiles define the interface, the appearance and behavior of the midlet on different devices is nearly identical. J2ME thus holds out the promise of becoming a universal front-end for client applications that can run on any mobile device.

The astute reader will have no doubt noticed the words “potential” and “nearly” in that last paragraph. Let’s face it: A few limitations in J2ME can hamper application portability. To begin with, subtle variations in each vendor’s J2ME implementation can cause incompatibilities.

Another issue is that if an API doesn’t exist for a specific device feature, you can’t use that feature at all. J2ME does specify optional standards, such as the MMAPI Mobile Media API, which provides support for audio playback and streamed video. Unfortunately, a developer can’t rely on an optional standard being available on every device.

However, J2ME is constantly evolving to address these concerns. In November 2002, the Java Community Process (JCP) released a significant revision to the MIDP specification. Called MIDP 2.0, it adds mission-critical APIs that support secure network connections (via HTTPS) and implements permissions with digital signatures to support trusted code. The specification also provides improved UI elements that offer enhanced form layout for business applications.

Unlike optional standards, a device that uses MIDP 2.0 must implement all the APIs that it describes, thus ensuring their availability to a program. Handsets that support the MIDP 2.0 specification should appear on the market later this year.

Down the road, the JCP proposes a new JTWI (Java Technology for the Wireless Industry) specification. In JTWI, a number of optional J2ME APIs — such as MMAPI and WMA (Wireless Messaging APIs) — become required services.

Even in its current state, J2ME offers developers the ability to write once and deploy a business application across the wide range of wireless gear currently available. J2ME’s abstraction layers also provide a hedge against vendor lock-in, and they help cope with the rapid changes in today’s wireless devices. Developers may have to craft the midlet’s interface to address the lowest-common-denominator display, but that’s a small price to pay compared with writing a custom client application for each device the corporation owns.

This chapter explains the Java 2 platform architecture and its security features as they apply to building Java applications. In particular, it describes the various Java platforms and the core security features that contribute to the end-to-end security of Java-based applications running on various systems—from servers to stand-alone computers, computers to devices, and devices to smart cards.

Sun"s Java philosophy of "Write Once, Run Anywhere" has been an evolving success story since its inception, and it has revolutionized the computing industry by delivering to us the most capable platform for building and running a wide range of applications and services. In general, the Java platform provides a general-purpose object-oriented programming language and a standard runtime environment for developing and delivering secure, cross-platform application solutions that can be accessed and dynamically loaded over the network or run locally.

With the release of the Java 2 Platform, Sun categorized the Java technologies under three key major editions in order to simplify software development and deployment. The Java 2 Standard Edition (J2SE) provides the runtime environment and API technologies for developing and executing basic Java applications, and it also serves as the secure foundation for running Java enterprise applications. The Java 2 Enterprise Edition (J2EE), or the J2EE Platform, is a set of standards and API technologies for developing and deploying multi-tier business applications. To support Java on microdevices and embedded systems, Java 2 Micro Edition (J2ME) provides the runtime environment and API technologies for addressing the needs of consumer electronics and devices. With its widespread adoption, today Java technology is enabled and executed from smart cards to microdevices, handhelds to desktops, workstations to enterprise servers, mainframes to supercomputers, and so on.

To facilitate end-to-end security of the Java platform-based application solutions, the Java runtime environment (JRE) and the Java language provide a solid security foundation from the ground up by imposing strong format and structural constraints on the code and its execution environment. This distinguishes the Java platform from other application programming languages—it has a well-defined security architectural model for programming Java-based solutions and their secure execution.

In this chapter, we will explore the various Java platforms and the intricate details of their security architecture that contribute to the end-to-end security of Java-based application solutions. In particular, we will study Java security and the inherent features of the following technologies:

Security has been an integral part of Java technology from day one. Security is also an evolving design goal of the Java community—building and running secure and robust Java-based network applications. The primary reason for Java"s success today as a secure execution environment is the intrinsic security of its architectural foundation—the Java Virtual Machine (JVM) and the Java language. This foundation achieves the basic Java security goal and its definitive ways for extending security capabilities to ensure features such as confidentiality, integrity, trust, and so forth. A second reason for its success is its ability to deliver an interoperable and platform-neutral security infrastructure that can be integrated with the security of the underlying operating system and services.

The JVM is an abstract computing engine that resides on a host computer. It is the execution environment for the Java programming language and has the primary responsibility for executing the compiled code by interpreting it in a machine-independent and cross-platform fashion. The JVM is often referred to as the Java runtime environment. While executing a Java program running on top of the JVM, the JVM insulates the application from the underlying differences of the operating systems, networks, and system hardware, thus ensuring cross-platform compatibility among all of the implementations of the Java platform.

The Java language allows creation of general-purpose programs called Java classes that represent a Java program or an application. The Java classes compile into a format called Java"s executable bytecodes, which are quite similar to the machine language that can run on top of a JVM. The JVM also allows users to download and execute untrusted programs and applications from remote resources or over a network. To support delivery of Java components over the network, the JVM controls the primary security layer by protecting users and the environment from malicious programs. To enable security, the JVM enforces stringent measures ensuring systems security on the host client machine and its target server environments.

Distributing the executable Java bytecode over a network or running automatically inside a Web browser or a client"s machine leads to different security risks and attacks, such as disclosure of the target environment to the untrusted applications and damage or modification of the client"s private information and data. For example, Java applets downloaded from a network are not allowed to have access to, read from, or write to a local file system. They are also not allowed to create network connections to any host system except the one where they are deployed. On the other hand, stand-alone Java applications that reside and run locally as trusted applications are not subjected to these security features. The key issue is that allowing untrusted applications such as Java applets to be downloaded from a network via a Web browser and letting them access certain resources on the host computer paves the way for security breaches and becomes a potential avenue for the spread of viruses. To prevent known security breaches and threats, the JVM provides a built-in Java security architecture model, configurable security policies, access control mechanisms, and security extensions. Because of the built-in JVM safety features, Java programs can run safely and are more securely protected from known vulnerabilities.

Java is a general-purpose object-oriented programming language similar to C++. It delivers platform-neutral compiled code that can be executed using a JVM and is intended for use in distributed application environments, heterogeneous systems, and diverse network environments. The Java language is also designed to provide for the security and integrity of the application and its underlying systems at all levels—from the Java language constructs to the JVM runtime and from the class library to the complete application.

The language defines all primitives with a specific size and all operations are defined to be in a specific order of execution. Thus, the code executed in different JVMs will not differ from the specified order of execution.

The language provides access-control functionality on variables and methods in the object by defining name space management for type and procedure names. This secures the program by restricting access to its critical objects from untrusted code. For example, access is restricted by qualifying the type members as public, protected, private, package, etc.

The Java language does not allow defining or dereferencing pointers, which means that programmers cannot forge a pointer to the memory or create code defining offset points to memory. All references to methods and instance variables in the class file are done via symbolic names. The elimination of pointers helps to prevent malicious programs like computer viruses and misuse of pointers such as accessing private methods directly by using a pointer starting from the object"s pointer, or running off the end of an array.

The Java language is a strongly typed language. During compile time, the Java compiler does extensive type checking for type mismatches. This mechanism guarantees that the runtime data type variables are compatible and consistent with the compile time information.

The language allows declaring classes or methods as final. Any classes or methods that are declared as final cannot be overridden. This helps to protect the code from malicious attacks such as creating a subclass and substituting it for the original class and override methods.

The Java Garbage Collection mechanism contributes to secure Java programs by providing a transparent storage allocation and recovering unused memory instead of deallocating the memory using manual intervention. This ensures program integrity during execution and prevents programmatic access to accidental and incorrect freeing of memory resulting in a JVM crash.

With these features, Java fulfills the promise of providing a secure programming language that gives the programmer the freedom to write and execute code locally or distribute it over a network.

In the previous two sections, we briefly looked at the basic security features provided by the JVM and the Java language. As part of its security architecture, Java has a built-in policy-driven, domain-based security model. This allows implementing security policies, protecting/controlling access to resources, rule-based class loading, signing code and assigning levels of capability, and maintaining content privacy.

In the first release of the Sun Java Platform, the Java Development Kit 1.0.x (JDK) introduced the notion of a sandbox-based security model. This primarily supports downloading and running Java applets securely and avoids any potential risks to the user"s resources. With the JDK 1.0 sandbox security model, all Java applications (excluding Java applets) executed locally can have full access to the resources available to the JVM. Application code downloaded from remote resources, such as Java applets, will have access only to the restricted resources provided within its sandbox. This sandbox security protects the Java applet user from potential risks because the downloaded applet cannot access or alter the user"s resources beyond the sandbox.

The release of JDK 1.1.x introduced the notion of signed applets, which allowed downloading and executing applets as trusted code after verifying the applet signer"s information. To facilitate signed applets, JDK 1.1.x added support for cryptographic algorithms that provide digital signature capabilities. With this support, a Java applet class could be signed with digital signatures in the Java archive format (JAR file). The JDK runtime will use the trusted public keys to verify the signers of the downloaded applet and then treat it as a trusted local application, granting access to its resources. Figure 3-1 shows the representation of a sandbox in the JDK 1.1 security model.

The release of J2SE [J2SE] introduced a number of significant enhancements to JDK 1.1 and added such features as security extensions providing cryptographic services, digital certificate management, PKI management, and related tools. Some of the major changes in the Java 2 security architecture are as follows:

In the Java 2 security architecture, all code—regardless of whether it is run locally or downloaded remotely—can be subjected to a security policy configured by a JVM user or administrator. All code is configured to use a particular domain (equivalent to a sandbox) and a security policy that dictates whether the code can be run on a particular domain or not. Figure 3-2 illustrates the J2SE security architecture and its basic elements.

): In J2SE, all local Java applications run unrestricted as trusted applications by default, but they can also be configured with access-control policies similar to what is defined in applets and remote applications. This is done by configuring a ProtectionDomain, which allows grouping of classes and instances and then associating them with a set of permissions between the resources. Protection domains are generally categorized as two domains: "system domain" and "application domain." All protected external resources, such as the file systems, networks, and so forth, are accessible only via system domains. The resources that are part of the single execution thread are considered an application domain. So in reality, an application that requires access to an external resource may have an application domain as well as a system domain. While executing code, the Java runtime maintains a mapping from code to protection domain and then to its permissions.

Protection domains are determined by the current security policy defined for a Java runtime environment. The domains are characterized using a set of permissions associated with a code source and location. The java.security.ProtectionDomain class encapsulates the characteristics of a protected domain, which encloses a set of classes and its granted set of permissions when being executed on behalf of a user.

): In essence, permissions determine whether access to a resource of the JVM is granted or denied. To be more precise, they give specified resources or classes running in that instance of the JVM the ability to permit or deny certain runtime operations. An applet or an application using a security manager can obtain access to a system resource only if it has permission. The Java Security API defines a hierarchy for Permission classes that can be used to configure a security policy. At the root, java.security.Permission is the abstract class, which represents access to a target resource; it can also include a set of operations to construct access on a particular resource. The Permission class contains several subclasses that represent access to different types of resources. The subclasses belong to their own packages that represent the APIs for the particular resource. Some of the commonly used Permission classes are as follows:

Example 3-1 shows how to protect access to an object using permissions. The code shows the caller application with the required permission to access an object.

Permissions can also be defined using security policy configuration files (java.policy). For example, to grant access to read a file in "c:\temp\" (on Windows), the FilePermission can be defined in a security policy file (see Example 3-2).

Policy: The Java 2 security policy defines the protection domains for all running Java code with access privileges and a set of permissions such as read and write access or making a connection to a host. The policy for a Java application is represented by a Policy object, which provides a way to declare permissions for granting access to its required resources. In general, all JVMs have security mechanisms built in that allow you to define permissions through a Java security policy file. A JVM makes use of a policy-driven access-control mechanism by dynamically mapping a static set of permissions defined in one or more policy configuration files. These entries are often referred to as grant entries. A user or an administrator externally configures the policy file for a J2SE runtime environment using an ASCII text file or a serialized binary file representing a Policy class. In a J2SE environment, the default system-wide security policy file java.policy is located at /lib/security/ directory. The policy file location is defined in the security properties file with a java.security setting, which is located at /lib/security/java.security.

Example 3-3 is a policy configuration file that specifies the permission for a signed JAR file loaded from "http://coresecuritypatterns.com/*" and signed by "javaguy," and then grants read/write access to all files in /export/home/test.

The J2SE environment also provides a GUI-based tool called "policytool" for editing a security policy file, which is located at "/bin/policytool."

The effective policy of the JVM runtime environment will be the union of all permissions in all policy files. To specify an additional policy file, you can set the java.security.policy system property at the command line:

): Each Java application can have its own security manager that acts as its primary security guard against malicious attacks. The security manager enforces the required security policy of an application by performing runtime checks and authorizing access, thereby protecting resources from malicious operations. Under the hood, it uses the Java security policy file to decide which set of permissions are granted to the classes. However, when untrusted classes and third-party applications use the JVM, the Java security manager applies the security policy associated with the JVM to identify malicious operations. In many cases, where the threat model does not include malicious code being run in the JVM, the Java security manager is unnecessary. In cases where the SecurityManager detects a security policy violation, the JVM will throw an AccessControlException or a SecurityException.

In a Java application, the security manager is set by the setSecurityManager method in class System. And the current security manager is obtained via the getSecurityManager method (see Example 3-4).

The class java.lang.SecurityManager consists of a number of checkXXXX methods like checkRead (String file) to determine access privileges to a file. The check methods call the SecurityManager.checkPermission method to find whether the calling application has permissions to perform the requested operation, based on the security policy file. If not, it throws a SecurityException.

If you wish to have your applications use a SecurityManager and security policy, start up the JVM with the -Djava.security.manager option and you can also specify a security policy file using the policies in the -Djava.security.policy option as JVM arguments. If you enable the Java Security Manager in your application but do not specify a security policy file, then the Java Security Manager uses the default security policies defined in the java.policy file in the $JAVA_HOME/jre/lib/security directory. Example 3-5 programmatically enables the security manager.

): The access controller mechanism performs a dynamic inspection and decides whether the access to a particular resource can be allowed or denied. From a programmer"s standpoint, the Java access controller encapsulates the location, code source, and permissions to perform the particular operation. In a typical process, when a program executes an operation, it calls through the security manager, which delegates the request to the access controller, and then finally it gets access or denial to the resources. In the java.security.AccessController class, the checkPermission method is used to determine whether the access to the required resource is granted or denied. If a requested access is granted, the checkPermission method returns true; otherwise, the method throws an AccessControlException.

Codebase: A URL location of class or JAR files are specified using codebase. The URL may refer to a location of a directory in the local file system or on the Internet. Example 3-7 retrieves all the permissions granted to a particular class that"s been loaded from a code base. The permissions are effective only if the security manager is installed. The loaded class uses those permissions by executing Class.getProtectionDomain() and Policy.getPermissions().

To ignore the default policies in the java.security file, and only use the specified policy, use "==" instead of "=". With the policy just presented, you may run the following:

Bytecode verifier: The Java bytecode verifier is an integral part of the JVM that plays the important role of verifying the code prior to execution. It ensures that the code was produced consistent with specifications by a trustworthy compiler, confirms the format of the class file, and proves that the series of Java byte codes are legal. With bytecode verification, the code is proved to be internally consistent following many of the rules and constraints defined by the Java language compiler. The bytecode verifier may also detect inconsistencies related to certain cases of array bound-checking and object-casting through runtime enforcement.

ClassLoader: The ClassLoader plays a distinct role in Java security, because it is primarily responsible for loading the Java classes into the JVM and then converting the raw data of a class into an internal data structure representing the class. From a security standpoint, class loaders can be used to establish security policies before executing untrusted code, to verify digital signatures, and so on. To enforce security, the class loader coordinates with the security manager and access controller of the JVM to determine the security policies of a Java application. The class loader further enforces security by defining the namespace separation between classes that are loaded from different locations, including networks. This ensures that classes loaded from multiple hosts will not communicate within the same JVM space, thus making it impossible for untrusted code to get information from trusted code. The class loader finds out the Java application"s access privileges using the security manager, which applies the required security policy based on the requesting context of the caller application.

With the Java 2 platform, all Java applications have the capability of loading bootstrap classes, system classes, and application classes initially using an internal class loader (also referred to as primordial class loader). The primordial class loader uses a special class loader SecureClassLoader to protect the JVM from loading malicious classes. This java.security.SecureClassLoader class has a protected constructor that associates a loaded class to a protection domain. The SecureClassLoader also makes use of permissions set for the codebase. For instance, URLClassLoader is a subclass of the SecureClassLoader. URLClassLoader allows loading a class or location specified with a URL.

Keystore and Keytool: The Java 2 platform provides a password-protected database facility for storing trusted certificate entries and key entries. The keytool allows the users to create, manage, and administer their own public/private key pairs and associated certificates that are intended for use in authentication services and in representing digital signatures.

We will take a look in greater detail at the usage of the Java keystore and keytool and how these tools help Java security in the section entitled "Java Security Management Tools," later in this chapter.

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Apps for cell phones already existed before the first Android smartphones or iPhones. As early as the early 2000s, Java was used as a programming language for small applications on cell phones. There was no app store, but many mobile network operators had their own mobile website where small applications and games could be downloaded via GPRS or UMTS. Later, applications and games were also downloaded to the phones via infrared, Bluetooth or USB as JAR files and shared between users. This article is about setting up a development environment for J2ME apps as well as a simple practical programming example for cell phones that were common at the time.

Caution: Some desktop applications used in this article are no longer under development and may therefore be incompatible with newer operating systems and contain security vulnerabilities. Installation on productive systems is not recommended!

J2ME is the Java Platform Micro Edition, which was developed especially for portable devices such as cell phones or PDAs. Over time, the end devices became more and more powerful, so that the Java Standard Edition also prevailed in this sector. The advantage of J2ME was also the independence of the JavaRuntime, so that J2ME applications could be executed on different operating systems and devices such as Nokia, Sony Ericsson, Siemens and so on. The basis for this is the CDC (Connected Device Configuration), which is not limited to cell phones, but is also available for DVB receivers and other multimedia devices. The application interfaces (API) of this runtime environment are made available as "profiles" and can provide new functions for later developments.

The MID profile (Mobile Information Device Profile) is often used for mobile phones. There are several versions of this profile, but there is little difference between them. In this case, the widespread J2ME applications are also often called MIDlets. In the high-level API, the MID profile provides I/O elements such as text fields, input fields, alarm boxes (alerts), progress indicators and much more. The low-level API, in turn, can be used to create pixel-level elements. When entering data, for example, the high-level API would be used to process entire strings (texts, numbers, etc.), while the low-level API would determine a single keystroke and its actuation period. The last example is mainly used in game development. The currently widely used MIDP 2.0 or MIDP 2.1 also supports HTTPS connections, streaming and multimedia features for game developers.

There are various options and tools that can be used to develop J2ME MIDlets apps. For the following practical example, an older JDK version (example: J2SE Development Kit 5.0-22) is required to run the Java WTK (Wireless Toolkit), which is also required. Optionally, a better text editor such as Notepad++ is also highly recommended, as it can clearly display Java code. More information about the necessary tools and the download links can be found in the Installation Guide of JDK & WTK..

The first J2ME MIDlet app is very simple and includes the minimum necessary packages and methods as well as a simple text field where a previously defined text is output. Following the successful simulator test, a JAR file can be generated from the Java source code, which is also executable on most cell phones. First, the Java Wireless Toolkit is opened and a new project is created. The project name and MIDlet class name are identical and are referred to as "MyFirstApp" in this case:

Clicking on "Create Project" creates the project and opens a window where the Target API platform has to be selected. Most cell phones support MIDP 2.1, so the default settings can be used directly:

Optionally, it is also possible to assign a different MIDlet name and to immortalize yourself as a MIDlet vendor (manufacturer name). This information is displayed on many cell phones under the app details. The parameters can be changed under the "Required" item:

After saving the Java file, a build can be created in WTC. This process takes a few seconds and the message "Build complete" is displayed in the console:

When the application is finished being programmed and tested, a JAR file can be generated from the project. The JAR file can be used to install the programmed application on a cell phone or to pass it on to other users. The compilation process is started under the menu item [Project]->[Package]->[Create Package]:

WTK generates a JAR file and a JAD file. For the installation and distribution of the app usually only the JAR file is needed. Both files are located in the "bin" folder in the main directory of the project folder:

[javax.microedition.lcdui]: This package is called the UI API and provides many functions for creating user interfaces in MIDP applications. This API consists of the high-level API and low-level API mentioned earlier. All elements that are displayed on the screen to the user are processed via the UI API.

[javax.microedition.midlet]: This package is responsible for exchanges between the application itself and the environment on which the application runs. For example, the runtime environment can pause the application during a call or close the application in a controlled manner. This package calls methods like startApp(), pauseApp(), resumeRequest(), destroyApp() and others.

After the app has been loaded, the method startApp() is called. This method now assigns a title for the current form and then appends the text "Hello World" to the form. The remaining two lines of this function now switch the form to the current screen.

Methods like pauseApp() or destroyApp() are called by the runtime in certain situations, but these methods do not contain any further commands. Therefore, no further processes take place.

Launch wireless toolkit via "Start" button ⇒ Programs ⇒ Sun Java Wireless Toolkit 2.5.2 ⇒ new Project... ⇒ In "Project Name", enter "Hello". In MIDlet Class Name, enter "HelloMIDlet" ⇒ Create Project ⇒ OK ⇒ Take note of the source file directory, e.g., "c:\WTK2.5.2\apps\Hello\src".

Write the following source code, using a programming text editor, and saved as "HelloMIDlet.java" in the source file directory noted in the previous step.

A midlet (or Java ME program) (#2) extends from the abstract class javax.microedition.midlet.MIDlet. This class declares three abstract methods: startApp(), pauseApp() and destroyApp(). These are call-back methods, that will be invoked by the runtime (or Application Management Software) at the appropriate instance. You need to override these methods to program the running behaviors of your midlet.

startApp() (#4) is called by the runtime when the midlet is starting, or resuming after it is being paused. pauseApp() (#5) is called by the runtime to pause the midlet. destroyApp() (#6) is called the the runtime when the midlet is about to be destroyed and removed from the memory.

In the startApp() (#4), we create the mainForm (of javax.microedition.lcdui.Form) to place the "Hello, world" StringItem UI component. We add a "Exit" command to the mainForm, and set this class as the command listener for handling the commands. We then retrieve the current display (of javax.microedition.lcdui.Display) of this midlet, and set the mainForm as the current display.

This class, as the command listener, needs to implement the javax.microedition.lcdui.CommandListener interface (#2). This interface declares an abstract method commandAction(). We override thismethod (#7) to invoke notifyDestroy() in response to the "Exit" command, which will inform the runtime that this midlet has entered the destroy state.

Compared with Sun Java Wireless Toolkit, NetBeans provides a GUI Builder (called Visual Mobile Designer) in addition to source coding. Follow the steps in "Quick Start Guide" to use the Visual Mobile Designer to write a Hello-world Java ME applications.

Java ME, is easier than the Java SE and Java EE, simply because the APIs is very much smaller (because mobile devices has limited capability). For example, the java.lang in Java ME (CLDC 1.1) has 17 classes and 1 interface, compared with the java.lang of Java SE (JDK 1.6), which has 35 classes and 8 interfaces.

Unlike Java SE, which is a single piece of software identical in all the operating platforms for developing desktop applications, Java ME is designed for mobile devices, which have limited and diverse computational power, memory, screen size, and networking capability. Due to the large variety and diversity of mobile devices, it does not make sense to design a one-size-fits-all solution and request all variety of mobile devices to support this piece of software.

Java ME is, therefore, divided into configurations, profiles, and optional APIs. A mobile device manufacturer is free to choose what to support, depending on the device"s capability.

A configuration specifies a Java Virtual Machine (JVM) and a base set of core APIs for a certain class of devices. There are currently two configurations defined:

CLDC (Connected & Limited Device Configuration): for intermittently networked (connected) and limited capability devices such as mobile phone, smart phone, PDAs. CLDC specifies a JVM and core API packages java.lang, java.io, java.util (which are subset of the Java SE counterparts) and javax.microedition.io (for networking support).

CDC (Connected Device Configuration): for robust networked and more capable devices, such as high-end PDAs, set-top boxes. (CDC is beyond the scope of this writing.)

Configuration (CLDC and CDC), by itself, is incomplete for application development. A profile is defined on top of a configuration to provide specific APIs (such as user interface, persistent storage) to make a complete application environment for developing applications. For example, MIDP (Mobile Information Device Profile) is build on top of CLDC; PDAP (PDA Profile) is build on top of CDC.

Mobile devices may choose to support optional APIs, such as Blue tooth API, Wireless Messaging API (WMA 1.0 or 2.0) and many others, depending on their capability.

Configurations and profiles are confusing. As a starter, you need not worry about how to distinguish them. You simply need to understand the combined APIs provided by the "MIDP for CLDC".

In this writing, we focus on the "MIDP for CLDC". Recall that CLDC is the base, and MIDP builds on top of CLDC to provide a complete application development environment. Currently, there are two versions of CLDC: 1.0 and 1.1 and three versions of MIDP: 1.0, 2.0, 2.1.

CLDC defines the JVM and a base set of core APIs for a class of devices with intermittent connectivity and limited capacity. CLDC specifies core APIs such as java.lang, java.io, java.util:

java.lang: CLDC 1.0 has 15 classes and 1 interface: Object, System, Thread/Runnable, Runtime, Class, String/StringBuffer, Math, wrapper classes (Integer/Byte/Short/Long/Character/Boolean). CLDC 1.1 added the floating-point support and wrapper classes Float/Double.

java.util: CLDC 1.0 has 7 classes and 1 interface: Random, collections (Vector/Hashtable/Stack), Calendar/Date/TimeZone, Enumeration. CLDC 1.1 added Timer/TimerTask.

java.io: CLDC has 11 classes and 2 interfaces: InputStream/OutputStream, DataInputStream/DataOutputStream, ByteArrayInputStream/ByteArrayOutputStream, Reader/Writer, InputStreamReader/OutputStreamWriter, and PrintStream.

CLDC does not provide a complete application development environment, e.g., it lacks the user interface. MIDP is built upon CLDC to provide a complete environment. MIDP specifies these APIs:

MIDP 2.0 added packages javax.microedition.media (multimedia support), javax.microedition.media.control, javax.microedition.lcdui.game (UI for game development), and javax.microedition.pki (public key infrastructure).

The latest MIDP 2.1 specification does not bring new features but instead it focuses to reduce device fragmentation by specifying a consistent set of Java technologies that must present in MIDP 2.1 capable phone.

A MIDP program is written by subclassing javax.microedition.midlet.MIDlet, and hence is called a midlet. A midlet is run and under the control of the Application Management Software (AMS or the runtime). The AMS can manage multiple midlets at the same time, and needs to content with other applications (such as SMS, MMS) for the limited system resources available in the mobile device.

A midlet extends abstract class MIDlet, which declares three abstract methods for managing its life cycle: startApp(), pauseAPP() and destroyApp(), and other non-abstract methods such as notifyPaused() and notifyDestroy().

A midlet could be in one of these states: constructed, paused, active, and destroyed. The life cycle of a midlet begins when the AMS invokes the constructor to create a new instance of the midlet. After it is constructed, the AMS places the midlet in the paused state. It then invokes Midlet::startApp() to start the midlet and place the midlet in the active state. The AMS may shift the midlet into the paused state by invoking Midlet::pauseApp() (e.g., there is a incoming call or MMS message which requires attention and system resources) and resume its operation by calling startApp() again. In the paused state, the midlet is not terminated, but it should release any resource it obtained in startApp(). The pause/active transition give you the ability to manage resources effectively. Typically, you shall use startApp() to allocate resources such as record stores, network connections, UI components, and use pauseApp() to release these resources. The AMS can put the midlet into the destroyed state from either paused or active state, via a call to destroyApp().

A midlet may voluntarily enter the paused state by invoking Midlet::notifyPaused(). Similarly, it could invoke Midlet::notifyDestroyed() to inform the AMS that it can now be considered destroyed.

The Display class provides the foundation for visual interaction with a midlet. There is only one instance of Display, which can be retrieved via static method Display.getDisplay(). You can then use Display::setCurrent() and Display::getCurrent() to get/set the current display for a particular Displayable.

The Displayable is an abstract class of objects that can be placed on the Display. A Displayable may have a title, a ticker, command(s) and an associated command listener. The following methods are provided:

Alert can be used for providing "dialog". There are two types of Alert: one display for a set period of time and does not require user"s interaction; the other interrupt the program until user provides a response. Alert is a Displayable. Hence, it can have title, ticker, command(s) and can register command listener. Alert has the following constructors:

The available AlertTypes are: ALARM, CONFIRMATION, ERROR, INFO, WARNING. You can use methods getTimeout() and setTimeout() to control the timeout (in milliseconds or FOREVER). Alert can be associated with an image or a sound effect.

off-screen graphics buffer for double-buffered rendering. You can retrieve the off-screen graphics handler via method GameCanvas::getGraphics() and transfer the contents to the on-screen buffer via GameCanvas::flushGraphics().

The GameCanvas class provides a off-screen graphics buffer for double-buffered rendering. You can retrieve the off-screen Graphics handle via method GameCanvas::getGraphics(). You can then perform your graphic rendenering on this off-screen buffer, and invoke GameCanvas::flushGraphics() to send the contents to the on-screen buffer.

This bouncing ball is straight-forward. However, I have problem writing multiple balls, because MIDP"s Math class does not provide arc-tangent function.

The Sprite class allow you to render an animated sprite with several image frames stored as one single image for better management of animated frames. Two examples of animated frame images are given below:

The Sprite class is able to extract the frames (given the frame width and height) and number them starting from 0 for the top-left frame, in row-major manner. Read the API documentation of the Sprite class for more illustration.

The Sprite class also provide basic transforms via the method Sprite::setTransform(type). The type includes TRANS_NONE, TRANS_ROT90, TRANS_ROT180, TRANS_ROT270, TRANS_MIRROR, TRANS_MIRROR_ROT90, TRANS_MIRROR_ROT180, TRANS_MIRROR_ROT270. For rotation, you can use setRefPixelPosition(x, y) to set the rotation center (by default, it rotates about (0, 0), which is the top-left corner of the image). Note that: (a) you cannot concatenate multiple transforms, (b) rotation of arbitrary degree in not supported, and (c) the rotational degree is measured clockwise (instead of the usual convection of counter-clockwise).

A sprite can detect collision with another sprite via the method Sprite::collidesWith(). You can set the collision detection at "pixel-level" or "bounding-box-level".

A TiledLayer is a special layer that is divided into grid of cells. Each cells can be filled with a selected tile frame. In this way, you can reuse the tile frames instead of providing a huge image. TiledLayer is often used for background.

To use a TiledLayer, you keep the tile frames in a single image (similar to sprite) and specify the width and height of the tile frame. The frames will be extracted and numbered starting from 1 (unlike sprite frame which starts from 0) for the top-left tile, in row-major manner. You then divide the tile layer into cells (of rows and columns). For each cell, you can assign a tile frame index (from 1 to n). Index 0 indicates that the cell is empty. You can use negative index for animated tile cells. Each negative index is associated with a positive tile frame index, and the associated tile frame can be changed on the fly. For example, the animated tile index -1 can be made alternate between tile frame index 5 and 6. Read the API documentation of TiledLayer for more illustrations.

The LayerManager can be used to manage a collection of Layers, and place them on the screen with a specific z-order. Each Layer is given an index corresponding to its z-order, with value of 0 closer to you, and highest number further away from you. Layers can be added into the LayerManager (via insert() or append()) and removed (via remove()). Recall that Layer is an abstract class, with implementations in Sprite and TiledLayer classes.