Exploring the soul of a Java mobile phone

Recently, mobile phones are always accompanied by a lot of proprietary terms and appear in various major media outlets: WAP, Bluetooth, and GPRS, recently, another thing that makes a cell phone "alive"-"Java 」! Java is so powerful that it can make Motorola's latest a6288 mobile phone "alive 」? Is java the same as the Java language we usually use on computers? Or is there any relevance? What is the difference between a living mobile phone and a general mobile phone and a WAP mobile phone? Now, let's take a look at the curiosity to visit the soul of the Java mobile phone-"j2's 」.

Java 2 Micro Edition is short for Java 2. It can be interpreted literally as a member of the Java family, its role in the Java family is positioned on "embedded system" applications.

We use a simple illustration to introduce this family:

Anyone who has written a process in C language will use the standard library of C language. Java also has a function library, and Java classifies the function library it uses more systematically, it is called "category library 」. Java's category library can also separate the core class and extended class ). Java is separated into several different versions to correspond to different performance device environments (such as computers, PCs, mobile phones, Information appliances, and PDAs. Therefore, according to the computing power and power supply status, the function library supporting all standard core categories is separated, and many extended category libraries for enterprise applications are also added, j2EE (Java 2 Enterprise Edition) for enterprise and server positioning; supports standard core category libraries and j2se (Java 2 Standard Edition) for client applications ); as well as applications with relatively weak computing capabilities, they only support one standard core-wish Library and the j2_supporting extended category library of embedded systems.

Now, what is an embedded system? If we use another term, we can use the Java version specifically designed to write consumer electronic product applications. This makes it easier for everyone to understand. Consumer electronic products can reach refrigerators, televisions, and cell phones and PDAs. Their common points are the use of embedded systems, so that they have the ability to process and operate logic; the difference lies in the volume and power supply mode. To address this difference, j2-based embedded devices are divided into cldc (connected limited device configuration) for devices that can be placed on the palm of the hand) and the CDC (connected device configuration) of the device that can be inserted into the wall.

At present, the CDC specification has not yet been finalized, and the official release is the cldc version. Literally speaking, cldc is designed for mobile phones and PDAs with limited power supply and computing capabilities. Due to the differences between mobile phones and PDAs, cldc has derived two types of profiles: PDA profile and mobile information device profile (MIDP) for mobile communication devices ). Therefore, the full name of the Java version used to write on the mobile communication device should be j2_cldc. MIDP.

To achieve the cross-platform capability of "write once, run anywhere", Java has an exclusive Virtual Machine (VM) in each version ). The Virtual Machine of j2m's is KVM (K is kilobytes ). Now we have a general understanding of j2's. Next we will talk about how to make the cell phone "alive 」!

The figure above is composed of two three-dimensional circles. The small circle above represents the function of a WAP mobile phone, which can provide voice services for calls, has additional application functions for standard phone book. It also has a WAP browser that allows users to access the WAP website. Although such a mobile phone is a bang, I always feel that something is missing. If the mobile phone can be like a PDA, you can download different application software as you like, or do not directly use TCP/IP to access the Internet through the WAP browser, enjoy applications such as the master-slave structure of the computer system, isn't that even better? Such a requirement must be achieved by a Java mobile phone. The figure above shows the Java Mobile Phone function. Therefore, compared with traditional mobile phones that cannot meet their own needs, Java mobile phones are more flexible, and application functions can also be increased or decreased as users need them. From this perspective, it seems reasonable to say that Java makes the mobile phone alive!
Do you want to test the Java program on your mobile phone? Don't worry! We will introduce them one by one in the following chapters. If you can't wait, you can go to the Motorola website to have a dedicated Developer Forum and some technical documents.

Most of the books related to j2's are for Palm, I believe that the "deep dive into kjava" published by "Zhi Cheng" and the "Palm application design-using Java" published by "Feng" are all well written, anyone who wants to know about j2-can buy it and have a look.