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Lesson 1:
Java Technology
This lesson introduces the basic building blocks of Java technology and the definitions used to construct Java platform.
After this lesson, you will be able to
- Understand the Java language and Java platform
- Describe how Java enables multiplatform computers to communicate easily
- Distinguish a Java Applet from a Java Application
- Understand the difference between Applets and Servlets
- Identify and distinguish the characteristics of Java Virtual Machine and Java Interpreter
- Describe the basic product functions of Java technology
- Describe the basic objectives of Java technology
- Understand how Java technology builds its Software Component Technology upon JavaBeans
- Understand why Java applet technology is safe for Internet
Estimated lesson time: 45 minutes
Java Technology
Java technology is both a programming language and a platform.
The Java Programming Language
The Java programming language is a high-level language that can be characterized by all of the following buzzwords:
Simple Architecture neutral Object oriented Portable Distributed High performance Interpreted Multithreaded Robust Dynamic Secure With most programming languages, you either compile or interpret a program so that you can run it on your computer. The Java programming language is unusual in that a program is both compiled and interpreted. With the compiler, first you translate a program into an intermediate language called Java bytecodes —the platform-independent codes interpreted by the interpreter on the Java platform. The interpreter parses and runs each Java bytecode instruction on the computer. Compilation happens just once; interpretation occurs each time the program is executed. The following figure illustrates how this works.
You can think of Java bytecodes as the machine code instructions for the Java Virtual Machine (Java VM). Every Java interpreter, whether it's a development tool or a Web browser that can run applets, is an implementation of the Java VM.
Java bytecodes help make "write once, run anywhere" possible. You can compile your program into bytecodes on any platform that has a Java compiler. The bytecodes can then be run on any implementation of the Java VM. That means that as long as a computer has a Java VM, the same program written in the Java programming language can run on Windows 2000, a Solaris workstation, or on an iMac.
The Java Platform
A platform is the hardware or software environment in which a program runs. We've already mentioned some of the most popular platforms like Windows 2000, Linux, Solaris, and MacOS. Most platforms can be described as a combination of the operating system and hardware. The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms.
The Java platform has two components:
- The Java Virtual Machine (Java VM)
- The Java Application Programming Interface (Java API)
You've already been introduced to the Java VM. It's the base for the Java platform and is ported onto various hardware-based platforms.
The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.
The following figure depicts a program that's running on the Java platform. As the figure shows, the Java API and the virtual machine insulate the program from the hardware.
Native code is code that after you compile it, the compiled code runs on a specific hardware platform. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.
The Product Functions of Java Technology
There three types of programs written in the Java programming language:
- applets,
- applications,
- Servlets.
If you've surfed the Web, you're probably already familiar with applets. An applet is a program that adheres to certain conventions that allow it to run within a Java-enabled browser. An applet can not do any kind of disk input/output operation to local client computers. It has very limited memory access capability to local computer so that it can not interfere with local computer's operation either. So applets are safe for the users on the Internet.
However, the Java programming language is not just for writing cute, entertaining applets for the Web. The general-purpose, high-level Java programming language is also a powerful software platform. Using the generous API, you can write many types of programs. An application is a standalone program that runs directly on the Java platform. A special kind of application known as a server serves and supports clients on a network. Examples of servers are Web servers, proxy servers, mail servers, and print servers.
Another specialized program is a servlet. A servlet can almost be thought of as an applet that runs on the server side. Java Servlets are a popular choice for building interactive web applications, replacing the use of CGI scripts. Servlets are similar to applets in that they are runtime extensions of applications. Instead of working in browsers, though, servlets run within Java Web servers, configuring or tailoring the server. Besides, servlet also has the powers of any application on the server side, and the safety features of applets on the client side.
How does the API support all these kinds of programs? It does so with packages of software components that provide a wide range of functionality. Every full implementation of the Java platform gives you the following features:
- The essentials: Objects, strings, threads, numbers, input and output, data structures, system properties, date and time, and so on.
- Applets: The set of conventions used by applets.
- Networking: URLs, TCP (Transmission Control Protocol), UDP (User Datagram Protocol) sockets, and IP (Internet Protocol) addresses.
- Internationalization: Help for writing programs that can be localized for users worldwide. Programs can automatically adapt to specific locales and be displayed in the appropriate language.
- Security: Both low level and high level, including electronic signatures, public and private key management, access control, and certificates.
- Software components: Known as JavaBeansTM, can plug into existing component architectures.
- Object serialization: Allows lightweight persistence and communication via Remote Method Invocation (RMI).
- Java Database Connectivity (JDBCTM): Provides uniform access to a wide range of relational databases.
The Java platform also has APIs for 2D and 3D graphics, accessibility, servers, collaboration, telephony, speech, animation, and more. The following figure depicts what is included in the Java 2 SDK.
The Objectives of Java Technology
Java is likely to make your programs better and requires less effort than other languages. It is believed that Java technology will help you do the following:
- Get started quickly: Although the Java programming language is a powerful object-oriented language, it's easy to learn, especially for programmers already familiar with C or C++.
- Write less code: Comparisons of program metrics (class counts, method counts, and so on) suggest that a program written in the Java programming language can be four times smaller than the same program in C++.
- Write better code: The Java programming language encourages good coding practices, and its garbage collection helps you avoid memory leaks. Its object orientation, its JavaBeans component architecture, and its wide-ranging, easily extendible API let you reuse other people's tested code and introduce fewer bugs.
- Develop programs more quickly: Your development time may be as much as twice as fast versus writing the same program in C++. Why? You write fewer lines of code and it is a simpler programming language than C++.
- Avoid platform dependencies with 100% Pure Java: You can keep your program portable by avoiding the use of libraries written in other languages.
- Write once, run anywhere: Because 100% Pure Java programs are compiled into machine-independent bytecodes, they run consistently on any Java platform.
- Distribute software more easily: You can upgrade applets easily from a central server. Applets take advantage of the feature of allowing new classes to be loaded "on the fly," without recompiling the entire program.
JavaBeans Technology
- JavaBeans Technology is a kind of Software Component Technology.
- The concept is to enable programmers to design applications by plugging together software components.
- The Core Java libraries and The JavaBeans API fundamentally support the client-level component assembly.
- The Enterprise JavaBeans specification supports the server-level component assembly.
Review Exercise 1.1: Java Definitions
Match the concepts in the numbered list with the definitions that follow it.Concepts
- Java Compiler
- Java Virtual Machine
- Byte Codes
- Java APIs
- Java Applet
- Java Application
- Java Servlet
- Java-enabled browser
- Java Platform
- Java Interpreter
Definitions
- parses and runs each Java bytecode instruction on the computer
- translates a program into an intermediate language called Java bytecodes
- is an Internet browser installed with Java Virtual Machine plugins
- is a large collection of ready-made software components
- is the machine code instructions for the Java Virtual Machine
- is a software development and execution environment
- is a program that adheres to certain conventions that allow it to run within a Java-enabled browser
- can be implemented by any Java interpreter, whether it's a development tool or a Web browser that can run applets.
- is a standalone program that runs directly on the Java platform
- can almost be thought of as an applet that runs on the server side, and has the powers of any application on the server side, and the safety features on the client side.
