Chapter 1 Java generation and characteristics
1.1 History and current history of Java
1.1.1 History of Java Production
Java came from Sun's project called Green, its original purpose was to develop a distributed code system for household consumer electronics, so we can send E-mail to refrigerators, TV sets, etc. Control, communicate with them. Start, ready to use C , but C is too complicated, the security is poor, and finally based on C develops a new language Oak (Java's predecessor), Oak is a language for the exquisite and secure language, Sun has bidding A interactive television project, but the result is defeated by SGI. Poor OAK almost homeless, happening MOSAIC and Netscape developed by Mark Ardresen in this time, the Oak project group members, they have prepared the Hotjava browser with Java, got the support of Sun CEO SCOTT MCNEALY, triggered Java Entering the Internet. Java is also known as one? Copy; Announcement, one day, a few Java member group members are discussing what name to this new language, when they are drinking Java (Java) coffee in cafe, there is a law to say Just how Java, I got the appreciation of others, so the name of Java spread this.
1.1.2 Java's current
Java is a new generation of SUN-picked-to-icon designer, which is specially adapted to the internet application. Its platform is unacceptable directly to the Wintel's monster position. One time, "Even the Internet, with Java programming", becomes a fashion for technicians. Although the report is reported in the press; if it is actually, Java is a revolutionary technology for software development, its status has been established, which is in the following aspects: 1. Many of the large companies in the computer industry have purchased Java's License, including IBM, Apple, DEC, Adobe, Silicon Graphics, HP, ORACEL, TOSHIBA, and the most reluctant Microsoft. This is said that Java has gotten the recognition of the industry. 2. A lot of software developers to start supporting Java software products. For example, Borland is developing Java-based fast application proceedings LATTE, and the pre-production product will be released in the middle of 1996. This taken of the Borland company pushed Java to enter the PC software market. Sun Secretary Self-employed Java Workshop has been released. Daviders such as: Illustra, Sysbase, Versant, Oracle are issued in the CGI interface, supporting HTML and Java. Today is a network-centered calculation era, does not support HTML and Java, application scope of application can only be limited to homogeneous environments (the same hardware platform? Copy ;. 3.intranet is becoming the best solution for corporate information systems And where Java will play an irreplaceable role. The purpose of whitranet is to use the Internet for information systems within the enterprise. Its advantage is: inexpensive, easy to use and manage. Users regardless of the type of machines and operating systems, The interface is a unified INTRNET browser, while the database, web page, app (applet with Java) exists on the WWW server, whether developers, or managers, or users can benefit from this solution. Java The language is constantly developing and improving. Sun is the main development and promoters, and more common compilation environments have JDK (Java Workshop). There are many other companies being developing Java language compilers and integrated environments. It is expected that the correctness and efficiency of the Java language will be improved, and the user is programmed with Java and now uses C programming. 1.2 Java language features
Is Java to the bottom of the language? Java is a simple and dynamic multi-threaded dynamic language of a simple structure of a simple face icon. 1.2.1 simple
Java is the first to set up a language set to a control system for home electrical appliances, so it must be simple. Java language's simple master main body is now under the following three squares: 1. Java's style is similar to C , because the C program is uncommon. In some sense, Java language is a variety of C and C language, because C programmers can quickly hold Java programming techniques soon. 2. Java abandoned the place where the C was easily incorrectly incorrect, such as pointer and memory management. 3. Java mentioned COPY; the rich library.
1.2.2 face-to-objective icon
Face-to-object can be said to be the most important characteristics of Java. The Java language is completely facing the object, which does not support the programming techniques for the face-to-face process. Java supports the quodes of static and dynamic styles and reuse. From the perspective of the object-oriented character, the Java class is similar to SmallTalk, but its characteristics, especially the characteristics suitable for partial formula calculation is far from smaltalk.
1.2.3 distribution
Java includes a sub-library that supports HTTP and FTP and other TCP / IP protocols. Therefore, the Java application can open and interview the object of the network with the URL, and its access method is completely equipped with the number of visitors. For the sub-ring, especially the Internet, COPY; the dynamic content is unusual is a non-often magnificent task, but Java's language characteristics make it easy to realize this goal.
1.2.4 Jian Zhuang
Java causes the interrogation of the inspection program in compiling and running. Type check help inspection and find out the error in the early period. Java has reduced memory error in memory. Java also implements a real array to avoid accessible data. This is the cycle of the development Java application program. JAVA mentioning ;: NULL pointer detection array edge detection isometric export word generation code check
1.2.5
In addition, for the establishment of Java as a network, Java translates its procedure into an intermediate file format in the structure. This intermediate code can only be implemented with a machine with a Java run system. Now, the Java running system has Solaris2.4 (SPARC), WIN32 system (Windows95 and Windows NT). The Java source program is compiled into a high-level BYTE-CODE format language that is unrelated to the machine. This language is designed in virtual Running on the machine, implemented by the machine-related operating debugger.
1.2.6
Java's security can be guaranteed from both. One side, in the Java language, C features such as pointers and release memory can be deleted, avoiding non-molbound operation. On the other hand, when Java uses to create a browser, language functions and one? Copy; browser, copy; Copy; make it more secure. Java language has passed many times before the machine is performed on your machine. It has passed the code check, checks the format of the code segment, detects the pointer operation, whether or not the icon operation is excessively and try to change a type of object.
1.2.6.1 Byte-Code check
If BYTE-CODE passes the code check, there is no return error. We can know that the code does not have the stack overflow and the overflow all the operating code parameter type is correct, if it is not generated non-manifold, such as converting an integer into a pointer. Interview with the objective operation
1.2.6.2 Loading
Class Loader maintains security through the names of the network and the name of the network. Because of the transfer of the class, it will always be inspected, which avoids the current icon of Toliima. The class downloaded from the network is tuned into a private name word that is related to the source. When a private class is interviewed, the Build-in (this class) is first checked, then check the associated class. This avoids the creation of the damage of the damage.
1.2.7 can be planted
The same characteristics of the same body is to make the Java applications can be operated in the operational system system equipped with the Java interpretation and the encyclopedia, which is a good foundation for the Java application software for the transfer. But only is not enough. If the basic data type design is dependent on the body, it will also be greatly incomplete. For example, in Windows 3.1 (Integer) is 16bits, the integer in Windows 95 is 32bits, integrals among the DEC Alpha 64bits, 32bits in Intel 486. The basis of the basic data type and its operations independently of the platform, and the Java data can be maintained on any hardware platform. The basic data type of Java language is as follows: byte 8-bit binary complement Short 16-bit binary complement INT 32-bit binary complement LONG 64-bit binary complement Float 32-bit IEEE 754 floating point Double 32 -bit IEEE 754 floating point char 16-bit unicode character
In any Java interpreter, the data type is based on the above standard. Because all CPUs used for almost playing can support the above-based data type, 8 ~ 64-bit integer format complement operation and single / double precision floating point operation. The Java Compiler is written in Java language. The Made of the Java Accounting System is based on the restriction of the POSIX Formula, and it is written in ANSI C. There is no "same physical achievement" in the Java language specification. 1.2.8 explained
The Java Union (Running System) can directly run the target code command. The chain connection is often limited to the translation program, and the program can take more time in the original source program.
1.2.9 high performance
If the descriptor speed is not slow, Java can directly translate the target code to translate the target code during operation. SUN can be adjusted 300,000 processes in a second of the direct release interpretation. The speed of translation of the target code is nothing from the performance of C / C .
1.2.10 multi-thread
Java mentioning; multi-threaded functionality enables multiple small tasks in one program. Thread - - There is also a small process - - is a small independent process that is outlined in a large process. Because of Java real multi-threaded technique, the ratio C and C are more bonded. Multi-threaded strokes come to a better interactivity and real-time control performance. When the real-time control, it will be detrimental to the system (UNIX, Windows, Macintosh et al., Etc.), which is better than single-threading. Anyone who has used the current browser feels that it is a very troublesome thing to adjust a pair of pictures. In Java, you can use a single-thread to adjust a panel, and you can visit other information in HTML without having to wait.
1.2.11 dynamic state
The dynamic characteristics of Java are their face-to-objective metering method? Copy; Exhibition. It allows the program to dynamically install the desired class in the operation process, which is not possible to realize the C language to facilitate the object program setting. In the C programming process, whenever an exemplary variable or a member function is added in the class, it must be re-translated, and it will lead to collapse. Java takes a measures from the following party? Copy; to solve this problem. The Java compiler is not compiled into a numerical reference to the reference to the instance variable and the member function, but the symbol reference information is passed to the interpreter in the bytecode, and then the interpreter is referenced by the interpreter after completing the dynamic connection class. The information is converted to a numerical offset. Thus, a pair's alignment is not determined by the interlayer when the computed translation process is determined, but is delayed to the run. In this way, it is not only the code in which the spread is not shown. When explaining the execution of the word code, the search for this symbol information and the transformation process only in a new name, and the code can be performed at full speed. It is possible to use a class that has been updated while running, without having to worry about the original code. If the program is connected to a certain class in another system in the network, all of this class can also be updated in the class, without making any of the programs of such a class. Java also simplifies the way to use an upgrade or new agreement. If your system is running Java program, I have encountered the procedure of I don't know how it is handled, and Java can automatically download the function program you need. 1.3 and C and C language differences
Java mentioned; a function of a strong language, but there is a little bit of a mixed feature. C is not good, but C and C are also accepted by the big house, and the Java is set to C form, so that the big house is easy to learn. Java has dropped a lot of functions in the C language, so that Java's language function is refined, and increases one? Copy; very useful features, Java removes the following C and C features and features: Pointer computing structure TypeDefs #define needs to be released Definition of memory global variable this? Copy; function is very easy to cause errors.
1.4 Java application brief
1.4.1 web browsing
Web View is the main use of the main network in the international network and even the local area. The document can be easily display text and various graphics, he can also mention? Copy; Super text link. This? Copy; browser calls HTML language written document, HTML / WWW browser technology is limited to text and image. If you want to broadcast a sound or run a performance program, you may not download it with the document on your machine and run the file format. Java program and its browser Hotjava, mentioned Copy; Make your browser running program. You can disconnect sound from your browser. You can also play the picture in the page. Java can also tell you how your browser is handling new type of document. When we can transfer video images on the 2400 BAUD line, Hotjava will be able to display this. Copy; video. 1.4.2 Network application system
Java is a language that is unlike platform, because this network application system developed by Java can run on a variety of platforms, greatly increasing the opening-up efficiency, reducing recrystall labor. And, JAVA integrated network function? REG; distributed network application system.
chapter summary:
1. Java's production and popularity is the objective requirement of today's Internet development. Safe, structural neutrality, portable, excellent performance, multi-threaded, dynamic. REG; score-fit system in the Internet environment. 3. Java can make a large number of network application procedures, and it is very good with the WWW views of the current flow line.
