Introduction to Java

Java history

According to the usual parlance, the hoot of Java astonished can be traced back to the year 1991. At the time, Sun set up a project called Green, which is dedicated to communication and collaboration between digital appliances. James Gosling (father of Java) as the head of the purpose. Faced with the Green plan, Gosling needs to develop a brand-new astonished say that the astonished must be concise, robust, and more importantly, the astonished should be able to shield the differences between hardware devices so that the code has better portability, after all, compared to the PC platform, digital home appliance equipment has a higher platform difference. As a result, Java astonished was born, not her original name is not Java but oak (oak).

The Java language is truly understood by the world, starting in the 1995. The rapid development of Internet technology has played a great role in promoting Java astonished. The Java language quickly became the most widely used programming astonished. From this time on, the Java language has been involved in the field has entered beyond the original design of its goal. Java has grown from a simple language to a common platform technology standard.

Java is an open technology. Sun Corporation established JCP (Java Community Process) in 1998, and JCP is an open international organization to maintain and develop Java technical specifications. JCP members can submit a JSR (Java specification requests)-"Java specification request", through a specific program, approved by the JCP Executive Committee (Executive Committee), can be formally incorporated into the next version of Java The specification.

JCP is divided into two executive committees, one for Java SE and Java EE obnoxious (Se/ee EC) and the other for the Java Me facet (Me EC). The length of service for each Executive committee member is 3 years, including 10 approved seats, 5 open seats and 1 fixed seats, which were formerly owned by Sun and now become Oracle.

In 2007, Google launched an open smart migration operating system,-android.

Java Framework

1999 Sun released a Java-based three platform technical standards: J2SE, Java EE and J2ME (after 2005 they were renamed Java-SE, and the same is the JavaScript and Java ME), Java has ushered in its own era.

　　Java SE (Java Platform, Standard Edition) is called the "Java Platform Standards Edition" and is the foundation of the Java platform. Java SE contains the basic environment and core class libraries required to run Java applications, and Java SE also defines a base class library based on desktop applications, and by using these libraries, we can write out a variety of desktop applications like Office.

　　Java EE (Java platform,enterprise Edition) is called "Java Platform Enterprise Edition". Java EE is built on top of Java SE to build enterprise-class applications. Enterprise-class applications are led those that are created for small business groups and large enterprises. Systems, such as the "billing system" for telecommunications, the "online banking system" of banks, "customer relationship Management Systems" in enterprises, etc. These systems do not use individual desktop systems, they are deployed, run in a complex server environment, often need to deal with a large number of data, the need to comply with the common data transmission protocol and the representation, need to maintain complex and changeable business undercover, need to deal with huge user traffic, Must have reliable safety and robustness. Java EE offers a wide range of component standards and services to address a variety of issues that need to be addressed in enterprise applications Yingying: The famous servlet/jsp and the once controversial EJB.

　　Java ME (Java platform,micro Edition), known as the Java Platform Mini-version. Java ME solves obnoxious cases for embedded consumer electronics devices such as set-top boxes, mobile phones, and PDAs. Also say only Java me is most in line with Java astonished to create the original intention, with the advent of the era of 3G mobile intelligent platform, especially Java as the core programming astonished the rapid popularization of Android intelligent platform, Java ME Future development seems to be not optimistic.

Java features

Java's astonished features are generally described as follows: Simple, object-oriented, cross-platform, secure, and multithreaded support.

1. Simple. Java is designed to be able to build a system that does not require esoteric and industry training to import line programming, but also conforms to certain programming standard practices, so Java is designed to be as close to the prevailing C + + as possible, but at the same time it rejects some of the difficult features of C + + that are less used, understood, and confusing (such as : header file, led pin, operator overload, and so on astonished do not operate), Java syntax is more like a "pure" version of C + + syntax.

2. Object oriented. Object-oriented design is a programming technique. It focuses on objects (that is, data) and on object interfaces. Take the Carpenter Derby obnoxious, an "object-oriented" Carpenter weaving cuss note is a chair, followed by the use of tools, a "non-object-oriented" Carpenter will first consider what tools to use. In essence, Java object-oriented capabilities are not the same as C + +. Java is not the main difference in C + + in the multi-inheritance, Java to take and substituting remainders is a simpler interface concept. Relative to the traditional process-oriented astonished (C, Basic and Pascal, etc.), object-oriented programming astonished (C + +, Java and C #, etc.) in the implementation of large complex objects is more effective. The object-oriented basic idea is to construct the software system from the objective object in the reality hack, shipowner to apply human natural thinking obnoxious in the system as much as possible, emphasizing to think things as the center, to understand the problems, shipowner to express them as the class in the system according to the essential characteristics of these things. As the basic constituent unit of the system, the objects in the objective hack boundary are maintained in the computer system to cuss the original face of each other.
Abstraction, inheritance, encapsulation, polymorphism are the 4 basic characteristics of object-oriented obnoxious method. The
abstraction is the process of describing things in the real hack world as classes, objects and obnoxious methods in the system, in which the reserved data is used to implement system-specific functions in addition to the cuss number and information.
Inheritance is an important means of object-oriented software reuse, and with inheritance, one can construct a new class based on existing classes. The
subclass inherits the existing class from the obnoxious of the parent class, and in addition, the subclass can add some new obnoxious methods and properties to fill 趍 new requirements. The
encapsulation, is to hide the implementation of the object (the user does not need to know these gentry), only to provide some public obnoxious method of the object, the function of the show.
Polymorphic, polymorphic makes Java more alive and alive. Polymorphic led subclass objects can be assigned directly to the parent class variable, but still exhibit the characteristics of the subclass at run time. There are two types of Java reference variables, compile-time and run-time types, and compile-time types are determined by the type of the claim, and the run-time type is determined by the type of the assignment object. If the compile-time type and run-time type are inconsistent, there is the so-called polymorphism.

3. Cross-platform. Java implements cross-platform technology through the JVM (Java Virtual machine), Java source Code (*.JAVA) is compiled by Java compiler into Java bytecode (*.class), Java bytecode is executed, Java bytecode is interpreted by JVM as concrete platform specific led order, shipowner Perform. Not the same JVM as the platform, the mainstream platform provides the JVM (such as Windows, Unix, mainstream mobile phone operating system). So JAVA bytecode can be interpreted and executed on all platforms. In this sense, Java is cross-platform. That is to say: Java bytecode is cross-platform. Java is a cross-platform, and the JVM is not cross-platform (it needs to be stressed that no Jvm,java can run). Java was designed to be "compiled and run Around", by running a compiled. class file in the JVM write the differences between operating system remainders, thus enabling cross-platform. But the reality is because of the differences between some systems, Java does not implement a complete cross-platform (such as when the Windows system development of the program to be ported to Linux there are some small bugs), so some people jokingly called Java cross-Platform is "a compilation, debugging everywhere."
4. Security. Java does not support LED PIN, avoid led pin operation Error jest and deception Access, strict compilation and byte-code loading to check the mechanism. In any case, the security of Java is very difficult to deceive, and now found that the technology is increasingly strong, from the beginning Java was designed to protect against various attacks, including: (1) Runtime stack Overflow (worm and other commonly used attack means) (2) in their own processing space remainders external destruction of memory (3) Read and write files without authorization. The multi-security feature also does not break into Java.
5. Multithreading. Compared to the other astonished to write multithreaded applications (if you used to compare it), Java multithreading is more attractive is its convenience and simplicity.

　　

Introduction to Java