java concept

Java is an object-oriented programming language that can be used to write cross-platform applications. . Simply put, Java is a language, but it is a language for jvm. Java is compiled into bytecodes, and jvm will interpret these bytecodes into machine codes and then take corresponding actions.

java virtual machine

Java Virtual Machine (JVM) is the abbreviation of Java Virtual Machine, which is built on hardware and operating system, using hardware or software means Implemented virtual computer.

The Java virtual machine has its own complete hardware architecture, such as processor, stack, registers, etc., and also has a corresponding instruction system. A virtual computer system that can run Java bytecode and has an interpreter component that enables communication between the Java bytecode and the computer operating system.

Java's garbage collection mechanism

GC, the garbage collection mechanism, refers to the memory used by the jvm to release the memory occupied by objects that are no longer used. In C/C, programmers need to manage memory by themselves. Java provides a system-level thread to track memory allocation. Memory areas that are no longer used will be automatically recycled. In different JVM implementations, the garbage collection mechanism is very different. For an object to be recycled, all references to it must be removed, usually by setting it to NULL. However, even with a garbage collection mechanism, memory overflow may still occur.

java running environment setup

JAVA_HOME

C:\Java\jdk1.7.0_55

PATH

%JAVA_HOME%\bin;%JAVA_HOME%\jre\bin;

CLASSPATH

.;%JAVA_HOME%\lib\dt.jar;%JAVA_HOME%\lib\tools.jar

Advantages of java

1. Platform independence

Platform independence means that Java can Runs on different platforms. Java introduces the principle of virtual machine and runs on the virtual machine to implement Java interfaces on different platforms. Programs written in Java can be shared around the world. A virtual machine is a software package used to interpret Java instructions, allowing Java to run on any machine, such as a virtual machine software package running on Mac or UNIX.

2. Security

Java programming is similar to C. Readers who have studied C will quickly grasp the essence of Java. Java abandons the direct operation of C pointers on memory addresses. When the program is running, the memory is allocated by the operating system, which can prevent viruses from invading the system through pointers. Java provides a security manager for programs to prevent illegal access to programs.

3. Object-oriented

Java absorbs the object-oriented concept of C, encapsulates data in classes, and uses the advantages of classes to achieve program simplicity and simplicity. Ease of maintenance. The object-related features such as class encapsulation and inheritance allow the program code to be compiled once and then used repeatedly through the above features.

4. Distributed

Java is built on the extended TCP/IP network platform. Library functions provide methods for transmitting and receiving information using HTTP and FTP protocols. This makes it as easy for programmers to work with files on the network as with local files.

5. Interpreted type

We know that languages ??such as C and C are compiled for CPU chips to generate machine code. The operation of this code is consistent with specific CPU related. Unlike C or C, Java does not compile for the CPU chip, but compiles the program into a kind of "intermediate code" called bytecode. Bytecode is a file that is very close to machine code and can be interpreted and executed on any system that provides a Java Virtual Machine (JVM).

6. Dynamic

The basic unit of Java program is class. Some classes are written by ourselves, some are introduced from class libraries, and classes are It is dynamically loaded at runtime, which allows Java to dynamically maintain programs and categories in a branch environment. Unlike C, after the class library is not scheduled to be upgraded, if you want the program to have the functions provided by the new class library, you need to Modify the program and recompile.

Use of java

1. Network application development, such as: online banking (Bank of China, China Construction Bank, Industrial and Commercial Bank of China's online banking are all developed using J2EE Yes, you can tell by looking at the suffix JSP of his online banking web page), web games, etc.

Enterprise management software, many of the now popular B/S (browser/server) architecture management programs are developed using J2EE.

3. Mobile games, have you seen the .jar extension of those mobile game installation files? This is the logo developed by JAVA. JAVA ME (J2ME) is generally used to develop these things.

4. Embedded development, such as writing programs for TV set-top boxes and small home appliances. Java se (j2se) is a foundation. If you want to rely on JAVA to make a living, then you should choose one of J2EE and J2ME to learn in depth. If you want to learn network programming, learn J2EE. If you want to learn mobile phone programming, if you want to learn embedded programming, learn J2ME. However, there are generally more job opportunities in J2EE.

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