is a high-level programming language launched by SUN in 1995. It is an Internet-oriented language and the preferred language for WEB applications (the bottom layer of Android, the big data hadoop framework is written in java, and Spark is written in Scala Written in Scala (written in Java), (relative to other languages) it is easy to learn, safe and reliable, completely object-oriented, and cross-platform (operating system, completely ignore the operating system, and can be used on any operating system after writing).
The concept of virtual machine is introduced in Java, which adds an abstract layer of virtual machine between the machine and the compiler. This virtual machine provides a common interface to compilers on any platform. The compiler only needs to be oriented to the virtual machine, generate code that the virtual machine can understand, and then the interpreter will convert the virtual machine code into machine code for a specific system for execution. In Java, this kind of code for the virtual machine to understand is called bytecode (ByteCode) (the content of the class file). It is not oriented to any specific processor, only to the virtual machine. The interpreter for each platform is different, but the virtual machine implemented is the same. The Java source program is compiled by the compiler and becomes bytecode. The bytecode is interpreted and executed by the virtual machine. The virtual machine sends each bytecode to be executed to the interpreter, and the interpreter translates it into a machine on a specific machine. code and then run it on a specific machine.
Principle of cross-platform
In other words, in Beijing, people generally speak Beijing dialect. In Shanghai, people generally speak Shanghai dialect, Guangdong, and Guangdong. If so...
An official document has been issued and needs to be implemented nationwide. What should we do? ——First translate into Mandarin. Various places are translating the Mandarin version into local dialects.
Here, Beijing and Shanghai are different types of machines, windows, linux...
Compilation (javac) is the process of translating official documents into Mandarin, and the compiled .class file is the Mandarin version of the official document. Version.
During execution, the translator in various places is jvm, which is responsible for converting .class into a dialect that can be understood by the local area for execution.
Java Virtual Machine---Java Virtual Machine---JVM, different Java virtual machines have been developed for different operating systems. A Java program does not run directly on the computer but runs On the Java virtual machine, the Java virtual machine translates Java programs into commands that the current system can recognize. The Java language is cross-platform, but the Java virtual machine is not cross-platform.
J2SE---Standard Edition
J2EE---Enterprise Edition
J2ME---Mobile Edition (Some people used it in the early days , almost no longer used now)
JVM---The premise that Java can be cross-platform
JRE---Java Runtime Environment---Java Runtime environment---JVM+core class library
JDK---Java Development Kit---Java Development Kit---Development Tools+JRE
java.sun.com
www.oracle.com
In 1994, it was called Oak---Oak
and later changed to Java---Java
JDK1.0(-JDK1.1 -1.2 (Applet/swing---GUI was later eliminated) -1.3-1.4-1.5 (JDK5.0, ten features)---JDK6---JDK7.0---JDK8 (can also be called 1.8/ten Features) By 1.8, there are more than 3,000 classes and interface methods
(75 after Jdk1.7.0_75 means 75 updates, no matter which version is installed, try not to use the first 50 minor versions)
C:\\Program Files(X86)
Note: Do not have spaces or Chinese characters in the installation path!
Compile Java The process of translating the code into a language that the computer can understand
Note:
1. The file name of the class file corresponds to the class name
2.After each class is compiled, it will Generate a class file
3. The class name of the public class must be consistent with the file name of the Java file
4. The entry point for the Java program to run is the main function. Without the main function, the Java program can be compiled
refers to the amount of some operating parameters specified for the operating system
(1)New->Variable name "JAVA_HOME", variable value "C:\Java\jdk1.8.0_05" (that is, the installation path of JDK)
(2) Edit -> Variable name "Path", add ";" at the end of the original variable value %JAVA_HOME%\bin;%JAVA_HOME%\jre\bin”
(3) New->Variable name “CLASSPATH”, variable value “.;%JAVA_HOME%\lib;%JAVA_HOME%\lib\dt.jar ;%JAVA_HOME%\lib\tools.jar”
refers to words that have been given special meanings in Java
There are 53 keywords in total, 51 of them are in use, and 2 are not currently used---const, goto---reserved words
Keywords used to define data types | |||||||
class |
interface |
byte |
short |
int |
long |
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float |
double |
char |
##boolean | ##voidenum | |||
false | null | ||||||
else | switch |
case |
default | while | |||
for | break | ##continue |
return |
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Keywords used to define access modifiers | ||
private | |||||||
protected |
##public | is used to define classes, functions, and variable modifiers Keywords | |||||
abstract | |||||||
final | static | synchronized | ##Keywords used to define relationships between classes |
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extends | |||||||
Keywords used to define the creation and judgment of instances | |||||||
new | |||||||
super | instanceof | Keywords used for exception handling | |||||
try | |||||||
finally | ##throw |
throws |
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Keywords used for packages | |||
import | |||||||
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##Other modifier keywords |
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strictfp | |||||||
transient | volatile | assert |
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