Introducing the basic principles of Java

Java

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 and the big data hadoop framework) Written in java, Spark is written in Scala, Scala is written in java), (relative to other languages) simple and easy to learn, safe and reliable, completely object-oriented, cross-platform (operating system, completely ignore the operating system, after writing Any operating system can be used).

java is compiled and run through the java virtual machine.

The Java virtual machine is the general term for various commands for compiling and running Java programs and their operating environment. After compilation, the Java source program generates a file with the suffix ".class", which is encoded in bytecode. This bytecode is actually a pseudocode that contains various instructions that are basically platform-independent instructions. The Java virtual machine interprets these bytecodes on the basis of bytecode files (and files with a .class suffix generated by compilation), converts these bytecodes into machine codes of the local computer, and hands them to the local computer for execution.

In this way, bytecode is actually a kind of platform-independent pseudo-code, which is turned into machine code on various platforms through Java commands. These pseudocodes are ultimately run on the local computer platform, but the Java program seems to run on the basis of these Java commands, so the collection of these Java commands seems to be a virtual computer implemented using software technology. This is where the name Java Virtual Machine comes from.

The process of Java virtual machine executing bytecode consists of a loop. It continuously family programs, performs legality and security testing, and interprets execution until the program is executed (including abnormal exit). The Java virtual machine first loads the bytecode from the file with the suffix ".class" into the memory; then it detects the legality and safety of the code in the memory, for example, detecting whether the array used by the Java program is out of bounds and the memory to be accessed Whether the address is legal, etc.; then interpret and execute the code that passes the test, and convert the bytecode into machine code for the corresponding computer platform according to different computer platforms, and then hand it over to the corresponding computer for execution. If the loaded code cannot pass the legality and security detection, the Java virtual machine executes the corresponding Exception handling program. The Java virtual machine continues to execute this process until the program execution ends. Although the Java language contains compilation commands, the Java virtual machine's interpretation and execution mechanism of bytecode determines that the Java language is an interpreted and executed language.

Cross-platform principle

Java Virtual Machine---Java Virtual Machine---JVM, different Java virtual machines have been developed for different operating systems. A Java program is not directly It runs on the computer but on the Java virtual machine. The Java virtual machine translates the Java program into commands that the current system can recognize. The Java language is cross-platform, but the Java virtual machine is not cross-platform.

Technical Structure of Java

J2SE---Standard Edition

J2EE---Enterprise Edition

J2ME---Mobile Edition (Some people used it in the early days , almost no longer used now)

JDK/JRE/JVM

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

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

Environment variables

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", in the original variable value Add ";%JAVA_HOME%\bin;%JAVA_HOME%\jre\bin" at the end
(3) New -> Variable name "CLASSPATH", variable value ".;%JAVA_HOME%\lib;%JAVA_HOME% \lib\dt.jar;%JAVA_HOME%\lib\tools.jar”

Keyword

means that it is given a special meaning in Java word

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 ifelseswitchdoKeywords used to define access modifiersprivateKeywords used to define classes, functions, and variable modifiersabstractKeywords used to define the relationship between classesextends is used for definition Keywords for creating instances and determining instancesnew##instanceofcatchfinallyimportstrictfp

##Keywords used to define data types
class	interface	byte		short	int		long
float	double	char		boolean	void		enum
Keywords used to define data type values
true	false	null
process control
		##case		## default	while
for	break	continue		return
protected	public
final	static	synchronized
implements
##this	super						Keywords used for Exception handling
try
	##throw	throws					Keywords used for packages
package
							Other modifier keywords
native
transient	volatile	assert

##Note: All keywords are in lowercase!

Identifier

The name customized in the program

Definition rules: 5 points

1 can be composed of letters, numbers,_,$- --It is not recommended to use the $ symbol

2You cannot use pure numbers, and numbers cannot be used as the beginning

3You cannot use keywords

4Java is a strictly case-sensitive language

5 In order to improve the readability of the program, try to be familiar with the name

Supports Chinese naming, but is not recommended

class A{}

class a{} can be written like this, but Windows does not allow it. Windows is not case-sensitive

Camel case naming method

Class name/interface name

If it consists of multiple words, The first letter of each word is capitalized HelloWorld

Variable name/method name

If it consists of multiple words, the first letter of the first word is lowercase, and the first letter of the remaining words is capitalized playGame

Package name

If it consists of multiple words, all letters must be lowercase and separated by .

Constant name

If it consists of multiple words, All letters are in capital letters, separated by _

Comments

Text used to explain or explain the program in the program

Format

//annotation text Single-line comments

/*&*/ Multi-line comments

/**&*/ Documentation comments---are often used to annotate classes, methods or constants

javadoc -d .\\document Demo.java---This command can only be used to extract the content of document comments in public classes

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