Java
Is the 1995 Sun Company launched a high-level programming language, is the Internet-oriented language, the language of choice for Web applications (Android bottom, Big Data Hadoop framework written in Java, Spark in Scala, Scala written in Java), (compared to other languages) easy to learn , safe and reliable, fully object-oriented, cross-platform (operating system, completely ignore the operating system, after writing any operating system can be used).
The principle of cross-platform
Java VM---Java Virtual machine---JVM, for different operating systems, the development of different Java virtual machines, a Java program is not directly run on the computer, but run on the Java Virtual machine, A Java Virtual machine translates a Java program into a command that the current system can recognize. The Java language is cross-platform, but the Java virtual machine is not cross-platform.
The technical structure of Java
J2SE---Standard Edition
Java EE---Enterprise Edition
J2ME---Mobile (early for someone, now almost no)
Jdk/jre/jvm
JVM---The premise that Java can cross the platform
JRE---Java Runtime environment---Java Run-time environment---jvm+ Core class Library
JDK---Java Development kit---Java SDK---development tools +JRE
Java.sun.com
Www.oracle.com
1994 Oak---Oaks
Java---After change
JDK1.0 (-jdk1.1-1.2 (applet/swing---GUI later eliminated) -1.3-1.4-1.5 (JDK5.0, 10 features)---JDK6---JDK7.0---JDK8 (can also be called 1.8/ 10 properties) to 1.8 there are more than 3,000 classes and interface methods.
(jdk1.7.0_75 back 75 means update 75 times, regardless of which version is installed, the first 50 smaller versions try not to use)
C:\\Program Files (X86)
Note: Do not appear in the installation path space and Chinese!
Compile
The process of translating Java code into a language that a computer can read
Attention:
1. The file name and class name of the class file correspond to
2. A class file will be generated after each type of compilation is completed
3. The class name of the public class must match the file name of the Java file
4. The Java program runs the portal is the main function, there is no main function, the Java program can compile
Environment variables
Refers to the amount of some running parameters specified for the operating system
(1) New variable name "Java_home", Variable Value "C:\Java\jdk1.8.0_05" (that is, the JDK installation path)
(2) Edit, variable name "Path", at the end of the value of the original variable, add ";%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 "
Key words
Refers to words that are given special meaning in Java
There are 53 keywords, 51 of them are in use, and 2 are not currently used---const, goto---reserved words
Keywords to define the data type |
Class |
Interface |
Byte |
Short |
Int |
Long |
Float |
Double |
Char |
Boolean |
void |
Enum |
Keywords to define data type values |
True |
False |
Null |
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Keywords to define Process Control |
If |
Else |
Switch |
Case |
Default |
While |
Do |
For |
Break |
Continue |
Return |
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Keywords used to define access rights modifiers |
Private |
Protected |
Public |
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Keywords for defining classes, functions, variable modifiers |
Abstract |
Final |
Static |
Synchronized |
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Keywords used to define the relationship between classes and classes |
Extends |
Implements |
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Used to define the keywords that create instances and judge instances |
New |
This |
Super |
instanceof |
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Keywords for exception handling |
Try |
Catch |
Finally |
Throw |
Throws |
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Keywords for the package |
Package |
Import |
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Other modifier keywords |
Native |
Strictfp |
Transient |
Volatile |
Assert |
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Note: All keywords are lowercase!
Identifier
Names that are customized in the program
Definition rule: 5 points
1 can be composed of letters, numbers, _, $---do not recommend the use of the $ symbol
2 cannot use pure numbers, numbers cannot start with
3 Cannot use keywords
4Java is a strictly case-sensitive language
5 in order to improve the reading of the program, as far as possible to see the name of understanding
Chinese naming is supported, but not recommended
Class a{}
Class a{} can write like this, but Windows does not allow Windows to be case-insensitive
Hump 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 is lowercase, and the remaining words are capitalized playGame
Package Name
If it consists of multiple words, all letters are lowercase and used in the middle. Separated
Constant name
If it is made up of multiple words, all letters are capitalized, separated by _
Comments
To interpret or explain a program's text in a program.
Format
Comment Text line comment
/* Comment Text */multi-line Comment
/** Comment Text */Document comment---often used for annotation classes, methods, or constants
javadoc-d. \\document Demo.java---This command can only be used to extract the contents of a document comment in a public class
Learn about the fundamentals of Java