Java basics and java

Java basics and java
1. Some simple doscommands:

-D: Switch the carriage return drive.

-Dir (directory): lists the files and folders in the current directory.

-Del: delete an object

-Md: Create a folder

-Rd: delete a folder

-Cd (change directory): change the specified directory (enter the specified directory)

• Go to the cd directory.
• Roll back cd...; cd \

-Cls: (clear screen) clear screen

-Exit: exit the doscommand line.

 

2. Basic data constants:

Basic data types: 4 categories, 8 Types
A: The integer occupies the number of bytes.

Byte 1
Short 2
Int 4
Long 8
B: Floating Point Number
Float 4
Double 8
C: character
Char 2
D: Boolean
Boolean 1

 

3. JVM, JRE, and JDK:

To put it simply:

JVM is a Java virtual machine.

JRE: Java Runtime Environment

JDK: Java Development Kit

 

JDK includes the runtime environment and development kit

JRE includes core class libraries and virtual machines

 

4. Implicit and forced conversions:

Forced conversion: Direct conversion from small to large

Example: int a = 1; byte B = 2; int c = a + B;

Implicit conversion: Small conversion is highly risky and mandatory.

Target type variable name = (target type) (converted data );

Example: byte B = 1; int I = 2; byte c = (byte) (B + I );

5. identifier:

Identifier: indicates the name of a package, class, method, or variable.


A: unicode characters
Numbers, English case and Chinese characters (Chinese characters are not recommended)
B: underline _
C: Dollar sign $

Note:
A: it cannot start with A number.
B: it cannot be a keyword in java.

Common naming rules:

A: basic requirements: click here
B: common naming rules
A: Package (actually a folder for class management), all in lower case, multi-level packages are separated.
Example: com, com. itheima
B: class. The first letter of a word is capitalized.
Example: Student, Car
Uppercase letters of each word for multiple words
Example: HelloWorld
C: Methods and variables
Lowercase letters of a word
Example: age, show ()
Multiple words start from the second word and the first letter of each word is capitalized.
Example: maxAge, getAge ()