Java series (3)-basic syntax and java syntax

Java series (3)-basic syntax and java syntax

1. Keywords

Features: All lowercase letters

2. identifier

(1) It is the character sequence that names such as class, interface, and method.

(2) composition rules:

   
A: uppercase/lowercase letters
    
B: Number
    
C: $ and _
 

(3) considerations:

   
A: it cannot start with A number.
    
B: it cannot be a keyword in java.
    
C: Case Sensitive
 

(4) common naming rules

   
A: The packages are all in lower case and are separated by multi-level packages.
    
B: class or interface: uppercase letters of each word
    
C: method or variable: lowercase for the first letter, and uppercase for the second word
    
D: constant: all uppercase. Multiple words must be separated by _ (STUDENT_MAX_AGE)
 

3. Notes

(1) category:

   
A: When the line comment ://
    
B: multi-line comment :/**/
    
C: document notes:
 

(2) functions of Annotations:

   
A: Explain the program and improve the code reading.
    
B: It can help us debug the program.
 

4. Constants

(1) The value of the program remains unchanged during execution.

(2) category:

   
A: literal value constant
    
B: Custom Constants
 

(3) literal Constants

   
A: String constant
    
B: integer constant
    
C: decimal constant
    
D: character constant
    
E: Boolean constant
    
F: null constant, null
 

(4) java provides four representations for Integer constants

   
A: binary (starting with 0 B)
    
B: octal (starting with 0)
    
C: decimal (the default integer is decimal)
    
D: hexadecimal (starting with 0x)
 

5. hexadecimal conversion

(1) convert others to decimal

(2) convert decimal to other hexadecimal

(3) Fast Conversion Method of hexadecimal conversion

   
A: The conversion between decimal and binary. The code is 8421.
    
B: Binary and octal, hexadecimal conversion
 

6. Variables

(1) amount of changes that can occur within a certain range during Program Execution

(2) Definition Format of variables:

   
A: data type variable name = initialization value
    
B: data type variable name;
    
Variable name = initial value
 

7. Data Type

(1) java is a strong type language that provides a corresponding data type for each type of data

(2) category:

   
A: basic data types: 4 categories, 8 Types
    
B: Reference Data Type: Class, interface, array
 

(3) Basic Data Types

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

8. Data Type Conversion

(1) The boolean type is not involved in conversion.

(2) Default conversion:

   
A: from small to big
    
B: byte, short, char → int → long → float → double
    
C: byte, short, and char are not converted to each other and are directly converted to the int type during calculation.
 

(3) forced conversion:

   
A: from big to small
    
B: It may cause loss of precision. It is generally not recommended.
    
C: Format: Target data type variable name = (target data type) (converted data)
    
[Example]