1 comment
Writing comments is a very good habit.
Single-line comments//
Multi-line comments
/*
Text
/
Document comments/ *
*Document comments
*/
Some interesting code comments
2Identifiers and keywords
All identifiers should start with a letter (AZ or az), dollar sign $, or underscore _.
After the first letter, any combination of letters , dollar signs , underscores, or numbers
can be used. Keywords cannot be used as variable names or method names.
Examples of legal identifiers: age, $salary, _value, __1_value
Examples of illegal identifiers: 123abc, -salary, #abc
Java identifiers are case-sensitive and
can be named in Chinese, but generally not recommended or recommended Pinyin, very LOW
3 data types
Java is a strongly typed language: the use of variables is required to strictly comply with regulations, and all variables must be defined before they can be used.
Strongly typed languages are safe and slow.
VBs and JAVAs are weakly typed languages.
String string
int number
Java data type classification
Primitive type
Integer types: byte (one byte); Short 2 bytes; int four bytes; Long 8 bytes
floating decimal is: float 4 bytes; Double 8 bytes long
character : Char occupies 2 bytes. //About 2^16?
Boolean type: occupies 1 bit, its value is only true, false
Reference type
Class
interface
array
byte
Bit: the smallest unit of computer internal data storage, 11001100 is an 8-bit binary number
Byte byte: the basic unit of computer data processing, it is customary to use a capital letter B to represent
1B (byte) = 8bit
characters: letters and numbers used in computers , Word and symbol
1bit: 1 bit
1B (byte)=8b (bit)
1024B=1KB=2^10B
1024KB=1M=2^20B
1024M=1G=2^30B
4 Expansion of data types and explanation of interview questions
It’s best to avoid using floating-point numbers to compare.
All characters are essentially numbers.
5 type conversion
Since Java is a strongly typed language, type conversion is needed when performing some operations.
Capacity: low…………………………high
byte, short, char->int->long->float-> In double (decimal priority is greater than integer)
operation, different types of data are converted to the same type, and then calculated
6 variables, constants, scope
6.1 Variables
A variable is a memory space.
Java is a strongly typed language, and each variable must declare its type.
Java variable is the most basic storage unit in the program. Its elements include variable name , variable type and scope
type varName [=value] [{, varName[=value]}]
//Data type variable name=value, you can declare multiple variables of the same type separated by commas.
note:
- Every variable has a type, and the type can be a basic type or a reference type.
- The variable name must be a legal identifier
- A variable declaration is a complete statement, so every declaration must end with a semicolon
6.2 Scope of variables
Class variable: write at the end of the class, keyword Static
instance variable: write in the middle of the class, no Static
local variable: write in the method
6.3 Constant
Constant: The value cannot be changed after initializing (initialize)! Unchanging value The
so-called constant can be understood as a special variable. After its value is set, it is not allowed to be changed during the running of the program.
Final constant name = constant value;
final double PI = 3.14;
constant names are generally capitalized character
6.4 Naming conventions for variables
- All variables, methods, and class names: see the name know what it means
- Class member variables: first letter lowercase and camel case principle: monthSalary Except for the first word, the first letter of the following words is capitalized lastname-lastName
- Local variables: lowercase initials and camel case
- Constant: uppercase letters and underscore: MAX_VALUE
- Class name: initial capitalization and camel case principle: Man, GoodMan
- Method name: lowercase initials and camel case principle: run(),runRun()
7 basic operators
Arithmetic operators: ± /,% (modulo operation: take the remainder), ++,-
assignment operators: =, a=10 (assign 10 to a)
Relational operators: >,<,>=,<=, ==,!= (not equal to), instanceof
logical operators: && and, || or,! Non-
bit operator: &,|,^,~,>>,<<,>>> (understand!!!)
Conditional operator:? :
Extended assignment operator: +=, -=, =, /=
priority()
8Increase and decrease operators, first acquaintance with the Math class
9 logical operators, bit operators, conditional operators
&& short-circuit operation
10 Three original operators and summary
11 pack mechanism
In order to organize classes better, Java provides a package mechanism, which is used to distinguish the namespace of the class name.
The syntax format of the package statement is:
package pkg1[.pkg2[.pkg3…]]
com.kuang.operator
generally uses the company domain name inversion as Package name
In order to be able to use a member of a package, we need to explicitly import the package in the Java program. Use the import statement to complete this function
import package1[.package2…].(classname|*);
12JavaDoc generates documentation
The JavaDoc command is used to generate your own API documentation
@author Author name
@version Version number
@since Indicate the jdk version that needs to be used earliest
@param Parameter name
@return Return value situation
@throws Exception thrown situation