Concept and understanding of variables

1. Variable

1.1 Concept and basic understanding of variables

The concept of a variable is a storage unit , For type reference , A variable refers specifically to a definite value

The variables are （ data type Variable name value ） form

int i = 15;

plastic Variable name value Declaration and assignment of variables

java It's a strongly typed language , The type of variable must be consistent with the type of data .

1.2 Declaration and assignment of variables

For variables, first declare them in the assignment , Is the basic operation of variables ,

int i ;

data type Variable name

i = 15;

Variable name value

Declare and assign values

int i = 15;

plastic Variable name value Declaration and assignment of variables

Declaration and assignment of multiple variables of the same type

data type Variable 1 , Variable 2 , Variable 3 = value 3 , Variable 4 , Variable 5 = value 5;

int i , o , p=1,s , f=5;

2 Java Data type of

Basic data type ： Integers decimal Boolean character

Reference data type ： character string Array object

Eight data types : byte short int long float double boolean char

1 byte 2 byte 4 byte 8 byte 4 byte 8 byte 1 byte 2 byte

8 position 16 position 32 position 64 position 32 position 64 position 8 position 16 position

The value range of the integer is minus the power of the positive and negative bits of the corresponding data type

such as byte The range of phi is zero Eight minus one power of two That's the seventh power of two Plus or minus -128~127 And so on

int The default form of a bit integer , If necessary long Type assignment You need to add... To the end of the value L ( Uppercase or lowercase Recommended capitalization )

1G=1024MB

1MB=1024KB

1KB=1024byte 1byte It's just a byte So a byte is 8 position

1byte=8bit 8bit intend 8 position

Floating point types have a large range

Floating point numbers are represented by scientific counting ：

2E3 Equivalent to 2 * 10 ^ 3（ result ：2000.0）3E5 Equivalent to 3 * 10 ^ 5（ result ：300000.0）

Be careful ：double Is the default type of floating point numbers , If necessary float Type assignment , You need to append... After the value “F”

boolean Boolean data type There are only two value ranges True or False describe “ really “ or ” false “

java Of boole Not involved in arithmetic operations

char Data type two bytes , The value range is 0~65535 char Data from Common letter Numbers And Chinese , And the languages in the world code

however char Only one value can be declared at a time

for example ：char ch1 = '2'; correct

char ch1 = ‘22’; error

char ch1 = 'a' 'A' 65 ' in ' It's all right But only one quantity can be declared at a time

3 Escape character

A newline \n You can wrap lines in any output statement amount to System.out.println()

tabs \t You can control spaces and align them up and down in the output statement

Backslash \\ Single quotation marks \’ If you want to output separately, you must insert an escape character before you can output But double quotation marks are optional

4 Reference data type

String Value range In any ” “ The middle value is OK

String str1 =” Hello “

String str1 =” The number “

String str1 =”helo“