2.2- data type

Constant

Definition

• In the process ⾏ In the process, its value cannot be sent ⽣ change
• Constant means we ⽣ Living ⽤ The numerical , word ⺟ etc.
• Constant ⽆ The law is changed ,⼀ Dan change , Its meaning is also changed

classification

• word ⾯ Value constants
• ⾃ Define constants (⾯ Learn again from the object part )

word ⾯ Classification of value constants

• character constants : ⽤ The content enclosed in single quotation marks ‘a’
• String constant : ⽤ What is enclosed in double quotation marks “abc” “ufc”
• Integer constant : All integers 1 0 3
• ⼩ Number constant : be-all ⼩ Count 1.2
• Boolean Literals : ⽐ More special , Only true and false
• Null constant : null( The array section will talk about )

System.out.println(" Character type constant :" + 'a');
System.out.println(" String type constant :" + "abc");
System.out.println(" Integer constant :" + 1);
System.out.println(" Floating point type constants :" + 12.32);
System.out.println(" Boolean type constants :" + true + "\t" + false);

Variable

Definition

• In the process ⾏ In the process of , Within a certain range, you can send ⽣ change
• in other words , Its value can be changed , At different times , Same as ⼀ Variables can represent different values
• With variables , We make ⽤ Just get up ⾮ Always flexible
• In computer memory ⼀ Block storage space , Is the basic unit for storing data

technological process

• Declare variables first , Then assign a value to the variable

  ​ Statement ⼀ A variable

  ​ data type Variable name ;

  ​ Assign values to declared variables

  ​ Variable name = value

• Declare the entry of variables ⾏ The operation of assignment

  data type Variable name = value

• Declare multiple variables of the same type and initialize at the same time – It can be disassembled
  data type Variable name 1, Variable name 2, Variable name 3, Variable name 4 = value

• The first ⼀ The second assignment operation is called initialization

    public static void main(String[] args) {
    
        int n;  // Statement int Variable of type ( Only integer ), Unassigned 
        n = 10;   // To the variable n assignment 
        int m = 20;   // Statement int Variable of type , assignment 
        int x = 1, y = 2, z = 3;  // Declare more than one int Variable of type 
        int a, b, c = 10;
    }

data type

byte 、 character 、 Character set and encoding method

• Bytes make up characters , Characters are the data we can see
• A character set is a set of characters
• The encoding method is the rule for characters to form a character set

Common coding methods

• ASCII code ：⼀ Ge Ying ⽂ word ⺟（ Regardless of ⼤⼩ Write ） Occupy ⼀ Bytes of space ,⼀ Among them ⽂ Chinese characters occupy two bytes .⼀ individual ⼆ Hexadecimal number
  Word sequence , As ⼀ Digital units ,⼀ General 8 position ⼆ Hexadecimal number , Conversion for ⼗ Base number . most ⼩ value 0, most ⼤ value 255. Such as ⼀
  individual ASCII Code is ⼀ Bytes .

• UTF-8 code ：⼀ Ge Ying ⽂ The character is equal to ⼀ Bytes ,⼀ Among them ⽂（ Including traditional Chinese ） Equal to three bytes .

• Unicode code ：⼀ Ge Ying ⽂ Equal to two bytes ,⼀ Among them ⽂（ Including traditional Chinese ） Equal to two bytes .
  Symbol ： Britain ⽂ Punctuation ⼀ Bytes , in ⽂ Punctuation takes up two bytes . give an example ： Britain ⽂ Full stop “.” Occupy 1 Bytes of ⼤⼩, in ⽂ Full stop “.” Occupy 2
  Bytes of ⼤⼩.

• GBK code ⽅ The formula is medium ⽂ Two bytes , Britain ⽂ Occupy 1 Bytes .

  The conversion relationship of information units between different orders of magnitude
  1B（byte, byte ）= 8 bit;
  1KB（Kibibyte, kilobytes ）=1024B= 2^10 B;
  1MB（Mebibyte, Megabytes , Megabytes , abbreviation “ mega ”）=1024KB= 2^20 B;
  1GB（Gigabyte, Gigabyte ,⼗ Gigabytes ,⼜ call “ Gigabit ”）=1024MB= 2^30 B;

Basic data type

Integer types ：

Create basic data type variables :

        byte b = 10;
        short s = 20;
        int i = 30;
        long l = 1000000000000L;
        double d = 12.22;
        float f = 5.1f;
        boolean b1=true;

Lack of precision

    public static void main(String[] args) {
    
        double a=1.0;
        double b=0.9;
        System.out.println(a-b);
        double c=(1.4-0.5)/0.9;
        System.out.println(c);
    }

Java The default of medium integer type is int , Floating point type defaults to double , double The accuracy of the ⽐float want ⾼, So if we're going to double Turn into float can
It can cause the problem of losing accuracy . So we're giving float When assigning a variable of type , want ⽤f Indicate the type of value

Use BigDecimal Deal with the lack of accuracy :

   BigDecimal b1=new BigDecimal("1.0");
    BigDecimal b2=new BigDecimal("0.9");
    System.out.println(b1.subtract(b2));

Character type

• char Two bytes 0~65535（ No negative number ）

• ASCII： American Standard Code for information exchange

• Unicode： In the field of computer science ⼀ Industry standard , Include character set .

public class CharDemo {
    
    public static void main(String[] args) {
    
        //Acsll code 
        char a = 97;
        System.out.println(a);
        //Unicode
        char b='\u4f01';
        System.out.println(b);
    }
}

Boolean type

Type conversion

Automatic type conversion

 Assign a value or variable representing a small data range to another variable representing a large data range 

Cast

 Assign a value or variable representing a large data range to another variable representing a small data range 
 Format ： Target data type      Variable name =（ Target data type ） Values or variables ;
 Example ：int k =(int)88.88;

public class TypeTranslateDemo {
    public static void main(String[] args) {
        // Automatic conversion 
        byte b = 10;
        int i = b;
        System.out.println(i);

        // Strong go 
        int x = 5;
        byte y = (byte) x;
        System.out.println(y);
    }
}