4. constant

Constant: refers to the fixed data in the Java program.

classification

type	meaning	Data examples
integer constant	integer	Positive and negative integers
Decimal constant	decimal	Positive and negative decimals
character constants	Enclosed in single quotation marks, there is only one character and cannot be empty	'word', 'a', '1'
string constant	Enclosed in double quotation marks, it can have multiple characters and can be empty	"String", "123", "abc", "“
Null constant	There is only one value	true,false
Boolean Literals	There are only two values	Null (not printable directly)

5. Variable and data type (variable)

5.1 data type

5.1.1 classification

Basic data types: integer, floating point, character, Boolean

Reference data types: string, array, interface, enumeration, annotation

1) Basic data type:

Integer, floating point, character, Boolean

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The default type in Java: integer type is int and floating point type is double

2) Character type

The range of integers that a single byte can represent is: - 128 ~ 127

float defaults to 7 significant digits

double defaults to 15 significant digits

ASCII coding table: all English letters, numbers and symbols are corresponding to the decimal system, so the world's first coding table ASCII is produced(
American Standard Code for Information Interchange.

character	numerical value
0	48
9	57
A	65
Z	90
a	97
z	122

**Unicode code table: * * universal code. It is also a comparison between numbers and symbols. The beginning 0-127 is exactly the same as ASCII, but it contains more characters from 128.

(Unicode is a universal fixed length character set in the world, and all characters are 16 bits)

3) Escape character

\"--------"

\'--------'

\\--------\

\t -------- tab

\n -------- line feed

5.1.2 data type conversion

1) Automatic conversion type (implicit)

Features: the code does not need special processing and is completed automatically.
Rule: data range from small to large (byte < short < char < int < long < float < double)

//On the left is the long type. On the left is the int integer by default. The left and right are different
//Indicates that the int constant 100 is assigned to the long variable for storage
//Automatic type conversion occurred
long num=100;
//Similarly, in accordance with the rule from small to large, automatic type conversion is carried out
double num1=1.55F;
/*
byte<short<char<int<long<float<double
*/
float num2=1.52L;

2) Cast type (explicit)

Features: the code needs special format processing and cannot be completed automatically.

Format: type with small range variable name with small range = (type with small range) data with large range;

format

Format: type with small range variable name with small range = (Type with small range) Originally a large range of data;

//Compilation failure will occur
//On the left is an int integer variable, and on the right is a floating-point variable of double type by default
//The value range of double type on the right is larger than that of int integer, so automatic type conversion cannot be performed
int i=1.5;
//Cast type
int i=(int) 1.5;

//Type int on the left and type long on the right
//Assign long type to int 		 byte<short<char<int<long<float<double		
//From large to small, automatic data type conversion cannot occur
int num=(int) 100L;

int num=(int)3.99;	//All decimal places will be discarded and output 3

matters needing attention

1. Cast is generally not recommended because it may cause precision loss and data overflow.
2. byte/short/char these three types can be used for mathematical operations, such as addition "+"
3. byte/short/char will be promoted to int type before calculation.
4. Data type conversion cannot occur for boolean type

byte<short<char<int<long<float<double

1. Floating point data calculation may have errors. If you want to realize accurate operation, you can use Java math. BigDecimal type

System.out.printf(0.1+0.2);		//0.3000 0000 0000 0000 4 

5.2 variables

5.2.1 concept

Variable: the amount of content that can be changed during program operation.

5.2.2 format:

Data type variable name = data value

//First declare a variable and then assign a value
int num1;
num1=5;
//Or one step
int num2=10;

5.2.3 precautions:

1. If you create multiple variables, the names between variables cannot be repeated.

2. For float and long types, don't throw away the letter suffixes F and L.

//Define single precision variable
float f=1.5F;
//Define long integer variables
long l=1.123456789L;

1. If you use a variable of type byte or short, the data value on the right cannot exceed the range of the type on the left.

2. There are variables for assignment, which cannot be used directly; It must be assigned before it can be used.

3. Variable usage cannot exceed the scope.

   Scope: from the line defining the variable to the end of the curly bracket to which it belongs.

4. You can create multiple variables with one statement, but it is generally not recommended.

6. Operator

6.1 arithmetic operators

operator	meaning
+	Addition operation
-	Subtraction operation
*	Multiplication
/	Division operation (quotient only, no remainder)
%	Modular operation (only remainder, no quotient)
++ --	Self increasing and self decreasing operation

matters needing attention:

1. If two integers are divided, only the integer part will be retained and the decimal part will be discarded

   System.out.println(5/2);	//The result is 2, and the decimal part will be discarded
   

2. If you want to keep the decimal part of the operation result, use the cast type to convert one of the operands to the double type for the operation

   System.out.println((double)5/2);	//2.5
   System.out.println(5/(double)2); 	//2.5
   System.out.println((double)5/(double)2);	//2.5
   

3. Neither 0 nor 0.0 can be divisor

4. byte+byte=int: (note)

   byte b1=10;
   //b1=b1+2; 	 byte+int=int; For incompatible types, conversion from int type to byte may cause precision loss
   //b1=b1+(byte)2; 	 byte+byte=int; Due to the optimization of the compiler, the operation result is automatically converted to int type
   b1=(byte)(b1+2)	//Cast type
   b1+=2;	//Really equivalent to b1=(byte)(b1+2)	
   //+=Self contained strong rotation
   

1) Self increasing and self decreasing operation (can only be used for variables, not constants)

Independent operation: + + i is no different from i + (the same principle of self subtraction)

Hybrid operation:

Variable i + +: first assign the value of a to b, and then add 1 to variable a. The final result is a=2, b=1 * * (use first and then add)**

public static void main (String[] args) {
    int a=1;
    int b=a++;
    System.out.println(a);//2
    System.out.println(b);//1
}

Variable + + i: add 1 to variable a, and then assign a to b. The final result is a=2, b=2 * * (add first and then use)**

public static void main (String[] args) {
    int a=1;
    int b=++a;
    System.out.println(a);//2
    System.out.println(b);//2
}

2) The plus sign "+" in the four operations has three common uses:

1. For numerical values, that is addition.

2. For character char type, char will be promoted to int before calculation.

   Comparison table between char type characters and int type numbers: ASCII and Unicode

3. For the String string (capitalized, not keyword), the plus sign represents the String concatenation operation.
   When any data type is connected with a string, the result will become a string

String str2 = "Java";
		// String + int --> String
		System.out.println(str2 + 20); //Java20

6.2 assignment operator

operator	meaning
=	be equal to
+=	Acceleration (with forced rotation)
-=	Subtraction
*=	Wait by
/=	Divide and so on
%=	Mold taking, etc

a+=1-------->a=a+1
b-=2-------->b=b-2
c*=3-------->c=c*3
d/=4-------->d=d/4
e%=5-------->e=e%5

6.3 comparison operators

operator	meaning
==	Equal to [two equal signs are equal, and one equal sign represents assignment]
>=	Greater than or equal to
<=	Less than or equal to
>	greater than
<	less than
!=	Not equal to

matters needing attention:

1. The result of the comparison operator must be a boolean value. If it is true, it is false
2. If you make multiple judgments, you can't write in succession.
   (in mathematics, for example, 1 < x < 3 [not allowed] in the program.)

6.4 logical operators

operator	meaning	remarks
&&	And (and)	If it is all true, it is true, otherwise it is false
||	Or (or)	At least one true is true; It's all false. It's false
!	Non (negative)	Originally true, it becomes false; It used to be false, but it became true

matters needing attention:

1. Logical operators can only be used with boolean values.

2. And, or need to have a boolean value on the left and right respectively, but the reverse can only have a unique boolean value.

3. And, or two operators. If there are multiple conditions, they can be written continuously.

Two conditions: condition a & & condition B

Multiple conditions: condition a & & condition B & & condition C

6.5 ternary operators

• Format:

  Data type variable name = expression ? Result 1 : Result 2;
  

• Calculation method

• technological process:

  First, judge whether the expression is true or false

  If it is true, assign the result 1 on the left to the variable

  If it is false, assign the result 2 on the right to the variable

  (choose one of the two)

  int num=1>2 ? 10:20;
  System.out.println(num);//20;
  
  int a=10;
  int b=20;
  int max=a<b ? a:b;
  System.out.println(max);//20
  
  int result = 3 > 4 ? 2.5 : 10; // Wrong writing!
  

  matters needing attention:

  1. Both expression A and expression B must meet the requirements of the data type on the left.
  2. The result of the ternary operator must be used.

6.6 shift operator (understand)

• < < shift left operator is used to move the binary bits of data to the left, supplemented by 0 on the right

• >>Shift right operator is used to move the binary bits of data to the right, and the left is supplemented by sign bits

• >>>Represents a logical shift right operator (unsigned shift right), which is used to move the binary bits of data to the right, and 0 is used on the left
  Supplement.

  byte b1=13;
  //The binary of 13 is: 0000 1101 = > the result of shifting one bit left is 0001 1010 (26)
  System.out.println(b1<<1);	//Moving one bit to the left is equivalent to multiplying by 2
  
  //Shift right: 0000 1101 = > the result of shifting right one bit is 0000 0110 (6)
  System.out.println(b1>>1)	//Moving one bit to the right is equivalent to dividing by 2
  
  byte b2=(byte)(byte<<1)；//To use strong rotation
  

6.7 bitwise operators (understand)

• &Indicates a bitwise and operator, which performs and operation according to binary bits. The same 1 is 1, and 0 is 0

    0000 1011
  & 0000 1101
  = 0000 1001
  

• |Represents a bitwise OR operator, which performs or operations according to binary bits. 1 is 1, and the same 0 is 0

    0000 1011
  | 0000 1101
  = 0000 1111
  

• ~Indicates the bitwise negation operator. Negation is carried out according to binary bits. 1 is 0 and 0 is 1

    0000 1011
  ~  
  = 1111 0100 
  

• ^Indicates the bitwise XOR operator, which performs XOR operation according to binary bits. The same is 0, but the difference is 1

    0000 1011
  ^ 0000 1101
  = 0000 0110
  

6.8 operator priority

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