Variables, constants, operators

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2.4 Variable 、 Constant

2.4.1 Variable

It's essentially a storage space , Determine the location , Value uncertainty . Access storage space by variable name , So as to operate the value stored inside .

java As a strongly typed language , Data type must be declared before definition , Storage space of corresponding length can be allocated only after declaration .

1、 Before each variable is defined, the type must be declared , It can be of any type 
2、 The variable name is a legal identifier 
3、 The definition of variables belongs to a complete statement , Must be ; ending 

​ Differences between static variables and instance variables ？

​ answer ： Static variables should be matched with keywords static Use . Create in class , Become a class variable , As long as such bytecode is loaded , You don't have to create objects , It will also be divided With storage space , It can be directly referenced by class name . No matter how many objects are created , The static variable of the operation is a , Generally use capital letters
​ Instance variables are declared outside the French sentence block of the class , Properties for class , When an instance object is created , Destroy when the object is destroyed .

2.4.2 Constant

​ The amount that will not be changed during the operation of the program . With keywords final Constants can only be assigned once in a program , Share among all objects .

​ grammar ：final data type Constant names = value ; （ Constant names are generally expressed in uppercase letters ）
​ When a constant is used as a member variable, it must be given an initial value .

JAVA 10 A new feature after ： Use var

Similar to JavaScript, Allows the compiler to use its initial value to infer the type of local variable .

1. var Cannot be declared as a variable , It is used by the compiler to determine the type during initialization , So you can't reallocate with different data types , Use var The initialized variable data type is inferred only according to the data type of the initializer at compile time , And can't change .
2. It cannot be used with method parameters and method return types . Because the compiler cannot infer which type of var Replaced at runtime .
3. It cannot be used as an instantiation variable .
4. Cannot be used for class variables .
5. It cannot be initialized to NULL value

2.5 Operator

2.5.1、 Arithmetic operator

   Arithmetic operator 
  	+	 The addition operator （ String addition , Mathematical plus sign ）	 Plus sign   
  	-	 Subtraction operators 	 Minus sign 
  	*	 Multiplication operators 
  	/	 Division operator （ The result will be an error due to the data type ）
  	%	 Seeking remainder 
  	++	 Self increasing :  The value of the operands increases 1
  		++1： First of all, increase by yourself , And then I'll take part in the calculation 		int a = 1;  int b = a++;  int c = ++a;   a=3 b=1 c = 3
  		1++： Take part in the calculation first , Since the increase again 
  	--	 Self reduction :  The value of the operands decreases 1
  		--1： First self reduction , And then I'll take part in the calculation 
  		1--： Take part in the calculation first , And then reduce 

2.5.2、 Assignment operator

   Assignment operator 
  	=	 Assign the value of the right operand to the left 
  	+=、-=、*=、/=	 Add the values of the left and right operands 、 reduce 、 ride 、 except 、 After assignment to the left operand 
  	％=： Modulo and assignment operators , It modulates the left and right operands and assigns them to the left 
  	<<=： Left shift assignment operator 		C <<= 2 Equivalent to C = C <<2
  	>>=： Right shift assignment operator 		C >>= 2 Equivalent to C = C >>2
  	＆=： Bitwise and assignment operators 		   C＆= 2 Equivalent to C = C＆2
  	^=： Bitwise XOR assignment operator 
  	|=： Bitwise or assignment operators 
  	

2.5.3、 Relational operator

   Relational operator 
  	==	 Determine if the values of two operands are equal , If they are equal then the condition is true .
  	!=	 Determine if the values of two operands are equal , If the values are not equal, the condition is true .
  	>	 Determines whether the value of the left operand is greater than the value of the right operand , If so, the condition is true .
  	>=	 Determines whether the value of the left operand is greater than or equal to the value of the right operand , If so, the condition is true .
  	<	 Determines whether the value of the left operand is less than the value of the right operand , If so, the condition is true .
  	<=	 Determines whether the value of the left operand is less than or equal to the value of the right operand , If so, the condition is true .
  	instanceof ： Check whether the object is of a specific type （ Class type or interface type ）.
  		String name = "Java"
  		name instanceof String    result ：true（name yes String Type of ）

2.5.4、 Logical operators

   Logical operators 
  	&&： And 、 And     A false is a false , True is true 
  	||： or 	    When it's true, it's true 
  	！： Not  	   Take the opposite 

2.5.5、 An operator

   An operator 
  	＆： If all the corresponding bits are 1, The result is 1, Otherwise 0
  		a：    0011 1100
  		b：    1111 1111
  		a&b：  0011 1100	
  	|： If all the corresponding bits are  0, The result is  0, Otherwise  1
  		a：   0011 1100
  		b：   1111 1111
  		a|b： 1111 1111	
  	^： If the corresponding bit values are the same , The result is 0, Otherwise 1
  		a：   0011 1100
  		b：   1111 1111
  		a^b： 1100 0011
  	〜： The bitwise negate operator flips every bit of the operands , namely 0 become 1,1 become 0.
  		a：   0011 1100
  		~a：  1100 0011
  	<<： Bitwise shift left operator . The left operand moves left by bits the number of bits specified by the right operand . amount to *2
  	>>： Bitwise shift right operator . Shift left operand right by bit number of bits specified by right operand . amount to /2  
  	>>>： Shift bit right to zero operator . The value of the left operand is shifted right by the number of bits specified by the right operand , Move the resulting space to zero .

​ & and && The difference between ：

​ && Is a logical operator , And 、 And what it means . If the expression on both sides is false, it will be false , True is true . Short circuit function , For example, the expression on the left is false, be The expression on the right does not participate in the operation

​ & It's a bit operator , Bitwise AND , The expressions on both sides of the equation are false When doing logic and operation , But both sides of the equation are involved in the calculation , No short circuit function .

​ 2*8 How to calculate the fastest ？

​ answer ： The bitwise shift left operator operates fastest , hold 2 Move left 3 position 2<<3

​ 2 convert to 2 Into the system for ：0000 0010

​ Shift left three places ：0001 0000 convert to 10 Into the system for 16

​ << >> It's very efficient

2.5.6、 Ternary operator

   Conditional operator （ Ternary operator ）※※※※※※
  	( Conditions )?a:b		 If the condition is true execute a, Otherwise execution b

2.5.7、 priority