Detailed explanation of this and static

static:
    1、static Translated into " static state "
    2、 be-all static Keywords are class related , Class level
    3、 all static Embellished , All use " Class name ." Mode of access
    4、static Decorated variable ： Static variables
    5、static The method of decoration ： Static methods

Classification of variables ：
     Variables are divided according to the declared location ：
       The variables declared in the method body are called ： local variable
       Variables declared outside the method are called ： Member variables

     Member variables can be divided into ：
       Instance variables
       Static variables

class VarTest{
	//  For the following example , Are object related , Use when visiting " quote ." Mode of access , It needs to be done first new object 
	//  Strength related , Must have object first , Ability to visit , A null pointer exception may occur 
	//  Instance variables in member variables 
	int i;
	//  Example method 
	public void m1(){		
	}
	
	//  Static variables in member variables 
	static int k;
	//  Static methods 
	public static void m2(){
		
	}
}

  When a variable is declared as an instance , When declared static ？
     If a property value of all objects of this type is the same ,
     It is not recommended to define as instance variable , Wasted memory space , The proposal is defined as
     Class level features , It's defined as a static variable , Only one... Is left in the method area
     Share , Save memory .

     An object is an instance variable
     One copy of multiple objects is a static variable

public class staticTest02
{
	public static void main(String[] args){
		 Chinese c1 =new Chinese("123456","junker"," China ");
		 Chinese c2 =new Chinese("00000","jun"," China ");
		 System.out.println(c1.id);
		 System.out.println(c1.name);
		 System.out.println(c1.country);
		 System.out.println(c2.id);
		 System.out.println(c2.name);
		 System.out.println(c2.country);		
}
}
//  Define a class ： Chinese 

class Chinese{
	//  ID number 
	//  Everyone has a different ID number , So the ID number should be an instance variable , One object, one share 
	String id;
	//  full name 
	//  Name is also a person a name , The name should also be an instance variable 
	String name;
	//  nationality 
	//  about " Chinese " For this class , All nationalities " China ", It doesn't change with the change of the object 
	//  Obviously nationality is not a characteristic of the object level 
	//  Nationality is a characteristic of the whole class 
	String country;
	
	//  No parameter 
	public Chinese(){
		
	}

	//  With parameters 
	public Chinese(String s1,String s2,String s3){
		id =s1;
		name =s2;
		country =s3;
	}
}

When variables country Not for static The static memory diagram is as follows ：

When variables country by static The code walkthrough and memory diagram for static variables are as follows ：

Code walkthrough 2：( by static Static variables , You should use " Class name ." Mode of access )

public class staticTest02
{
	public static void main(String[] args){
		
		//  Visiting Chinese nationality 
		//  Nationality is static Static variables , You should use " Class name ." Mode of access 
		 Chinese c1 =new Chinese("123456","junker");
		 System.out.println(Chinese.country);
		 
		 //  Report errors [id,name Is instance variable , Should first new An object   adopt " quote ." Mode of access ]
		 //System,out.println(Chinese.id);
		 
		 System.out.println(c1.id);
		 System.out.println(c1.name);
		 
		 Chinese c2 =new Chinese("00000","jun");
		 System.out.println(c2.id);	
		 System.out.println(c2.name);
		 System.out.println(Chinese.country);
		 		
}
}
//  Define a class ： Chinese 

class Chinese{
	//  ID number 
	//  Everyone has a different ID number , So the ID number should be an instance variable , One object, one share 
	String id;
	//  full name 
	//  Name is also a person a name , The name should also be an instance variable 
	String name;
	//  nationality 
	//  Focus on five stars : Plus static Variables are called static variables 
	//  Static variables are initialized when the class is loaded , Unwanted new object , The space for static variables is opened up 
	//  Static variables are stored in the method area 
	//  Static variables usually give a value 
	static String country =" China ";
	
	//  No parameter 
	public Chinese(){
		
	}

	//  With parameters 
	public Chinese(String s1,String s2){
		id =s1;
		name =s2;
	}
}

  Code walkthrough 3：

Example of ： You have to use " quote ." To visit
Static ： It is recommended to use " Class name ." To visit , But use " quote ." It's fine too , If you use " quote ." Visiting will make others confused ： Programmers think it is an instance

public class staticTest02
{
	public static void main(String[] args){
		 Chinese c1 =new Chinese("123456","junker");
		 
		 //  Report errors [id,name Is instance variable , Should first new An object   adopt " quote ." Mode of access ]
		 //System,out.println(Chinese.id);
		 
		 System.out.println(c1.id);
		 System.out.println(c1.name);
	 //  Visiting Chinese nationality 
	//  Nationality is static Static variables , You should use " Class name ." Mode of access 
		 System.out.println(Chinese.country);
		 
		 Chinese c2 =new Chinese("00000","jun");
		
		 System.out.println(c2.id);	
		 System.out.println(c2.name);
		 
		 //  Use " quote ." Is still accessed through  
		 System.out.println(c2.country);	//  China 
		 		
}
}
//  Define a class ： Chinese 

class Chinese{
	String id;
	
	String name;

	static String country =" China ";
	
	//  No parameter 
	public Chinese(){
		
	}

	//  With parameters 
	public Chinese(String s1,String s2){
		id =s1;
		name =s2;
	}
}

Null reference access static will not null pointer exception

public class staticTest02
{
	public static void main(String[] args){
		 Chinese c1 =new Chinese("123456","junker");
		 System.out.println(c1.id);
		 System.out.println(c1.name);
		 System.out.println(Chinese.country);
		 
		 Chinese c2 =new Chinese("00000","jun");	
		 System.out.println(c2.id);	
		 System.out.println(c2.name);
		 
		 // c2  It's a null reference 
		 c2 =null;

		 //  There will be no null pointer exceptions 
		 //  Because static variables do not require the existence of objects 
		 //  In fact, the following code runs , still ：System.out.println(Chinese.country);
		 System.out.println(c2.country);//  Still pass 	

		 //  A null pointer exception occurred   because name Is instance variable 
		 // System.out.println(c2.name);
		 		
}
}
//  Define a class ： Chinese 

class Chinese{
	String id;
	
	String name;

	static String country =" China ";
	
	//  No parameter 
	public Chinese(){
		
	}

	//  With parameters 
	public Chinese(String s1,String s2){
		id =s1;
		name =s2;
	}
}

Add 【 master 】

public class staticTest03
{
	public static void main(String[] args){
		User u =new User();
		int i =u.getId();
		System.out.println(i);
	}
}


class User
{	
	//  Instance variables are also called object variables 
	private int id; // id Object level variables 
	
	/*
	//  Will report a mistake 
	//  Static methods cannot be used   Because the returned id、id It is an object level requirement new object 
	//  The static method only needs " Class name ." Access to   So there's an error 
	public static int getId(){
		return id;
	}
	*/
	public int getId(){
		return id;
	}

}

When a static method is used, the variables should also be static ：

One 、 Static code block  

    1、 Use static Keywords can be defined : Static code block

    2、 What is a static block of code , What is grammar ？
       static{
        java sentence ;
        ...
       }

    3、static When do static code blocks execute ？
       characteristic ： When the class is loaded, execute , And only once   
    
    4、 Be careful ： Static code blocks are executed when the class loads , And in main Method before execution

    5、 Static code blocks are generally executed in top-down order

    6、 What do static code blocks do , What's the usage? ？

     Specific business ：
         The project manager said ： All the programs we write , As long as the class is loaded , Please record the log information of class loading
         ( In which year, month, day, hour and minute , Which class is loaded into JVM In the middle ) You need to use static code blocks

Code walkthrough ： 

public class staticTest04
{	
	//  Static code block 
	static{
		System.out.println("A");
	}
	//  Multiple static code blocks can be written in a class 
	static{
		System.out.println("B");
	}
	static{
		System.out.println("C");
	}

	//  Program entrance 
	public static void main(String[] args){
		System.out.println("come on~");
		}

	//  Write a static code block 
	static{
		System.out.println("D");
	}
}

Running results ：( Conclusion ： Static code blocks are executed when the class is loaded )

Two 、 Code execution order    

( Conclusion ： Static code block 1 and Static code block 2 In order 、 Static code blocks and static variables also have order )

Code demonstration ：

public class staticTest05
{	
	//  When static variables are initialized ？	 When the class loading 
	//  Where are static variables stored ？	 Method area 
	static int i =100;
	
	//  When static code blocks are executed ？	 Class loading time 
	static{
		System.out.println("i:"+i);
	}
	
	/*
	 Instance variables  k Variables are instance variables , In the construction method ( Hidden object methods ) Execution time (new When ) Memory space will be opened up 
	 error :  Cannot reference non static from a static context   Variable  k
	int k =6;
	static{
		// static The static code block executes when the class is loaded ,k Is instance variable   So it will report a mistake 
		System.out.println(k);

		System.out.println("name:"+name); //  An error is also reported here ： Illegal forward reference 
	}
	

	//  Report errors ： Illegal forward reference 
	//static{System.out.println("name:"+name);}
	//static String name ="junker";
	
	*/
	// Program entrance 
	public static void main(String[] args){
		System.out.println("main begin~");
	}	
}

3、 ... and 、this Memory structure

   1、this It's a keyword , All lowercase
    
    2、this What is it? , In terms of memory ？
         One object, one this.
         this It's a variable , It's a reference ,this Save the memory address of the current object , Point to their own ,
         therefore , Strictly speaking ,this It stands for " The current object "
         
         this Inside objects stored in heap memory
    
    3、this Can only be used in instance methods , Who calls this instance method ,this Who is the
         therefore this It stands for ： The current object
    
    4、this In most cases, it can be omitted

    5、 Why? this Cannot be used in static methods ？？？
        this Represents the current object , There is no object in the static method

Code walkthrough ：[ master ] 

public class ThisTest01
{	
	public static void main(String[] args){
	// new An object 
	Customer c1 =new Customer("junker");
	Customer c2 =new Customer("jun");
	c1.shopping();
	c2.shopping();
	
	}
}




//  Customers 
class Customer
{
	//  attribute 
	//  Instance variables ( You have to use " quote ." Mode of access )
	String name;

	//  Construction method 
	public Customer(){
		
	}
	public Customer(String s){
		name =s;
	}

	//  How customers shop 
	//  Example method 
	public void shopping(){
		//  there this Who is it ？ this Is the current object 
		// c1 call shopping();,this Namely c1
		// c2 call shopping();,this Namely c2
		System.out.println(name+" Shopping ");
		//  doubt ： String name Is instance variable ( You have to use " quote ." Mode of access )
		//  Why is there (name+" Shopping "); No ( quote .name+" Shopping ") Well  
		//  Why not in this program (c1.name+" Shopping "); Well ？
		// [ Be careful ：new The citation that comes out c1  Is in ThisTest01 Method body   This is Customer Class body  ];

		//  answer ： Actually (name+" Shopping "); The current object is hidden this ( See the memory diagram for details )
		//System.out.println(this.name+" Shopping ");
	}
}

Expand ：

/*
	 analysis ：i Variable in main Method ？？？
*/

public class ThisTest02{	

	int i =100;	//  Instance variables are object level   Must first new Objects can only be used 
	static String k; 
	public static void main(String[] args){
		//  error :  Cannot reference non static from a static context 
		//System.out.println(this.i);
		//System.out.println(i);	//  The same mistake   because i Is instance variable   and main The method is static 
		
		//  If you really want to main Method to access instance variables   Then you need to manually new An object 
		ThisTest02 t =new ThisTest02();
		System.out.println(t.i);
		//  Access static variables  " Class name ." Mode of access 
		System.out.println(ThisTest02.k);
	}
}

this In which cases can not be omitted ：

/*
1、this You can use in instance methods , Cannot be used in static methods .
2、this Keywords can be omitted in most cases , When can't we omit ？
*/

public class ThisTest06
{
	public static void main(String[] args){
		//  No arguments 
		Student s1 =new Student();
		s1.setName("jun");
		s1.setNo(11111);
		System.out.println(s1.getNo());
		System.out.println(s1.getName());
		//  Ginseng 
		Student s2 =new Student(22222,"junker");
		System.out.println(" Student number ："+s2.getNo());
		System.out.println(" full name "+s2.getName());

		
	}
}

//  Analyze what is not well written in the following code ？
//  The formal parameters are not clearly described 
class Student
{	
	private int no;
	private String name;
	
	//  Constructors 
	public Student(){}
	public Student(int no,String name){
		this.no =no;
		this.name =name;
	}
	
	/*
	//  Report errors 
	public Student(int no,String name){ //  Inside the formal parameters are local variables 
		no =no;	//  Sinister no Is instance variable   Both virtual machines may think that the variables are the same , therefore 
		name =name;//  Instance variables are object level variables   need " quote ." Form visit  this just : The current object 
	}
	*/

	//  Construction method 
	public int getNo(){
		return no;
	}
	public void setNo(int no){	//  Shape parameter n The meaning of the expression is not obvious   Some don't know no
		this.no =no;
	}

	public String getName(){
		return name;
	}
	public void setName(String name){
		this.name =name;
	}
}

Running results ：

this In addition to being used in instance methods , It can also be used in construction methods

New syntax ： Call another constructor of this class through the current constructor , You can use the following syntax format ：
         this( List of actual parameters );
         Through a construction method 1 To call the constructor 2, Code reuse can be achieved
         Be careful ：" Construction method 1" and " Construction method 2" They are all in the same category

  demand ：
    1、 Define a date class , It can represent the year, month and day information .
    2、 Requirements in requirements ：
     If you call a parameterless constructor , The default creation date is ：1970 year 1 month 1 Japan .
     Of course , In addition to calling the parameterless constructor , You can also call a parameter constructor to create a date object .

public class ThisTest07
{
	public static void main(String[] args){
		//  No parameter 
		Date d1 =new Date();
		d1.doSome();
		//  With parameters 
		Date d2 =new Date(1949,3,6);
		d2.doSome();
	}
}

class Date
{
	private int year;
	private int month;
	private int day;
	
	/*
		 It is found that there are no parameter constructors and parameter constructors this.year this.month this.day  repeat 
		 Then you can call a parameterized constructor with a parameterless constructor   grammar ：this( List of actual parameters );
		 The actual parameter list corresponds to the parameters of the parameter construction method 
	*/
	//  Parameterless constructor 
	public Date(){
		/*
		this.year =1970;
		this.month =1;
		this.day =1;
		*/
		this(1970,1,1);
	}
	//  There are parameter constructors 
	public Date(int year,int month,int day){
		this.year =year;
		this.month =month;
		this.day =day;
	}
	//  Set up checkpoints 
	//  Reading data 
	public int getYear(){
		return year;
	}
	public void setYear(int year){
		this.year =year;
	}
	
	
	public int getMonth(){
		return month;
	}
	public void setMonth(int month){
		this.month =month;
	}

	public int getDay(){
		return day;
	}
	public void setDay(int day){
		this.day =day;
	}
	
	public void doSome(){
		
		//System.out.println(year); //  Encapsulation in this class is invalid  1970( There is a default constructor to pass in 1970)
		//  When there is no constructor, the result is output int The type is 0 String-> null

		System.out.println(year+" year "+month+" month "+day+" Japan ");
	}
}

  Running results ：