Classic cases of static keywords and block blocks

Static Key words and block Classic case of block

public class StaticTest02 {
    
    public static int id; 
    // Class variables | Static variables ---> The static variables of the class are initialized in the static area of the method area id=0
    public int age; // Member variables 
    public static void main(String[] args) {
    
        StaticTest02 staticTest02 = new StaticTest02();
  // Instantiation staticTest02 Make room in heap memory 0X001--> Member variables follow the object into heap memory for initialization  age=0
        staticTest02.age=10;// Object references are member variables age The assignment is 10
        System.out.println(StaticTest02.id);
        //  Output the static variable value of the class id=0
        System.out.println(staticTest02.age);
        // Object calls member variables age Output is 10

        staticTest02.change(); 
// Call the member method for the first time , Make room in heap memory 0x002-->id=1 age=10 //id=1 staticTest02.age=1  Method used , Stack frame release 
        System.out.println(StaticTest02.id); //1
        System.out.println(staticTest02.id); 
        // The object here refers to 0X001---> therefore id yes 1
        System.out.println(staticTest02.age);  
        // The object here refers to 0X001--> age=10
        staticTest02.change();  
    // Second call change Method to open up heap space -->0X003  Print the first sentence  ：id=2 age=10（ Here age Still 0X001 Of age value ） 
     //  Print the second sentence ：id=2 staticTest02.age=1
        System.out.println(id);  //id=2
        System.out.println(staticTest02.age);  
        //age=10
       // When main After the method is executed, the stack frame is released 
    }
    public void change() {
    
        StaticTest02 staticTest02 = new StaticTest02();
    // Instantiate class objects staticTest02, Member variables follow the object into heap memory for initialization  age=0
        staticTest02.age++;// Member variables of class objects age：1
        StaticTest02.id++;// Of class variables of class objects id by 1
        System.out.println("In Change(): id = " + id + " age = " + age);  
        // First call id=1 age=10
        System.out.println("In Change(): id = " + id + " staticTest02.age = " + staticTest02.age);
        // First call  id=1 staticTest02.age=1
        //id=2 staticTest02.age=10
    }
}

1. block block {}-> Scope
{} Local code block | Ordinary statement block –> Method or statement block Execution opportunity : Method call
Define a piece of code with the same logic in a block , It's semantic , Self forming scope
2. {} Tectonic block --> Outside the method in the class Execution opportunity : new When you're with someone
Make some initial actions for member content
3. static{} A static block —> Class
Initialize static content
For some configurations … It only needs to be executed once , Subsequent reusable code can be defined in static blocks , Do some initial behavior , Do not encapsulate method calls

4. Synchronized block
Be careful :
static Member modifier Only member variables can be modified, not local
Local variables cannot be called externally , Because it exists in the stack frame of the method , Follow method calls , The building block code is executed before the constructor code , If there are multiple building blocks , From top to bottom
Initialize after the class is loaded for the first time , And only once
If there are multiple static blocks , The plex is executed from top to bottom .

  Execution order ： A static block   ->  Main method  ->  Tectonic block  ->  Constructors 

/**  The code of the building block will be compiled into the constructor code during compilation , Before the constructor code  debug  Debugging tools  1. Track the execution of the program  2. location , Debug the location where the exception occurs  3. Observe the changes of variables during program execution  4. Learn the source code and the execution process   Use steps : 1)  Breaking point   When the program executes to the breakpoint , stop , The subsequent execution is controlled by the program ape   Click the break point after the line number  2) debug Environment is running  IDEA Common shortcut key : step over F8  Go on to the next step , If the next step is to call the method , Do not enter  step into F7  Go on to the next step , If the next step is to call the method , How to get in , If the source code method does not enter  Force step into alt+shift+F7  Go on to the next step , If the next step is to call the method , Forced entry method , If the source code method is forced to enter  step out shift+f8  Call up to the location of the method call  **/
public class BlockTest02 {
    
    public static int a = 0;
    {
    
        a = 10;
        System.out.println("3 、 Non static code block execution a=" + a); //10
    }
    static {
    
        a = 6;
        System.out.println("1 、 Static code block execution a=" + a); //6
    }
    public BlockTest02() {
    
        this(a); //6
        System.out.println("6、"+a);  //10
        System.out.println("7、 The parameterless construction method performs a=" + a); //10
    }
    public BlockTest02(int n) {
      //n=6
        System.out.println("4 、"+n); //6
        System.out.println("5、"+a);// 10
    }
    public static void main(String[] args) {
    
        System.out.println("2 、main"); //main
        BlockTest02 tsc = new BlockTest02();
        System.out.println(tsc);
    }
}