Deep understanding of static keyword

outline ：static Modifiers have four functions in general ： Decorate a field or method 、 Decorated code block 、 Static import package 、 Construct static inner classes . In fact, my understanding static It is for convenience that we do not create instances of classes , You can call a method or variable directly through the class name

One 、 Decorate a field or method
1. Decorate fields
When using static When modifying a field , Then this field is a static field , There is only one such field in each class , Namely be static Decorated variables are shared by all instances of the class , Other non static fields , Each instance of this class has its own copy .

for instance ： In the student class Student There are two fields in , Namely ： School id——schoolId（ Static field ）、 Student id——studentId（ Non static fields ）. Every Student Objects have their own students id, But these students all have the same school id.
in other words , If there is 1000 A student object , Then there are 1000 A student id, Corresponding to each student object . however , There is only one static field school id, Even if there is no student object , Static field school id There is also .
Static variables are used less , Generally, static constants are used more . for example ,Java Of Math Class Π static const .

public static final double PI = 3.14159265358979323846;

2. Modification methods
By static The method of decoration , It's called a static method . Static methods are methods that can be executed without instantiating objects . When calling a normal method , After instantiating the object , Call this method through this object ; But when you call a static method , There is no need to instantiate the object , because static The method of decoration belongs to class , Call this method directly through the class name .
matters needing attention ：
After being defined as a static method , Methods can no longer be used this Keyword. ;
Non static methods cannot be used in static methods , But static methods can be called in non static methods ;
for example ：
Math Class max Method , Pass in two integer values , Return maximum , Expression for ：

Math.max(int a, int b)// Use Math Class directly calls max The method can , No need to instantiate objects 

Scenarios using static methods ： Method only needs to access the static field of the class or it is used when the calling method does not need to instantiate the object .
Two 、 Decorated code block
If the static field of a class needs complex initialization code , You can use static code blocks .
Put the code in a block , And mark the block as static, Then a static code block is constructed .

// Randomly generate a value less than 100 The integer of 
private static int id = 1;
static{
    
  Random random = new Random();
  id = random.nextInt(100);
}

3、 ... and 、 Static import package
When using static fields or methods , You need to add the class name before the field or method , For example, when using the method of comparing the maximum value in the mathematical class , Need to be in max Methods before adding Math Class name , When static fields or methods are used less , There's no problem with that , But if more static fields or methods are used , You can statically import packages in the header of the class file .

for example ：
Before and after the static import package , Use the maximum comparison function , How to write ：

// The head is not statically introduced Math class 
Math.max(1,2);

// Static head introduces Math class 
import static java.lang.Math.*;
max(1,2);
// Introduce classes statically , Can make the code clearer , The amount of code is also reduced 

Four 、 Construct static inner classes
When an inner class is just to hide inside a class , When there is no need for the internal class to have a reference to the external class , You can declare an inner class as static.
matters needing attention ：
As long as the inner class does not need to access the outer class object , Then you should use static inner classes
Unlike regular inner classes , Static inner classes can have static fields and methods
The inner class declared in the interface is automatically static and public