Static keyword

java Medium static Keyword is mainly used for memory management .

static The purpose of keywords

Easy to call without creating an object ( Method / Variable ). By static Keyword decorated methods or variables do not need to rely on objects for access , As long as the class is loaded , You can access it through the class name .static Can be used to modify member methods of a class 、 Member variables of class , You can also write static Code blocks to optimize program performance .

static Variable

static Variables are also called static variables , The difference between static and non static variables ：

Static variables are shared by all objects , There is only one copy in memory , The class is initialized when it is first loaded

Non static variables are owned by objects , Is initialized when the object is created , There are multiple copies , The copies owned by each object do not affect each other

public class Humanity {
    static String name;

}



public class Student extends Humanity{
    void show(){
        System.out.println(" I am a "+name+" Student ");
    }

}



public class Teacher extends Humanity{
     void show(){
         name=" laozhao ";
        System.out.println(" I am a "+name+" teacher ");
    }
}



public class Test {
    public static void main(String[] args) {
        Teacher teacher = new Teacher();
        teacher.show();
        Student student = new Student();
        student.show();

    }
}

static Method  

static Methods also become static methods , Since static methods do not depend on any object, they can be accessed directly , So for static methods , It's not this Of , Because it is not attached to any object , Since there is no object , I can't talk about this 了 , And because of this feature , Non static member variables and non static methods of a class cannot be accessed in a static method , Because non static member variables and non static methods must depend on specific objects to be called .

 

As you can see from the code above ：

Static methods test2() Calling non static member variables age, Compile failed . This is because , No objects were generated at compile time ,age Variables don't exist at all .

Static methods test2() Calling non static method test1(), Compile failed . This is because , The compiler cannot predict methods in non static members test1() Whether non static member variables are accessed in , It is also forbidden to call non static member methods in static methods.

Non static member methods test1() Access static member methods test2()/ Variable name There is no limit

therefore , If you want to call a method without creating an object , You can set this method to static. The most common static method is main Method , That's why main If the method is static, it will be clear at a glance , Because the program is executing main Method, no objects are created , Accessed only by class name .

static block ( Static code block )

Execute as the class loads , And only once

How to write it ,

static{

}

Non static code block ：

When executing, if there is a static initialization block , Execute the static initialization block first, and then execute the non static initialization block , It is executed once every object is generated , It can initialize instance variables of a class .

How to write it ：

{

}

The execution order of static code blocks ： Static code block -----> Non static code block --------> Constructors

Common interview questions

public class Test extends Base{

        static{
            System.out.println("1");
        }

        public Test(){
            System.out.println("2");
        }

        public static void main(String[] args) {
            new Test();
        }
    }

    class Base{

        static{
            System.out.println("3");
        }

        public Base(){
            System.out.println("4");
        }
    }

  Output results ：

You can see that the execution order is   Parent static code block ----> Subclass static code block ----> Parent class constructor -----> Subclass construction method