1 static static variable

1.1 Use of static variables

static variables are also called static variables, also called class variables

Static variables are shared by all objects and have only one copy in memory

If and only if the class is initialized when it is first loaded

Static variables belong to class

Static variables can be called directly through the class name

It can also be called by object name.static variable name

static variable

public class Student{ 
    private String name; 
    private static String schoolName; 
    private static int count; 
    public Student(String name){ 
        this.name = name; 
        count++; 
    } 
    public void showStuInfo(){ 
        System.out.println("Student’s name is " + this.name + ", the name of the school is " + Student.schoolName); 
    } 
    
    public static void main(String[] args){ 
        Student.schoolName = "Experimental Middle School"; 
        Student s1 = new Student("Zhang San "); 
        Student s2 = new Student("李思"); 
        Student s3 = new Student("王五"); 
        
        s1.showStuInfo(); 
        s2.showStuInfo(); 
        s3.showStuInfo();
        
        System.out.println("The number of students is: " + Student.count); 
        System.out.println("The number of students is: " + s1.count); 
    } 
}

1.2 The difference between instance variables and static variables

Static variables belong to a class, which does not produce objects. Static variables can be called through the class name;

Instance variables belong to objects of this class, and an object of this class must be generated before instance variables can be called.

Static variables exist in the method area as the class is loaded;

Instance variables exist in the heap memory as the object is created.

Static variables are created at the beginning of the program and destroyed at the end of the program;

Instance variables are created when the object is created and destroyed when the object is destroyed.

2 static methods

A method modified by static is called a static method, also called a class method.

In static methods, non-static member variables and non-static member methods cannot be directly accessed.

In static methods, the this keyword cannot be used

Static methods can be called directly through the class name

It can also be called by object name.static method name

Static methods and static method access

public class Student{ 
    private String name; 
    private int age; 
    private int studentId; 
    private static String classRoom; 
    public static void showClassRoom(){ 
        //Silent method used this 关键字
        System.out.println("Classroom 3301"); 
    } 
    public static void main(String[] args){ 
        Student.showClassRoom(); 
    } 
}

3 static blocks

3.1 Use of static blocks

Static code blocks are executed when the class is loaded and only executed once

The this keyword cannot be included in static code blocks

There can be multiple static blocks, which are executed in order

public class Emp{ 
    static{ 
        System.out.println("Welcome"); 
    } 
    static{ 
        System.out.println("Come again"); 
    } 
}

3.2 Static blocks, anonymous construction blocks, constructors

Execution order

public class Son{ 
    static{ 
        System.out.println("static block 1"); 
    } 
    
    private String name; 

    public Son(String name){ 
        this.name = name; 
        System.out.println("Constructor"); 
    } 
        
    { 
        System.out.println("Anonymous construction block"); 
    } 
    static{ 
        System.out.println("static block 3"); 
    } 
    
    public static void main(String[] args){ 
        Son s1 = new Son("张三"); 
        Son s2 = new Son("李四"); 
        Son s3 = new Son("王五"); 
    } 
}

The output is as follows:

static block 1

static block 3

anonymous building blocks

Construction method

anonymous building blocks

Construction method

anonymous building blocks

Construction method

The order of execution is: static block --> anonymous block --> constructor method

4 static introduction

If we need to frequently reference methods under a certain class, such as related data calculation classes under java.lang.Maththe class , or org.junit.Assertvarious assertion methods under the class used for unit testing, we can use static references. This saves the need to repeatedly write the previous class name.

import static java.lang.Math.random; 
import static org.junit.Assert.assertTrue; 

public class test { 
    public static void main(String[] args) { 
        double random = random(); //Method in Math 
        System. out.println(random); 
        assertTrue(random > 0.3); //Method in Assert 
        
        new test().normalMethod(); 
    } 

    //Static introduced method, also used in ordinary methods 
    public void normalMethod(){ 
        double random = random(); 
        System.out.println(random); 
        assertTrue(random > 0.7); 
    } 
}

Methods introduced statically can be used in both ordinary methods and static methods.

static in Java