Class initialization and instance initialization interview questions

Output program print order

public class Father {
    
    private int a = testA();

    private static int b = testB();

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

    Father() {
    
        System.out.println("(2)");
    }

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

    private static int testB() {
    
        System.out.println("(4)");
        return 0;
    }

    public int testA() {
    
        System.out.println("(5)");
        return 0;
    }
}

public class Son extends Father {
    

    private int a = testA();

    private static int b = testB();

    static {
    
        System.out.println("(6)");
    }

    Son() {
    
        System.out.println("(7)");
    }

    {
    
        System.out.println("(8)");
    }

    private static int testB() {
    
        System.out.println("(9)");
        return 0;
    }

    public int testA() {
    
        System.out.println("(10)");
        return 0;
    }

}

1. Class initialization process

1 To create an instance of a class, you need to load the initialization class first ,main The class of the method needs to be loaded and initialized first ;
2 A word class needs to be initialized and the parent class needs to be initialized first ;
3 A class initialization is execution 《clinit》() Method ：
《clinit》() Method consists of static class variable display assignment code and static code block ,
《clinit》() Methods and execute them in order from top to bottom ,
《clinit》() Method is only executed once .

    public static void main(String[] args) {
    
		// perform main Method , Initialization class , Do not initialize instance , (4) (1) (9) (6)
    }

2. Instance initialization process

perform () Method ：
1《init》() Method may overload more than one , There are several constructors, there are several 《init》() Method ;
2《init》) Method displays the assignment code and the non static code block by the non static instance variable , Composition of corresponding constructor ;
3 The non static instance variable shows that the assignment code and the non static code block code are executed from top to bottom , The constructor code finally executes ;
4 Every time you create an instance object , Call the corresponding constructor , Execute corresponding 《init》() Method ;
5 The constructor of the subclass must call the constructor of the parent class ;

public static void main(String[] args) {
    
        Father father = new Son();
        father = new Son();
         perform main Method , Initialization class , Initialize instance , (4) (1) (9) (6)(10)(3) (2) (10) (8) (7) (10)(3) (2) (10) (8) (7)
    }
    

3. Method rewriting

1 Methods that cannot be overridden :
final
static
private
2 Polymorphism of objects ：
Subclass if the method of the parent class is overridden , Through the subclass object call must be subclass rewritten code ;
The default call object for non static methods is this;
this The object is in the constructor or () Method is the object being created .