How to initialize objects in java

Step one: Load the class. A java object will be class loaded before initialization, and a Class object will be generated in the JVM. Loading a class will perform the following operations, and the recursive description is given below.

If the class has a parent class, its parent class is loaded first.

i Initialize the static members of this class

ii Execute the static code block of this class

The second step: Create an object. If the class has a parent class, when creating the object, the object of its parent class will be created first, the properties and methods of the subclass will be wrapped in the outer layer, and then the reference of the subclass will be returned. The recursive description is given below.

If the class has a parent class, create the object of the parent class first.

i Initialize the ordinary members of this class

ii Execute the ordinary code block

iii Call the Class construction method

Example:

This class object is used as a member variable

public class Info{    public Info(String s) {
        System.out.println(s);
    }
}

Parent class

public class Parent {
    public static Info info = new Info("Parent static member");      //静态成员
    public Info info2 = new Info("Parent common member");            //普通成员
    static {                                                         //静态代码块
        System.out.println("parent static block");
    }
    {                                                                //普通代码块
        System.out.println("parent common block");
    }
    public Parent() {                                                //父类构造方法
        System.out.println("Parent.Parent()");
    }
}

Subclass

public class Child extends Parent{
    public static Info info = new Info("Child static member");       //静态成员
    public Info info2 = new Info("Child common member");             //普通成员
    static {                                                         //静态代码块
        System.out.println("Child static block");
    }
    {                                                                //普通代码块
        System.out.println("Child  common block");
    }
    public Child() {                                                 //子类构造方法
        System.out.println("Child.Child()");
    }
}

The following test class loading process, we do not create objects, but directly load the class, and load subclasses

public class InitObjectTest{
    public static void main(String[] args) {
        try{
            //Class.forName("Parent");
            Class.forName("Child");
        }catch(Exception e){
            
        }
        //System.out.println("=============== now , we create an Object below ===========");
        //new Parent();
    }
}

Test results:

The test results comply with the rules for loading classes written above. First initialize the parent class static members, then execute the parent class static block, then initialize the subclass static members, and finally execute the subclass static block. . We can see that static members are indeed initialized when the class is loaded.

Note: The class is loaded only once, and the class will no longer be loaded when the object is created later. This is why the static code block is only executed once.

Recommended tutorial: java introductory tutorial

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