When is an Object created after a constructor is called

问题

Consider the following code classes.




     public class A
    {
        public A()
        {
            callCreation();
        }

        protected void callCreation()
        {
            System.out.println("A Created!!");
        }
    }




    public class B extends A
    {
        protected void callCreation()
        {
            System.out.println("B Created!!");
        }
    }


    public class C extends B
    {
        protected void callCreation()
        {
            System.out.println("C Created!!");
        }

        public static void main(String[] args)
        {
            A a = new A();
            A b = new B();
            A c = new C();
        }
    }

The output of running the class C is given below.

A Created!!
B Created!!
C Created!!

The first output line in the output

A Created!!

is printed because when the constructor of class A is called, it calls the super class's constructor (java.lang.Object) implicitly before calling the callCreation() method in the class A's constructor. And this will be the case for B and C classes too. In that case when the constructor of B is called the call flow should be typically : B's constructor -> A's Constructor -> java.lang.Object's Constructor -> come back to A's callCreation() method to finish calling A's constructor. If so how is the overridden value printed and not the super class's value is printed? So the question is 'when is an object of a class created exactly? to put it in other words, the object of a class should be created only after the constructor finishes calling/initializing all the elements within itself. If so how can a method be called from a child class and not from the parent class?

回答1:

The callCreation methods in B and C override the method from A. So when the method is called in the constructor A the implementations in B and C will be run, even though the constructors of B and C have not been executed yet. This is possible because constructors don't actually create the object, rather they are called to initialize some moments after the JVM has created it.

In general it's a very bad idea to call methods that can be overridden from a constructor for this very reason. A method in B or C may assume that the constructor and object initializers have already been run, and read an unexpected value from a field. For example the following ends up printing "B Created!! null" because the field still has not be assigned its value.

public class B extends A
{
    final String msg = "Yes!";

    protected void callCreation()
    {
        System.out.println("B Created!! "+msg);
    }
}

回答2:

thinking in this way makes it more obvious: when an object of type B is being created super() keyword calls the A constructor, then in A constructor "this.callCreation()" is executed, which refers to the current object which is B, so callCreation corresponding to the current object(B) is called. the same process is done for C.

public class A {

public A() {
    this.callCreation();
}

protected void callCreation() {
    System.out.println("A Created!!");
}

}

class B extends A {

public B() {
    super();
}

protected void callCreation() {
    System.out.println("B Created!!");
}

}

class C extends B {

public C() {
    super();
}

protected void callCreation() {
    System.out.println("C Created!!");
}

public static void main(String[] args) {

       A a = new A();
       A b = new B();
       A c = new C();

}

}

来源：https://stackoverflow.com/questions/9381959/when-is-an-object-created-after-a-constructor-is-called

标签

java

object

constructor

creation