我不了解这种继承功能

Question

Consider the following illegal code :-

class WrongCode{
    int i;
    static int i;
}

Here, the compiler says that we have duplicate fields in the same class.

Now, consider the following classes in the same file.

class Parent{
    int i = 10;
}

class Child extends Parent{
    static int i = 100;
}

public class Main{
    public static void main(String ... aaa){
        Parent ob = new Child();
        System.out.println(ob.i);   // This prints Parent's i
    }
}

Since the actual object is of Child, shouldn't ob refer to Child's i? And if it is refering to Parent's "i", then in a way it is also having Parent's "i" in its own class along with its own static "i" which is NOT ALLOWED.

Child static i overshadows Parent i. And Parent's i is not static, so then how is it accessed directly using instance and not className?

Answer 1

You have instance field i in Parent class and it remain an instance field in Child class.

System.out.println(ob.i);  // must be 10

Have a look at - Oracle Java Tutorial - Hiding Fields

Answer 2

It is important to realize here that there is no way System.out.println(ob.i); could print Child 's i : it only knows that ob is of declared type Parent , not that it was instantiated with an actual Child . Thus, if Parent did not have any i , there would be a compile error. If parent has an i , this is printed.

I have seen it mentioned on SO that access of class variables via instances (ie ob.i being equivalent to Parent.i ) should be considered a serious design flaw of Java. I agree it can be sometimes confusing. Anyway, both your parent and child could also have a non-static i and it need not be the same. The argument above should be applicable to reasoning which one would be printed in which situation.

Answer 3

In ob , the static int i of child is never visible since ob is of type Parent, irrespective of how it was instantiated( base class or derived class).

That's why you have the value as 10 , that Parent s i value.

Answer 4

Java lets your class have its own variables that have the same name as a variable in the parent. But it can't just let you randomly redefine parent variables, as that would cause other stuff to break. So what it does...when you have a variable obj that's declared as the parent class, even if it holds an instance of a child class, obj.i will refer to the parent class's i rather than the child's.

Answer 5

When you access class member fields (instance variables) like ob.i. you'll get the results from the class that's known at compile time, not what is known at run time. Thats why you have value as 10 which is parents value.

For method calls they are dispatched at run time to an object of the actual class the reference points.

Regarding shadowing here is what Java lang spec says:

If the class declares a field with a certain name, then the declaration of that field is said to hide any and all accessible declarations of fields with the same name in superclasses, and superinterfaces of the class.

A hidden field can be accessed by using a qualified name (if it is static)

language spec

You may refer "Field Declarations" section.

Answer 6

实际上，它的多态性和ob只能访问父类字段和行为（如果有）。