method overriding Vs class variable overriding in java

问题

I was just trying some sample code for checking class variable overriding behavior in Java. Below is the code:

class A{
  int i=0;

  void sayHi(){
    System.out.println("Hi From A");
  }
}

 class B extends A{
  int i=2;

  void sayHi(){
    System.out.println("Hi From B");
  }
}


public class HelloWorld {
 public static void main(String[] args) {
    A a= new B();
    System.out.println("i->"+a.i); // this prints 0, which is from A
    System.out.println("i->"+((B)a).i); // this prints 2, which is from B
    a.sayHi(); //  method from B gets called since object is of type B
  }
}

I am not able to understand whats happening at these two lines below

System.out.println("i->"+a.i); // this prints 0, which is from A
System.out.println("i->"+((B)a).i); // this prints 2, which is from B

Why does a.i print 0 even if the object is of type B? And why does it print 2 after casting it to B?

回答1:

i is not a method - it's a data member. Data members don't override, they hide. So even though your instance is a B, it has two data members - i from A and i from B. When you reference it through an A reference you will get the former and when you use a B reference (e.g., by explicitly casting it), you'll get the latter.

Instance methods, on the other hand, behave differently. Regardless of the the type of the reference, since the instance is a B instance, you'll get the polymorphic behavior and get the string "Hi From B" printed.