JAVA inheritance ( can't find out the difference ) [duplicate]

Question

I am doing tests to prepare for OCA and I got confused, while doing this task:

class A {
   private int i = 10;

   public void f() {}
   public void g() {}
}

class B extends A {
   public int i = 20;
   public void g() {}
}

public class C {
   A a = new A(); // 1
   A b = new B(); // 2
}

So I can't understand difference between A a = new A(); and A b = new B();

I know.. this might be easy for you guys, but I cannot figure this out.

Thanks in advance

Answer 1

A a = new A() creates an object A and using this object you can access f() and g() methods in class A.

A b = new B() can be done because class B extends class A. The concept here is called polymorphism. Java allows user to assign a child object (B in this case) to parent reference (A in this case). Now using the object b you can access f() of class A as well. Note that it wouldn't have been possible if you had reference to class B (if you created B b = new B()).

Further, you can access g(), but since g() is present in class B, that takes precedence. So, g() of class B gets executed.

Try this example to understand more:

class A{
   private int i = 10;
   public void  f(){System.out.println(i);}
   public void g(){System.out.println(i+"**");}
}

class B extends A{
   public int i = 20;
   public void g(){System.out.println(i+"**********");}
}

public class C{
    public static void main(String[] args) {
        A a = new A();//1
           A b = new B();//2
           a.f();
           b.f();
           b.g();
           a.g();

    }

}

Answer 2

A a = new A();
Here a can access all data members and member functions of class A.
Whereas
A b = new B(); here b can access only those member of class A which are present in class B.
This concept is called upcasting and it is done to achieve abstraction in java where we want child class method to be executed by parent class.