Base b = new Base(); versus Base b = new Derived() ??

Question

I want to clarify the following, please help!!!
class A
{ public void f()
{ System.out.println("A->f");
class B extends A
{ public void g()
{ System.out.println("B->g");
public class Test
{ public static void main(String[] args)
{ A a = new A();
A a = new B();
a.f();
//a.g();
A a = new A(); //(1)
means create an instance of class A of type A. Correct??
A b = new B(); //(2)
means create an instance of class B of type A
In both (1) and (2), instance "a" can access any methods of class A, but
cannot access any methods of class B
What's the difference then???

Answer 1

This is an example of dynamic function calling of java (polymorphism).
consider the following :
class A {
void print(){System.out.println("A");}
class B extends A {
void print(){System.out.println("B");}
public class Test {
public static void main(String args[]){
A a = new A();
A a1 = new B();
a.print();
b.print();
a.print() will print A.
a1.print() will print B.
Virtual Machine at run time, recognized at run time, the type of reference, variables a and a1 are having.
a holds a reference to object of class A hence a.print() calls print() in A.
a1 holds a reference to object of class B. B overrides the print() in A. The Virtual Machine will determine at run-time, what method to call. It will call print() method in B as a1 holds the reference to that object. If print() was not present in B, it would call print() in A as B inherits that method.
For our case it will print B.
I hope this helps.
The main advantage of polymorphism in this case will be, suppose we have 5 classes extending A and all of them overriding print() method.
Without polymorphism we will have 1 variable for class A and 5 variables for classes extending A. And then we can call print() methods. But with polymorphism, only one variable can refer to all of those objects and call appropriate methods.
Another example :
class A {
void print(){(System.out.println("A"));}
class B extends A {
void print(){(System.out.println("B"));}
public class Test {
public static void main(String args[]){
A a = new A();
a.print();
a = new B();
a.print();
the result will be :
A
B