How object references work

Question

Hi guys,
class A{ public void aM(int i) { 
System.out.println(this.toString()+" Iam a " + i);} }
class B extends A{ public void bM(int i) { 
System.out.println(this.toString()+" Iam b "+ i); } }
class MyClass{ public static void main(String arg[]){
A a = new A(); B b = new B();
a.aM(1); //calls the method in object 'a'
b.bM(2); //calls the method in object 'b'
a = b; // compiles properly
a.aM(3); //calls the method in object 'b'
a.bM(); // compilation error cannot resolve symbol
b = a; // compilation error incompatible types
b = (B)a; // compiles properly
b.aM(4); // calls the method in object 'b'
b.bM(5); // calls the method in object 'b'
How object references works
in the above class if i assign subclass reference to superclass reference variable it gets compiled, but superclass reference to subclass reference variable give compilation error without explicit casting;
why this behaviour?
what compiler checks at compilation time and what JVM checks at runtime?
In the method calls also it is calling methods on the object 'b', why?
Thanks.

Answer 1

Sorry, but I don't want to read that code. The
formatting makes it very hard to follow the flow.Sorry , please check this code
class A{
  public void aM(int i) {
   System.out.println(this.toString()+" Iam a " + i);
class B extends A{
  public void bM(int i) {
   System.out.println(this.toString()+" Iam b "+ i);
public class MyClass{
  public static void main(String arg[]){
     A a = new A();
        B b = new B();
     a.aM(1); //calls the method in object 'a'
     b.bM(2); //calls the method in object 'b'
     a = b; // compiles properly
     a.aM(3); //calls the method in object 'b'
     a.bM(); // compilation error cannot resolve symbol
     b = a; // compilation error incompatible types
     b = (B)a; // compiles properly
     b.aM(4); // calls the method in object 'b'
     b.bM(5); // calls the method in object 'b'
}