Java Tidbits......................(Polymorphism)

This is the topic that had shown me stars during day ! Every time when i read any java book i felt that i am pretty clear with this & can handle  any type of Qs but every time i failed .

polymorphism  is simple concept of deciding  at run time which object's method to be called .
Here i will paste few examples where i was most confused

case 1

class ABC {
           private int a=10;
           int getA ( )
            {return a;}
}

class BCD extends ABC{
            private int a=20;
            int getA ( )
            {return 2*a;}
}

public static void main(Strings args[])
{

ABC a = new ABC();
s.o.p ( a.getA( ) )  ;  
BCD b = new BCD();
s.o.p ( b.getA( ) ) ;

//   b=a;                  Compiler Error

b = ( BCD ) a;            //  Need to type cast explicitly
s.o.p ( b.getA( ) ) ;      // ClassCastException at run time


}

Reason 1

Compiler will error because upcasting is not automatic in java
ClassCastException will come when the object reference of base class point to base class object .

to remove that
a = new BCD ( ) ;
b= ( BCD ) a;

....to be continued ....