What Is The Meaning Of "B Ob=new A( )?

You are creating an Object named 'Ob'
'B' reference, but a 'A' Object

It's used in Polymorphism.

consider a classic example...
Animal a = new Animal();    // generic Animal Eating Generically
a.eat();

Horse h = new Horse();      // Horse eating Hay
h.eat();

Animal ah = new Horse();   // Horse eating Hay. Polymorphism works! The actual object type (Horse),
ah.eat();            //not the reference type(Animal), is used to determine which eat() is called.


well you need to understand the concepts of Polymorphism, methos overloading and method overriding. I suggest  you take a good book and read the topic.
use SCJP book by Kathy Sierra and Bert Bates. Its an excellent book for Fundamentals

msg me if in doubt.
Hope it helps...