Question

Dog d=new Dog();

In the above line which is object and which is reference variable? or Whether object and reference variable are the same?

Answer 1

d is a reference to a Dog object instance.

Object instances and references are not the same; references point to object instances.

To illustrate, you can do

 Dog d2 = d;

now you have 2 references, d and d2, that point to the same underlying instance of Dog.

Now if you do

d = new Dog();

d2 points to the first dog you created, and d points to the second (where it originally pointed to the first.) This statement creates a new object instance, and assigns it to the original Dog reference.