Question: I want to understand this:

```
ParentClass p=new ChildClass(); 
//Here ChildClass extends ParentClass

```

Is `p` object of `ParentClass` or of `ChildClass`?

Answer: 
Think about `p` as a view on the given object. You can view a `Dog` as an `Animal` (in which case you can e.g `feed()` it), or you can view it as a `Dog` (so you can get it to `bark()`). In other words, the variable's type that references an object will restrict what you can call on it, but does not change the type of the object.
`p` is not instance at all. it is just reference of type `ParentClass` that is pointing to object of `ChildClass` on heap memory.

`Objects` live on heap while references can be on stack memory or in other objects on heap. think `reference` as pointers in c/c++ with some additional functionality. 

Your example is of inheritance where reference of type `ParentClass` can point to `ChildClass` object.