Question: I want to understand this:

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

```

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

Answer: 
p is an instance of `ChildClass` which of type `ParentClass` and 
using p you can call all the methods of ParentClass but you cannot call any method of ChildClass.
Example 

```
public class ParentClass{
public Person(){}
public void p(){
System.out.println("from p()");
}
}

public class ChildClass extends ParentClass{
public void c(){
System.out.println("from c()");
}

public static void main(String[] args) {
ParentClass p = new ChildClass();
ChildClass c = new ChildClass();
p.p(); //possible
p.c(); //not possible
c.c();  //possible
}

```

}
P is a reference in the stack of type ParentClass pointing to a childClass instance in the heap ,this enables you to just call class members defined in ParentClass and take advantage of Polymorphism on runtime through method overriding.