Hi Pradeep,
What timothy says is exactly right.
To augument that, Think of how garbage collector works. GC invokes on the superclass object.
If you create an object of human named babu, after using the behaviours i.e after reference is lost, it has to be garbage collected. How does JVM knows what object(babu) to collect. But
java has single rooted hierarchy, it knows that object babu is a subclass of Object, so i collect him as garbage using the object type handle.
Hope this wud help you