Need a clarification on below code, why it is printing the output 10 instead of 20

Because variable 'a' is overridden in child class with value 20 so when you create the object like this:

Parent obj  = new Child();

You are somewhat saying that I am interested in the fields and methods of Parent but if they are overridden in Child then through the concept of Inheritance, I am going to get the child class implementation only and if their implementation is not available in child then of course in that case only, I will get the implementation of Parent.

Ohh got it, if implementation is missing in child class then it will pick the data or value from Parent class. Thanks

Here child class is over rides with variable a 20 then 20 should be printed right

Yes. Right