Hi Rajesh,
You can refer "thinking in java" book for practice.

I would say read "Kathy Sierra" for SCJP certification.. This book is really good and explained all core Java concepts in simple words and good examples & chapters are not very big so you can easily read them in shorter time.

Also Mapper & Reducer used in hadoop are "Static inner classes" inside the program i.e. "WordCount", so you can also need to know about "INNER CLASSES" in Java.

Hope this helps!

here are some good books

- Hadoop: The Definitive Guide by Tom White
- Map Reduce Design Patterns by Donald Miller and Adam Shook
- Hadoop in Action by Chuck Lam
- Hadoop in Practice by Alex Holmes

A slightly more advanced guide to running Hadoop. It includes chapters that detail how to best move data around, how to think in Map Reduce, and (importantly) how to debug and optimize your jobs.

- Pro Hadoop by Jason Venner

This A-Press book claims it will guide you through initial hadoop set up while also helping you avoid many of the pitfalls that usual Hadoop novices encounter. Again it is similar in contents to Hadoop in Action and The Definitive Guide
- Real World Hadoop Solutions Cookbook by Jonathan Owens, Brian Femiano & Jon Lentz

A book which aims to provide real-world examples of common hadoop problems. It also covers building integrated solutions using surrounding tools (hive, pig, girafe, etc)

- Hadoop Map Reduce Cookbook by Srinath Perera

- Instant Map Reduce Patterns by Srinath Perera

This book is built around seven map reduce ‘recipes’ to learn from. It aims to be a consise, practical guide to get you coding.