No it is not hard as a large extent of Big Data is nothing but Java because most of the concepts of Big Data were written in Java. So you can easily move into Big Data simply by building Java Solutions that run on top of Hadoop. If you are looking for a place eto learn this technology I will suggest you Intellipaat which is offering the best Big Data Hadoop Course in collaboration with IBM. This program help you to master all the required skills and to bag some handsome salaries in this field.
Along with that do check out this video on how Intellipaat helped its learner Pankaj to become an Big data developer being a Java developer -