I'd be explained in simple way Java is used for storing, analysing and processing large data sets. The choice of using Java as the programming language for the development of hadoop is merely accidental and not thoughtful.
Java Hadoop is mandatory for intelligent technologists to pick up Hadoop quickly with the Hadoop ecosystem getting bigger day by day. Professionals need to consider their skills before they begin to learn Hadoop. For most professionals who are from various backgrounds like Java, PHP, .net, mainframes, data warehousing, DBAs, data analytics and want to get into a career in Hadoop and Big Data. Hadoop is open-source software built on Java thus making it necessary for every Hadoop to be well-versed with at least java essentials for Hadoop. I hope this information helps you.