If you ever wonder how google does its fancy searching lightning fast, this should give an idea.
It is interesting how the idea was derived from Lisp(functional programming languages are good for computations with no state attached, am i right?) and iterators were used to read massive amounts of data. It again shows us that being equipped with knowledge about different programming languages and paradigms pays off.
Also if you are a Java developer, you are lucky. Because 90% of any functionality is implemented in terms of libraries and frameworks, and a simple search in google will reveal the java project. Apache has an implementation of Map-Reduce called Hadoop.
Also you might want to check out the different Java Frameworks that allow Grid Computing.