Hadoop 0.20 and refactoring of the Yahoo sample code on reversed index

There are several MapReduce snippets to test and learn about Hadoop.
One of these samples is the reversed index, i.e. for each word we want to know which file it comes from. Thus the ouptut file should look like this:
hello                                     test.txt
formation formation.txt        test.txt

This example is mentioned on the Yahoo developer network, but it doesn’t work as is on version 0.20 of Hadoop.
We decided to rewrite parts of the code in order to make it compatible. This is what you will find in this blog article.

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