Hadoop underwent a major API change in its 0.20 release, which is the primary interface in the 1.0 version we use in this book. Though the prior API was certainly functional, the community felt it was unwieldy and unnecessarily complex in some regards.
The new API, sometimes generally referred to as context objects, for reasons we'll see later, is the future of Java's MapReduce development; and as such we will use it wherever possible in this book. Note that caveat: there are parts of the pre-0.20 MapReduce libraries that have not been ported to the new API, so we will use the old interfaces when we need to examine any of these.
The 0.20 and above versions of MapReduce API have most of the key classes and interfaces either in the org.apache.hadoop.mapreduce package or its subpackages.
In most cases, the implementation of a MapReduce job will provide job-specific subclasses of the Mapper and Reducer base classes found in this package...