title: Using set/list data types for intermediate keys in Hadoop
body: <p>In an Apache Hadoop map-reduce program, what are the options for using sets/lists as keys in the output from the mapper?</p>
  
  <p>My initial idea was to use <code>ArrayWritable</code> as key type, but that is not allowed, as the class does not implement <code>WritableComparable</code>. Do I need to define a custom class, or is there some other set like class in the Hadoop libraries that can act as key?</p>