I was writing a new interface layer in Java 7 today where I'm being handed a bunch of key/value pairs to use to look up and return various items of data.
So I wrote an immutable class with a constructor that processes the key/value pairs and loads up the keys into the correct locations in the object.
But I need to sort the queries so that all of the Key A queries are grouped together and all of the Key B queries are grouped together within the Key A queries. So I wrote a compareTo method for the immutable class that works through all of the data elements in the class, starting with Key A, then Key B, working down through Key N. Then I tossed the keys into a TreeSet, which happily sorted them so that when I walk through the elements, all of the matching values for Key A come first, all of the matching entries for Key B within Key A come next, and so on.
Then I created a class that would load an instance of the big in-memory dataset that Key A refers to. And I wrote a moderately long switch/case statement for each of the different kinds of data that I might return from the big dataset based on the original class that loaded up all of the key/value pairs.
I think this might work.
Of course, I still have to test it... :)