Saturday, July 30, 2005

The complexity of Map in Java

I just posted an updated version of my util library of extensions to Java Collections in the course of which I added a ListMap interface to replace UnionMap and an implementation, ArrayHashListMap.

If one were to write read-only Map implementations, the procedure is straight-forward and well-documented in the Javadocs for Map. However, writing a modifiable map is much more of an undertaking.

Consider all the places that "modifiability" can be leaked:

entrySet()
This is burdensome as the Set has to provide modifiable operations which effect the original Map and all methods of Set which themselves return modifiable collections or iterators need to tie back to the original Map all the way down to Map.Entry.
keySet()
The same remarks for entrySet() apply to keySet() (exception Map.Entry).
values()
Similarly for values() although it is a Collection rather than a Set, a distinction of very little.
layers()
A method particular to ListMap, it returns a list of maps which comprise the PATH-like layers which are collapsed to present a single Map view.

What did I do in face of this? I introduced a new interface, Refreshing (would Refreshable have been better?), which has one method, void refresh(), implemented versions of Map List, Set and Iterator which take a Refreshing object to signal after modifiable operations complete, and extended AbstractMap to return refreshing versions of collection classes for each of the possible points where modifiability leaks. You can see the result in ListMap and ArrayHashListMap.

Thank goodness for mock objects and unit tests!

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