Sunday, May 20, 2012

##dark corners within java collections

* after you put a "key" object into a hashset/map, you can shoot
yourself in the foot by modifying the object's state. Its hash code
will change!
** 2 objects equals() relationship will change too.

* for sorted set/map, the compareTo() method can be inconsistent with
equals(). These sorted containers use compareTo() exclusive and ignore
equals(), but developers generally expect these containers to be
consistent with equals(). Javadoc says

It is strongly recommended (though not required) that natural
orderings be consistent with equals(). Sorted sets (and sorted maps)
without explicit comparators behave "strangely" otherwise. In
particular, such a container violates the general contract for
Set.java or Map.java, which is defined in terms of the equals().

* the equals/compareTo consistency requirement is looser than the
equals/hashcode contract.

* space efficiency of hashset (or treeset) vs hashtable (treemap). A
HashSet instance uses a HashTable instance so not so space-efficient.
Same for TreeSet vs TreeMap

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