Returns a string value that is used as a hash value for MapCollections
such as Table, Relation, Set, Bag, and Directory. MapCollections use this
string value to hash an object for hash table-based searches.
Object implementations are expected to abide by a general contract for hash code usage:
Whenever hashCode is invoked on the same object more than once, hashCode must return the
same hashcode value, provided than none of the internal information the object uses for an "==" comparison has changed.
If two object instances compare equal using the "==" operator, the hashCode methods for both object instances must return
the same value.
It is not required that two object instances that compare unequal using "==" return different hash code
values.
Returning a wide range of hash values will produce better performance when an object is used as an index for a
MapCollection. A return value of 4 string characters is recommended. The characters in the hash value may be any
characters from '00'x to 'ff'x, inclusive.
