>Any large HashTable in Java starts to yield the problem of duplicate keys, it's just a weird situation, like you can 99.999% trust something ... but can't ever fully trust it so that over time, you're guaranteed to have something wrong.

hashCode() is a prehash function the outputs of which need to be mapped further to the (typically much smaller) number of buckets in a hash table of certain size (which would depend on the number of objects currently in the table), those "duplicate keys" are not a problem, they're how hash tables work in any language. Objects' hashcodes are used to find the relevant bucket, then this bucket is properly examined using equals(). HashMap and Hashtable are backed by arrays which have the max size of Integer.MAX_VALUE (minus some change) in JVM anyway, so those would need to be indexed by an int. I hope this helps to overcome the trust issues you have with Java data structures.