How can i generate a long hash of a String?

问题

I have a java applciation in which I want to generate long ids for strings (in order to store those strings in neo4j). In order to avoid data duplication, I would like to generate an id for each string stored in a long integer, which should be unique for each string. How can I do that ?

回答1:

long has 64 bits. A String of length 9 has 72 bits. from pigeon hole principle - you cannot get a unique hashing for 9 chars long strings to a long.

If you still want a long hash: You can just take two standard [different!] hash functions for String->int, hash1() and hash2() and calculate: hash(s) = 2^32* hash1(s) + hash2(s)

回答2:

This code will calculate pretty good hash:

String s = "some string";
long hash = UUID.nameUUIDFromBytes(s.getBytes()).getMostSignificantBits();

回答3:

Why don't you have a look a the hashcode() function of String, and just adopt it to using long values instead?

Btw. if there was a way to create a unique ID for each String, then you would have found a compression algorithm that would be able to pack every String into 8 bytes (not possible by definition).

回答4:

There are many answers, try the following:

• http://stackoverflow.com/questions/415953/generate-md5-hash-in-java EDIT: removed, I've missed the long requirement. Mea culpa.
• http://en.wikipedia.org/wiki/Perfect_hash_function

Or, as suggested before, check out the sources.

PS. One more technique is to maintain a dictionary of strings: since you're unlikely to get 2⁶⁴ strings any time soon, you can have perfect mapping. Note though that that mapping may as well become a major bottleneck.

来源：https://stackoverflow.com/questions/9309723/how-can-i-generate-a-long-hash-of-a-string