is the int value of String.hashCode() unique?

Question

I encountered a problem days ago.Now i have tens of millions of words,type of string. now i decide to keep them in database and use index to keep them unique.And i do not want to compare the original words to keep them unique. I would like to make sure whether the hashCode() method of a string can be unique , will it not be changed if a use another laptop or different time or something like that?

Answer 1

Unique, no. By nature, hash values are not guaranteed to be unique.

Any system with an arbitrarily large number of possible inputs and a limited number of outputs will have collisions.

So, you won't be able to use a unique database key to store them if it's based only on the hash code. You can, however, use a non-unique key to store them.

In reply to your second question about whether different versions of Java will generate different hash codes for the same string, no.

Provided a Java implementation follows the Oracle documentation (otherwise it's not really a Java implementation), it will be consistent across all implementations. The Oracle docs for String.hashCode specify a fixed formula for calculation the hash:

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

You may want to check this is still the case if you're using wildly disparate versions of Java (such as 1.2 vs 8) but it's been like that for a long time, at least since 1.5.

Answer 2

No,

Because a string in java can have maximum 2,147,483,647 (2^31 - 1) no of characters and all characters will vary so it will produce a very large no of combinations, but integer have only a range from -2,147,483,648 to 2,147,483,648. So it is impossible, and using this method the hash code of a string is computed

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1].

Example :

If you create two string variables as "FB" and "Ea" there hash code will be same.