Bad idea to use String key in HashMap?

Question

I understand that the String class\' hashCode() method is not guarantied to generate unique hash codes for distinct String-s. I see a lot of usage of putting Strin

Answer 1

This is not an issue, it's just how hashtables work. It's provably impossible to have distinct hashcodes for all distinct strings, because there are far more distinct strings than integers.

As others have written, hash collisions are resolved via the equals() method. The only problem this can cause is degeneration of the hashtable, leading to bad performance. That's why Java's HashMap has a load factor, a ratio between buckets and inserted elements which, when exceeded, will cause rehashing of the table with twice the number of buckets.

This generally works very well, but only if the hash function is good, i.e. does not result in more than the statistically expected number of collisions for your particular input set. String.hashCode() is good in this regard, but this was not always so. Allegedly, prior to Java 1.2 it only inlcuded every n'th character. This was faster, but caused predictable collisions for all String sharing each n'th character - very bad if you're unluck enough to have such regular input, or if someone want to do a DOS attack on your app.