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 String keys into HashMap-s (using the default String hashCode() method). A lot of this usage could result in significant application issues if a map put displaced a HashMap entry that was previously put onto the map with a truely distinct String key.

What are the odds that you will run into the scenario where String.hashCode() returns the same value for distinct String-s? How do developers work around this issue when the key is a String?

Answer 1

I strongly suspect that the HashMap.put method does not determine whether the key is the same by just looking at String.hashCode.

There is definitely going to be a chance of a hash collision, so one would expect that the String.equals method will also be called to be sure that the Strings are truly equal, if there is indeed a case where the two Strings have the same value returned from hashCode.

Therefore, the new key String would only be judged to be the same key String as one that is already in the HashMap if and only if the value returned by hashCode is equal, and the equals method returns true.

Also to add, this thought would also be true for classes other than String, as the Object class itself already has the hashCode and equals methods.

Edit

So, to answer the question, no, it would not be a bad idea to use a String for a key to a HashMap.

Answer 2

You are talking about hash collisions. Hash collisions are an issue regardless of the type being hashCode'd. All classes that use hashCode (e.g. HashMap) handle hash collisions just fine. For example, HashMap can store multiple objects per bucket.

Don't worry about it unless you are calling hashCode yourself. Hash collisions, though rare, don't break anything.

Answer 3

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.