hashtable

As specified in JDK documentation, Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values. Why is this? Hashtable is the older class, and its use is generally discouraged. Perhaps they saw the need for a null key, and more importantly - null values, and added it in the HashMap implementation. HashMap is newer, and has more advanced capabilities, which are basically just an improvement on the Hashtable functionality. When HashMap was created, it was specifically designed to handle null values as keys and handles them as a special case. Edit From

How is the rehashing process done in a hashmap or hashtable when the size exceeds the maxthreshold value? Are all pairs just copied to a new array of buckets? EDIT: What happen to the elements in the same bucket (in linked list) after rehashing? I mean will they remain in same bucket after rehashing? dharam The maximum threshold in the question is called the load factor. It is advisable to have a load factor of around 0.75. Load factor is defined as (m/n) where n is the total size of the hash table and m is the preferred number of entries which can be inserted before a increment in size of the

Why is that there is no Hashtable support as part of Standard C Library? Is there any specific reason for this? paxdiablo There is no hashtable in the standard C library because either: no-one has submitted a proposal to the working group; or the working group has deemed it unnecessary. That's the way ISO works. Proposals are put forward and accepted or rejected. You have to be careful what you add to the standard library since you have two conflicting groups. As a user, you might want every data structure under the sun to be added to the standard to make the language more useful. But, as a

I have sets of hashes (first 64 bits of MD5, so they're distributed very randomly) and I want to be able to see if a new hash is in a set, and to add it to a set. Sets aren't too big, the largest will be millions of elements, but there are hundreds of sets, so I cannot hold them all in memory. Some ideas I had so far: I tried just keeping it all in sqlite table, but it becomes really really slow once it cannot fit everything in memory. Bloom filters sound like they would have very high error rate. I don't mind tiny error rate (64 bit hash gives 1 collision on 4G element set already), but error

Can somebody explain the main differences between (advantages / disadvantages) the two implementations? For a library, what implementation is recommended? Richard Barrell Wikipedia's article on hash tables gives a distinctly better explanation and overview of different hash table schemes that people have used than I'm able to off the top of my head. In fact you're probably better off reading that article than asking the question here. :) That said... A chained hash table indexes into an array of pointers to the heads of linked lists. Each linked list cell has the key for which it was allocated