hashtable

I am trying to understand what should drive the choice of the access method while using a BerkeleyDB : B-Tree versus HashTable. A Hashtable provides O(1) lookup but inserts are expensive (using Linear/Extensible hashing we get amortized O(1) for insert). But B-Trees provide log N (base B) lookup and insert times. A B-Tree can also support range queries and allow access in sorted order. Apart from these considerations what else should be factored in? If I don't need to support range queries can I just use a Hashtable access method? When your data sets get very large, B-trees are still better

Why does Google sparsehash open-source library has two implementations: a dense hashtable and a sparse one? The dense hashtable is your ordinary textbook hashtable implementation. The sparse hashtable stores only the elements that have actually been set, divided over a number of arrays. To quote from the comments in the implementation of sparse tables: // The idea is that a table with (logically) t buckets is divided // into t/M *groups* of M buckets each. (M is a constant set in // GROUP_SIZE for efficiency.) Each group is stored sparsely. // Thus, inserting into the table causes some array

I am trying to iterate through a list of keys from a hash table using enumeration however I keep getting a NoSuchElementException at the last key in list? Hashtable<String, String> vars = new Hashtable<String, String>(); vars.put("POSTCODE","TU1 3ZU"); vars.put("EMAIL","job.blogs@lumesse.com"); vars.put("DOB","02 Mar 1983"); Enumeration<String> e = vars.keys(); while(e.hasMoreElements()){ System.out.println(e.nextElement()); String param = (String) e.nextElement(); } Console output: EMAIL POSTCODE Exception in thread "main" java.util.NoSuchElementException: Hashtable Enumerator at java.util

I'm trying to implement my own hash function, i add up the ASCII numbers of each string, using java. I find the hash code by finding the mod of the size of the hash table and the sum. size%sum. I was wondering if there was a way to use the same process but reduce collisions, when searching for the string? Thanks in advance. I would look at the code for String and HashMap as these have a low collision rate and don't use % and handle negative numbers. From the source for String public int hashCode() { int h = hash; if (h == 0 && value.length > 0) { char val[] = value; for (int i = 0; i < value

I want my hash to sort in descending order according to the values. How do I do that in Java? A HashMap (and its legacy predecesor Hashtable ) is by nature unordered. Even if you sort it, it will remain unordered. If you want to maintain insertion order, then use LinkedHashMap instead. If you want an automatic sort on keys , regardless of insertion order, then use SortedMap instead. If you want to sort a Map on values , then you basically need to put the key/value pairs in another kind of a sortable data structure, e.g. List<Entry<K, V>> , then sort it using Collections#sort() with help of a