ConcurrentHashMap VS. HashTable

Question

Hi all,
I need to use a thread-safe map and am not sure if I can safely use java.util.concurrent.ConcurrentHashMap instead of HashTable w/o external synchronization. The Java docs says:
"A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable.
*+However, even though all operations are thread-safe, retrieval operations do+ +not+ +entail locking, and there is+ +not+ +any support for locking the entire table in a way that prevents all access. This class is fully interoperable with+ **+Hashtable+** +in programs that rely on its thread safety but not on its synchronization details.+*
*+Retrieval operations (including+ **+get+**+) generally do not block, so may overlap with update operations (including+ **+put+** +and+ **+remove+**+). Retrievals reflect the results of the most recently+ +completed+ +update operations holding upon their onset. For aggregate operations such as+ **+putAll+** +and+ **+clear+**+, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration+*.
They do +not+ throw [ConcurrentModificationException|http://java.sun.com/javase/6/docs/api/java/util/ConcurrentModificationExceptio
*+However, iterators are designed to be used by only one thread at a time.+*"
I do use iterators on my map; and it will be used by many threads at the same time; so does that mean I need to externally "synchronize" my map?
The parts in bold and italic made me not sure about using the ConcurrentHashMap...

Answer 1

JavaFunda wrote:
So can anybody give a example where we hashtable can not be replace by ConcusrrentHashMapPersonally, other than Peter's first suggestion, I can't think of a single one. I believe that EJP's post (other than line 3) was aimed at why you might prefer a HashMap ( not Hashtable) over CHM; and he's dead right.
Winston