Hashmap vs hashtable ....

[Andres Gonzalez]

Hey ranchers.. just wanna make sure I know pretty much all (or the main) differences between hashmap and hastables.
1o.) according to RHE book:
Hashtable does not allow the null value to be stored, although it makes some efforts to support multithreaded use...
2o.) reading a post at javaranch
hashtable are synchronized, hashmaps are not.
any others??
thanks

[shweta mathur]

HashMap extends AbstractMap
HashTable Extends Dictionary

[Valentin Crettaz]

You may also want to have a look at the following article from the August '02 Javaranch newsletter:
Storing Objects in Java Part 3 - The Map Interface by Thomas Paul
See also the following discussion in the Performance forum:
Hashtable vs Hashmap by Max Habibi
And this too:
Utilizing the Map Interface from the Collections Framework
[ October 21, 2002: Message edited by: Valentin Crettaz ]

[Valentin Crettaz]

One more article that appeared in the July 02 edition of Java Pro:
Using Hashtables in Java

[M.C. Horn]

Another important difference is that Hashtable is synchronized and Hashmap is not. This is critical in knowing, because of Thread access issues. A Thread can obtain a monitor on a Hashtable without any extra coding. Any code utilizing a Hashmap that is needed to be Thread safe must provide synchronized code block when accessing Hashmap. Same thing applies when talking about Vector versus an ArrayList. The newer implementations such as Hashmap and ArrayList do not burden the programmer with the overhead of synchronization. With Vector and Hashtable you have no choice.

[Kathy Sierra]

Howdy -
I'm too lazy to follow all the links, so although I'm sure this is mentioned in the other references, I just wanted to summarize what's important for the exam:
* Hashtable methods are synchronized, HashMap are not.
* Hashtable does not accept null anything (so, you can't have a null key and you can't have any null values)
* HashMap allows both a null key (obviously, just one) and null values
And... bonus reply here
Don't forget LinkedHashMap -- big difference between it and the other two (as far as the exam is concerned) is that there's no guaranteed iteration order for Hashtable and HashMap (and remember, you're not iterating over the actual Hashtable or HashMap, you're iterating over just the *values*) --
but with LinkedHashMap you get a choice of two different orderings: by insertion order, or by access order.
Cheers,
Kathy "too lazy to link" Sierra :roll: