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Java Forum / General / January 2006Few java thread and collection questoins
A. what is the difference between Hashtable and HashMap and when would you use in instead of the other. It seems to me that HashMap should never be used for it provides same functionality as hashtable but slower. B. Is it possible all running threads in the jvm? to make it more concrete: I have main thread that spawns many child threads. A child thread would like to know all threads spawned by the father, Thanks > A. what is the difference between Hashtable and HashMap and when would > you use in instead of the other. It seems to me that HashMap should > never be used for it provides same functionality as hashtable but > slower. All the methods in Hashtable are synchronized and they arent in hash map. I guess if you dont have a contention then dont use Hashtable as it would slower the execution. > B. Is it possible all running threads in the jvm? to make it more > concrete: I have main thread that spawns many child threads. A child > thread would like to know all threads spawned by the father, You can always do this programmatically while spawning threads or use ThreadGroup > Thanks Sanjay > > A. what is the difference between Hashtable and HashMap and when would > > you use in instead of the other. It seems to me that HashMap should [quoted text clipped - 12 lines] > You can always do this programmatically while spawning threads or use > ThreadGroup I was wrong in saying programatically, I was only thinking about the parent thread here. You can use ThreadGroup though. > > Thanks > > Sanjay > I guess if you dont have a contention then dont use Hashtable as it > would slower the execution. Hmm. What is the difference in implementation of these datastructures? Isn't HashMap uses some sort Black tree and those get time isO( Log n)? Whereas in hashtable it is O(1)? > You can always do this programmatically while spawning threads or use > ThreadGroup a. Thread groups are deprecated and not recommended for use. b. In my case, I only have access to spawned thread and I'd like to know what other threads exist, that should include the main thread as well. I have a feeling that it might be a security violation. Thanks a lot. >> I guess if you dont have a contention then dont use Hashtable as it >> would slower the execution. > > Hmm. What is the difference in implementation of these datastructures? Almost nothing. > Isn't HashMap uses some sort Black tree and those get time isO( Log > n)? Whereas in hashtable it is O(1)? No HashMap is a hash table/map. TreeMap is a red-black tree implementation. If you stick to declaring references as Map, you can switch between the different implementations as you like. HashMap is approximately O(1) in the normal case. However in the worst case it is O(n). >> You can always do this programmatically while spawning threads or use >> ThreadGroup > a. Thread groups are deprecated and not recommended for use. ThreadGroup is not deprecated. It's generally not recommended, because it isn't recommended to do that sort of thing. > b. In my case, I only have access to spawned thread and I'd like to > know what other threads exist, that should include the main thread as > well. I have a feeling that it might be a security violation. You should be alright if a SecurityManager is not set. If it does throw, then that's because you're not allowed at that information, rather than the particular approach you take. If it's just for debugging, ThreadGroup should be fine, as is ctrl-\ (ctrl-break in Windows, IIRC) from the console, jdb or other debugger, or the JVM JMX (java.lang.management) possibly remotely through the likes of jstack (not Windows) and jconsole. Tom Hawtin  SignatureUnemployed English Java programmer http://jroller.com/page/tackline/ > A. what is the difference between Hashtable and HashMap and when would > you use in instead of the other. It seems to me that HashMap should > never be used for it provides same functionality as hashtable but > slower. I don't know where you got the idea that HashMap is slower than Hashtable. HashMap provides constant time operations for simple operations (put, get) - that's the whole point of hashing (if it's a good hashing algorithm). The main differences are that HashMap is not synchronized, and permits null values. The synchronisation theoretically makes Hashtable slower. > > A. what is the difference between Hashtable and HashMap and when would > > you use in instead of the other. It seems to me that HashMap should [quoted text clipped - 7 lines] > The main differences are that HashMap is not synchronized, and permits null > values. The synchronisation theoretically makes Hashtable slower. Yes, I made a mistake, I meant TreeMap. When would you use TreeMap is opposed to HashMap? Thanks. > Yes, I made a mistake, I meant TreeMap. When would you use TreeMap is > opposed to HashMap? > > Thanks. When you want the keys to be sorted. Sorting is done with the Comparable interface, or a Comparator object. Yes this is slower, but it can be faster than having to sort them each time you need a sorted view. > > Yes, I made a mistake, I meant TreeMap. When would you use TreeMap is > > opposed to HashMap? [quoted text clipped - 4 lines] > interface, or a Comparator object. Yes this is slower, but it can be > faster than having to sort them each time you need a sorted view. That does it - just like in cpp. Thanks. > > Yes, I made a mistake, I meant TreeMap. When would you use TreeMap is > > opposed to HashMap? [quoted text clipped - 4 lines] > interface, or a Comparator object. Yes this is slower, but it can be > faster than having to sort them each time you need a sorted view. That does it - just like in cpp. Thanks. >I don't know where you got the idea that HashMap is slower than Hashtable. >HashMap provides constant time operations for simple operations (put, get) >- that's the whole point of hashing (if it's a good hashing algorithm). HashMap is slightly faster since it is does not have the thread-safety overhead. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Nice link, gives you most of the information you need about hasmap, hashtable, treemap.. http://www.javaranch.com/newsletter/Aug2002/mapinterface.html Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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