Thread-safe HashSet with Guava Collections

Question

Like the title says, i would like to get a thread-safe HashSet using Guava Collections.
Are any available?

There would be no sense in making in threadsafe when it is already immutable, isn't it? — whiskeysierra, Sep 02 '10 at 17:13
You're righy Willi. It will not be immutable. Several threads will mutate it concurrently. — santiagobasulto, Sep 02 '10 at 17:21
By the way, don't prefer collections from Guava over JDK ones just because they're from Guava. Use each when they make the most sense for what you're doing. — ColinD, Sep 02 '10 at 17:25
@Colin: Totally agree, and in fact, my answer doesn't use any Guava classes at all, just what comes with the Java Concurrency Framework. :-P — C. K. Young, Sep 02 '10 at 17:29
@CollinD: Yeah, that's absolutely true. But i know well the java.util and java.concurrent collections. I've a hard program running and was trying to test the Guava Collections. — santiagobasulto, Sep 02 '10 at 17:30
@santiago: If you really want to watch Guava shine, try the `Multimap` and `Multiset`, and the soft/weak keys/values with `MapMaker`. :-) (There are of course many other features, too many for me to cover.) — C. K. Young, Sep 02 '10 at 17:39
I've used this soft referenced map in a cache system and works real well new MapMaker().softKeys().makeMap() — santiagobasulto, Sep 02 '10 at 17:41
@whiskeysierra: Never assume a "de facto" immutable class to be thread-safe. Such a class is not protected against memory visibility and code reordering issues and therefore it is NOT thread-safe. — 30thh, Mar 10 '18 at 10:36
@30thh What's the point in having and using an immutable class if I can't rely on it? — whiskeysierra, Mar 12 '18 at 08:18
@whiskeysierra immutability has in general nothing to do with tread-safety. Only simple immutable objects with all the fields declared as final are guaranteed thread-safe. Please google for "JMM Safe Initialization". — 30thh, Mar 12 '18 at 10:00

score 28 · Answer 1 · answered Sep 02 '10 at 17:05

28

Set<K> set = Collections.newSetFromMap(new ConcurrentHashMap<K, Boolean>());

answered Sep 02 '10 at 17:05

C. K. Young

219,335
46
382
435

1

Btw, all your points belong to me as i answered this question on the guava mailing list, 8 minutes before it was posted here ;) – whiskeysierra Sep 02 '10 at 17:20
@Willi: If I could assign them to you, I would, because I've already hit my rep cap for the day, so I'm getting absolutely no rep for this answer. :-P – C. K. Young Sep 02 '10 at 17:21
2

You'd think there'd be a little bit more straightforward way of doing this. – ColinD Sep 02 '10 at 17:24
1

@Colin: Welcome to Java! If you want straightforward, code in Ruby or Scheme. :-P – C. K. Young Sep 02 '10 at 17:26
@Colin: it may seem like a monster code. But once you get in the API and familiarize with it, you get amazed how simple it get. It happened to me with the Streams Framework. – santiagobasulto Sep 02 '10 at 17:28
I'm pretty familiar with most core Java APIs, this trick just strikes me as particularly unintuitive as the best way to accomplish this. – ColinD Sep 02 '10 at 17:45

score 7 · Accepted Answer · answered Sep 06 '10 at 18:37

7

This would be the right answer, Using the Sets class from Guava. Anyway the answer from @crhis was good intended.

Sets.newSetFromMap(new ConcurrentHashMap<V, Boolean>());

answered Sep 06 '10 at 18:37

santiagobasulto

11,320
11
64
88

Actually, this is now deprecated, currently `Collections.newSetFromMap(new ConcurrentHashMap());` should be used. – sarsonj Oct 15 '19 at 12:45

score 6 · Answer 3 · answered Jun 14 '18 at 12:59

6

Java 8 introduces new way to create concurrent hash set - ConcurrentHashMap.newKeySet()

Set<K> set = ConcurrentHashMap.newKeySet();

It's shorter than wrapping in Collections.newSetFromMap

answered Jun 14 '18 at 12:59

turbanoff

2,439
6
42
99

score 4 · Answer 4 · edited May 23 '17 at 11:48

Google Collections had a factory method named Sets.newConcurrentHashSet() for a while.

Its implementation was similar to Chris's suggestion:

public static <E> Set<E> newConcurrentHashSet() {
  return newSetFromMap(new ConcurrentHashMap<E, Boolean>());
}

They had a newSetFromMap() method inside the com.google.common.collect.Sets class (written by Doug Lea with assistance from members of JCP JSR-166). That method was added to java.util.Collections in java 1.6.

It was withdrawn in Google Collections 1.0rc1, since there are plans to better support concurrent sets in Guava (more information here).

This post expands on the use of the "newSetFromMap" method to construct concurrent sets.

Google guava still has newConcurrentHashSet() method BTW. https://google.github.io/guava/releases/27.0-jre/api/docs/com/google/common/collect/Sets.html#newConcurrentHashSet-- — turbanoff, Nov 02 '18 at 13:26

Thread-safe HashSet with Guava Collections

4 Answers4

Linked