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Editorial Team
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Asked: 2026-06-07T03:05:30+00:00

Apparently, there are two ways to obtain a thread-safe HashSet instance using Java’s Collections

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Apparently, there are two ways to obtain a thread-safe HashSet instance using Java’s Collections utility class.

I ask:

  • How do they differ?
  • Which, and under what circumstances, is to be preferred over the other?
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1 Answer

  1. Editorial Team

    What you may be thinking of is

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

    This supports concurrent updates and reads. Its Iterator won’t throw ConcurrentModicationException. where as

    Set<Type> set = Collections.synchronizedSet(new HashSet<Type());

    Is more light weight but only allows one thread at a time to access the set. You need to lock the set explicitly if you want to Iterator over it and you can still get a CME if you don’t update it in a safe way (while iterating over it)

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