Collections.newSetFromMap(»ConcurrentHashMap«) vs. Collections.synchronizedSet(»HashSet«)

Question

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

• Collections.newSetFromMap( ConcurrentHashMap )
• Collections.synchronizedSet( HashSet )

I ask:

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

Answer 1

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)