Question

HashSet is based on HashMap.

If we look at HashSet<E> implementation, everything is been managed under HashMap<E,Object>.

<E> is used as a key of HashMap.

And we know that HashMap is not thread safe. That is why we have ConcurrentHashMap in Java.

Based on this, I am confused that why we don’t have a ConcurrentHashSet which should be based on the ConcurrentHashMap?

Is there anything else that I am missing? I need to use Set in a multi-threaded environment.

Also, If I want to create my own ConcurrentHashSet can I achieve it by just replacing the HashMap to ConcurrentHashMap and leaving the rest as is?

Answer 1

There’s no built in type for ConcurrentHashSet because you can always derive a set from a map. Since there are many types of maps, you use a method to produce a set from a given map (or map class).

Prior to Java 8, you produce a concurrent hash set backed by a concurrent hash map, by using Collections.newSetFromMap(map)

In Java 8 (pointed out by @Matt), you can get a concurrent hash set view via ConcurrentHashMap.newKeySet(). This is a bit simpler than the old newSetFromMap which required you to pass in an empty map object. But it is specific to ConcurrentHashMap.

Anyway, the Java designers could have created a new set interface every time a new map interface was created, but that pattern would be impossible to enforce when third parties create their own maps. It is better to have the static methods that derive new sets; that approach always works, even when you create your own map implementations.