The java.util.concurrent package provides a fine-designed ConcurrentHashMap which enables efficient concurrent access to objects in a map container. However, there is a lack of concurrent Set in this package.
Fortunately, we can make our own ConcurrentHashSet based on ConcurrentHashMap by the following codes.
According to Collections.newSetFromMap()’s documentation:
Returns a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map.
Therefore, our ConcurrentHashSet will exhibit the same concurrency properties as the underlying ConcurrentHashMap does. Since our set’s interface doesn’t have defined methods corresponding to the features ConcurrentHashMap brings over Map, like putIfAbsent() and numerous other new Java 8 methods like computerIfAbsent(), it is just a Set that happens to be concurrent instead of designed to be.