Question

I ran into some shady piece of code in which a HashSet is accessed and modified by several threads at the same time, and I wanted to know what effects this could have had, considering the Javadoc only explains the behavior is “non-deterministic” under concurrent, unsynchronized accesses.

Specifically, is it possible that a call to remove(element) returns true, but that the effects of the call are never ever seen by other threads (ie. contains(element) will always return true in other threads) ?

PS: Just to reassure you, I know this code is bad and I will replace the HashSet with an equivalent synchronized collection.

Answer 1

Accessing HashSet in multiple threads can.

• result in a change you make not being seen by another thread.
• an Iterator to fail with a ConcurrentModifcationException.
• a thread to go into an infinite loop inside HashSet code (rare but pretty bad when it does)

Note: a simple replacement is to use

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