is Java HashSet thread-safe for read only?

Question

If I have an instance of an HashSet after I ran it through Collections.unmodifiableSet(), is it thread-safe?

I\'m asking this since Set documentation states that it\'s n

Answer 1

HashSet will be threadsafe if used in a read-only manner. That doesn't mean that any Set that you pass to Collections.unmodifiableSet() will be threadsafe.

Imagine this naive implementation of contains that caches the last value checked:

Object lastKey;
boolean lastContains;

public boolean contains(Object key) {
   if ( key == lastKey ) {
      return lastContains;
   } else {
      lastKey = key;
      lastContains = doContains(key);
      return lastContains;
   }
}

Clearly this wouldn't be threadsafe.