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 not, but I'm only performing read operations.

Answer 1

From the Javadoc:

Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally

Reading doesn't modify a set, therefore you're fine.

Answer 2

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.