Is Guava's ImmutableList.Builder thread safe?

Question

What are the thread safety guarantees for Guava's ImmutableList.Builder? The javadocs don't say.

Answer 1

While the Guava Immutable classes are threadsafe, their builders are not. For most applications, only one thread will interact with any particular Builder instance.

While the absence of thread-safety usually doesn't need to be documented, such Javadoc might make sense for the Immutable collection builders. People may be surprised that ImmutableList is threadsafe while ImmutableList.Builder isn't.