What's the difference between Collections.unmodifiableSet() and ImmutableSet of Guava?

Question

JavaDoc of ImmutableSet says:

Unlike Collections.unmodifiableSet, which is a view of a separate collection that can sti

Answer 1

A difference between the two not stated in other answers is that ImmutableSet does not permit null values, as described in the Javadoc

A high-performance, immutable Set with reliable, user-specified iteration order. Does not permit null elements.

(The same restriction applies to values in all Guava immutable collections.)

For example:

ImmutableSet.of(null);
ImmutableSet.builder().add("Hi").add(null); // Fails in the Builder.
ImmutableSet.copyOf(Arrays.asList("Hi", null));

All of these fail at runtime. In contrast:

Collections.unmodifiableSet(new HashSet<>(Arrays.asList("Hi", null)));

This is fine.