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

Consider this:

Set x = new HashSet();
x.add("foo");

ImmutableSet guava = ImmutableSet.copyOf(x);
Set builtIn = Collections.unmodifiableSet(x);

x.add("bar");
System.out.println(guava.size()); // Prints 1
System.out.println(builtIn.size()); // Prints 2

In other words, ImmutableSet is immutable despite whatever collection it's built from potentially changing - because it creates a copy. Collections.unmodifiableSet prevents the returned collection from being directly changed, but it's still a view on a potentially-changing backing set.

Note that if you start changing the contents of the objects referred to by any set, all bets are off anyway. Don't do that. Indeed, it's rarely a good idea to create a set using a mutable element type in the first place. (Ditto maps using a mutable key type.)