How does Guava Set difference work?

Question

when are two objects considered to be different in Sets.difference.When they have a different hash code or when object.equals returns a false.

Answer 1

The javadoc says:

The returned set contains all elements that are contained by set1 and not contained by set2

It thus means that the rule depends on the type of the two sets. If the Set is a HashSet, for example, equals() will be used. If the set is a TreeSet, compareTo() (or the comparator's compare() method) will be used. If an IdentityHashSet is used, the identity of the object will be used.

hashCode() will never be used by any (correct) Set implementation to determine equality, because two unequal objects may have the same hashCode.