In Java 8 I would use:

Set<String> set = Stream.of( "a", "b" ).collect( Collectors.toSet() );

This gives you a mutable Set pre-initialized with "a" and "b".

In Java 8 I would use:

Set<String> set = Stream.of("a", "b").collect(Collectors.toSet());

This gives you a mutable Set pre-initialized with "a" and "b". Note that while in JDK 8 this does return a HashSet, the specification doesn't guarantee it, and this might change in the future. If you specifically want a HashSet, do this instead:

Set<String> set = Stream.of("a", "b")
                        .collect(Collectors.toCollection(HashSet::new));