android/guava/src/com/google/common/collect/Sets.java
public static <E> TreeSet<E> newTreeSet(Comparator<? super E> comparator) {
    return new TreeSet<E>(checkNotNull(comparator));
  }
public static <E> TreeSet<E> newTreeSet(Comparator<? super E> comparator) {
    return new TreeSet<E>(checkNotNull(comparator));
  }
Creates a <i>mutable</i>, empty {@code TreeSet} instance with the given comparator.

<p><b>Note:</b> if mutability is not required, use {@code
ImmutableSortedSet.orderedBy(comparator).build()} instead.

<p><b>Note for Java 7 and later:</b> this method is now unnecessary and should be treated as
deprecated. Instead, use the {@code TreeSet} constructor directly, taking advantage of the new
<a href="http://goo.gl/iz2Wi">"diamond" syntax</a>. One caveat to this is that the {@code
TreeSet} constructor uses a null {@code Comparator} to mean "natural ordering," whereas this
factory rejects null. Clean your code accordingly.

@param comparator the comparator to use to sort the set
@return a new, empty {@code TreeSet}
@throws NullPointerException if {@code comparator} is null