Here you can find the source of toTreeSet(Collection
public static <T> TreeSet<T> toTreeSet(Collection<T> collection, Comparator<T> comparator)
//package com.java2s; //License from project: Apache License import java.util.*; public class Main { public static <T> TreeSet<T> toTreeSet(Collection<T> collection, Comparator<T> comparator) { final TreeSet<T> treeSet = new TreeSet<T>(comparator); for (T t : collection) { treeSet.add(t);/*from w w w . ja v a 2 s. c om*/ } return treeSet; } }