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.Collection; import java.util.Comparator; import java.util.TreeSet; 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);// w w w. jav a 2 s .co m } return treeSet; } }