Java Collection - 如何从TreeSet(排序集合)获取值

import java.util.NavigableSet;
import java.util.TreeSet;

public class Main {
  public static void main(String[] args) {
    NavigableSet<Integer> ns = new TreeSet<>();
    ns.add(0);
    ns.add(1);
    ns.add(2);
    ns.add(3);
    ns.add(4);
    ns.add(5);
    ns.add(6);

    // Get a reverse view of the navigable set
    NavigableSet<Integer> reverseNs = ns.descendingSet();

    // Print the normal and reverse views
    System.out.println("Normal order: " + ns);
    System.out.println("Reverse order: " + reverseNs);

    NavigableSet<Integer> threeOrMore = ns.tailSet(3, true);
    System.out.println("3 or  more:  " + threeOrMore);
    System.out.println("lower(3): " + ns.lower(3));
    System.out.println("floor(3): " + ns.floor(3));
    System.out.println("higher(3): " + ns.higher(3));
    System.out.println("ceiling(3): " + ns.ceiling(3));

    System.out.println("pollFirst(): " + ns.pollFirst());
    System.out.println("Navigable Set:  " + ns);

    System.out.println("pollLast(): " + ns.pollLast());
    System.out.println("Navigable Set:  " + ns);

    System.out.println("pollFirst(): " + ns.pollFirst());
    System.out.println("Navigable Set:  " + ns);

    System.out.println("pollFirst(): " + ns.pollFirst());
    System.out.println("Navigable Set:  " + ns);

    System.out.println("pollFirst(): " + ns.pollFirst());
    System.out.println("Navigable Set:  " + ns);

    System.out.println("pollFirst(): " + ns.pollFirst());
    System.out.println("pollLast(): " + ns.pollLast());
  }
}