Problem 79 - Java, aka fun with HashSets

So, as it was free, I solved problem 79 in Java. Things weren't that bad, and my aversion to Java for the past year and a half or so made me actually pleasantly surprised to find that it wasn't too bad to work with the language, even though I will admit that Vector<HashSet<Integer>> is a lot of boilerplate, but nonetheless, having sets available was good for this problem. Nothing too special, runs in .134 seconds on my machine. Also, notably, my solution depends on the fact that there are no duplicate numbers in the answer, things would be a bit harder if those were allowed.

import java.util.Scanner;
import java.util.HashSet;
import java.util.Vector;

public class e79 {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        Vector<HashSet<Integer>> preceding = Vector<HashSet<Integer>>();
        HashSet<Integer> used = new HashSet<Integer>();
        for (int i = 0; i < 10; ++i) {
            preceding.add(new HashSet<Integer>());
        } 
        while (scan.hasNextInt()) {
            int entry = scan.nextInt();
            Integer d1 = entry / 100;
            Integer d2 = (entry / 10) % 10;
            Integer d3 = entry % 10;
            preceding.get(d2).add(d1);
            preceding.get(d3).add(d1);
            preceding.get(d3).add(d2);
            used.add(d1);
            used.add(d2);
            used.add(d3);
        }
        String ans = "";
        int i = 0;
        while (i < 10) {
            if (preceding.get(i).isEmpty() && used.contains(i)) {
                ans += i;
                used.remove(i);
                for (HashSet<Integer> hs : preceding) {
                    hs.remove(i);
                }
                i = 0;
            } else {
                ++i;
            }
        }
        System.out.println(ans);
    }
}