Omino's santa list

package santaslist;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashSet;
import java.util.Set;

//This piece of code SHOULD return a list of unique strings with certain characteristics. The excercise was taken from
//adventofcode.com, more specifically, from day 5. This code does NOT return the correct answer.
public class SantasList {

    static FileReader file;
    static BufferedReader br;
    static Set<String> set = new HashSet<>();
    static String[] letters = {"aa", "bb", "cc", "dd", "ee", "ff", "gg", "hh", "ii", "jj", "kk", "ll", "mm", "nn", "oo", "pp", "qq", "rr", "ss", "tt", "uu", "vv", "zz", "xx", "yy", "ww"};
    static String[] vocali = {"a", "e", "i", "o", "u"};

    public static void main(String[] args) throws FileNotFoundException, IOException {

        file = new FileReader("list/santaslist.txt");
        br = new BufferedReader(file);
        String line;
        int nice = 0;

        while ((line = br.readLine()) != null) {
            String fp = firstPass(line);
//I tried doing the following in a single loop. The result was exactly the same, but at least it looks sorta cleaner :p
            if (fp != null) {
                String sp = secondPass(fp);
                if (sp != null) {
                    String tp = thirdPass(sp);

                    if (tp != null) {
                        set.add(tp);
                    }
                }
            }

        }

        for (String s : set) {
            nice++;

            System.out.println(s + " " + nice); //prints the final list of "good" strings with an increment.

        }
    }

    public static String firstPass(String s) {

        for (String str : letters) {
            if (s.contains(str)) {
                return s;
            }

        }
        return null;

    }

    public static String secondPass(String s) {
        if ((!s.contains("ab")
                && (!s.contains("cd"))
                && (!s.contains("pq"))
                && (!s.contains("xy")))) {

            return s;
        }
        return null;

    }

    public static String thirdPass(String s) {
        int voc = 0;
        for (String v : vocali) {

            if (s.contains(v)) {
                voc++;

                if (voc >= 3) {
                    voc = 0;
                    return s;
                }

            }
        }

        return null;
    }
}