PINs generator problem

Main.java

import java.util.Collection;
import java.util.Objects;
import java.util.TreeSet;

public class Main {
  public static void main(String[] args) {
    assertEquals("0,5,7,8,9", getPossiblePins("8"));
    assertEquals("11,12,14,21,22,24,41,42,44", getPossiblePins("11"));
    assertEquals("13,15,16,19,43,45,46,49,53,55,56,59,73,75,76,79", getPossiblePins("46"));
  }

  private static void assertEquals(String expected, String actual) {
    if (!Objects.equals(expected, actual)) {
      throw new AssertionError("expected: " + expected + " but was: " + actual);
    }
  }

  public static String getPossiblePins(String pin) {
    Collection<String> pins = new TreeSet<>();
    generatePins("", pin, pins);
    return String.join(",", pins);
  }

  private static void generatePins(String prefix, String suffix,
                                   Collection<String> pins) {
    pins.add(prefix + suffix);
    if (suffix.isEmpty()) {
      return;
    }

    String closeDigits = getCloseDigits(suffix.charAt(0));
    suffix = suffix.substring(1);
    for (int i = 0; i < closeDigits.length(); i++) {
      char nextDigit = closeDigits.charAt(i);
      generatePins(prefix + nextDigit, suffix, pins);
    }
  }

  private static String getCloseDigits(char c) {
    switch (c) {
      case '0':
        return "08";
      case '1':
        return "124";
      case '2':
        return "1235";
      case '3':
        return "236";
      case '4':
        return "1457";
      case '5':
        return "24568";
      case '6':
        return "3569";
      case '7':
        return "478";
      case '8':
        return "57890";
      case '9':
        return "689";
      default:
        throw new IllegalArgumentException("illegal number");
    }
  }
}