*
   * @since 20.0 (varargs overload since 2.0)
   */
  public static void checkArgument(
      boolean expression, String errorMessageTemplate, char p1, char p2) {
    if (!expression) {
      throw new IllegalArgumentException(lenientFormat(errorMessageTemplate, p1, p2));
    }
  }

  /**

    Iterator<String> iterator = Collections.singletonList("foo").iterator();
    assertEquals("foo", Iterators.getOnlyElement(iterator, "bar"));
  }

  public void testGetOnlyElement_withDefault_empty() {
    Iterator<String> iterator = Iterators.emptyIterator();
    assertEquals("bar", Iterators.getOnlyElement(iterator, "bar"));
  }

  public void testGetOnlyElement_withDefault_empty_null() {

   *         .collect(
   *             flatteningToMultimap(
   *                  str -> str.charAt(0),
   *                  str -> str.substring(1).chars().mapToObj(c -> (char) c),
   *                  MultimapBuilder.linkedHashKeys().arrayListValues()::build));
   *
   * // is equivalent to
   *
   * static final ListMultimap<Character, Character> FIRST_LETTER_MULTIMAP;
   *

    assertEquals(Object[].class, TypeToken.of(Object[][].class).getComponentType().getType());
    assertEquals(char.class, TypeToken.of(char[].class).getComponentType().getType());
    assertEquals(char[].class, TypeToken.of(char[][].class).getComponentType().getType());
    assertEquals(byte.class, TypeToken.of(byte[].class).getComponentType().getType());