*
   * @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() {

    set.add(new Derived("foo"));
    set.add(new Derived("bar"));
    assertThat(set).containsExactly(new Derived("bar"), new Derived("foo")).inOrder();
  }

  public void testNewTreeSetEmptyNonGeneric() {
    TreeSet<LegacyComparable> set = Sets.newTreeSet();
    assertTrue(set.isEmpty());
    set.add(new LegacyComparable("foo"));
    set.add(new LegacyComparable("bar"));
    assertThat(set)

    List<Object> result = doConcurrentGet(cache, "bar", count, startSignal);

    assertEquals(1, callCount.get());
    for (int i = 0; i < count; i++) {
      assertThat(result.get(i)).isInstanceOf(InvalidCacheLoadException.class);
    }

    // subsequent calls should call the loader again, not get the old exception
    try {
      cache.getUnchecked("bar");
      fail();

    List<Object> result = doConcurrentGet(cache, "bar", count, startSignal);

    assertEquals(1, callCount.get());
    for (int i = 0; i < count; i++) {
      assertThat(result.get(i)).isInstanceOf(InvalidCacheLoadException.class);
    }

    // subsequent calls should call the loader again, not get the old exception
    try {
      cache.getUnchecked("bar");
      fail();

    TreeMap<Derived, Integer> map = Maps.newTreeMap();
    assertEquals(Collections.emptyMap(), map);
    map.put(new Derived("foo"), 1);
    map.put(new Derived("bar"), 2);
    assertThat(map.keySet()).containsExactly(new Derived("bar"), new Derived("foo")).inOrder();
    assertThat(map.values()).containsExactly(2, 1).inOrder();
    assertNull(map.comparator());
  }

  public void testTreeMapNonGeneric() {

    TreeMap<Derived, Integer> map = Maps.newTreeMap();
    assertEquals(Collections.emptyMap(), map);
    map.put(new Derived("foo"), 1);
    map.put(new Derived("bar"), 2);
    assertThat(map.keySet()).containsExactly(new Derived("bar"), new Derived("foo")).inOrder();
    assertThat(map.values()).containsExactly(2, 1).inOrder();
    assertNull(map.comparator());
  }

  public void testTreeMapNonGeneric() {

    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());

  }

  private static CharMatcher.IsEither isEither(char c1, char c2) {
    return new CharMatcher.IsEither(c1, c2);
  }

  /** Implementation of {@link #anyOf(CharSequence)} for exactly two characters. */
  private static final class IsEither extends FastMatcher {

    private final char match1;
    private final char match2;

    IsEither(char match1, char match2) {
      this.match1 = match1;

  }

  private static CharMatcher.IsEither isEither(char c1, char c2) {
    return new CharMatcher.IsEither(c1, c2);
  }

  /** Implementation of {@link #anyOf(CharSequence)} for exactly two characters. */
  private static final class IsEither extends FastMatcher {

    private final char match1;
    private final char match2;

    IsEither(char match1, char match2) {
      this.match1 = match1;