assertTrue(isNumber.apply(2.0f));
    assertFalse(isNumber.apply(""));
    assertFalse(isNumber.apply(null));
  }

  @GwtIncompatible // Predicates.instanceOf
  public void testIsInstanceOf_interface() {
    Predicate<@Nullable Object> isComparable = Predicates.instanceOf(Comparable.class);

    assertTrue(isComparable.apply(1));
    assertTrue(isComparable.apply(2.0f));