guava/src/com/google/common/collect/EnumMultiset.java
void checkIsE(@Nullable Object element) {
    checkNotNull(element);
    if (!isActuallyE(element)) {
      throw new ClassCastException("Expected an " + type + " but got " + element);
    }
  }
void checkIsE(@Nullable Object element) {
    checkNotNull(element);
    if (!isActuallyE(element)) {
      throw new ClassCastException("Expected an " + type + " but got " + element);
    }
  }
Returns {@code element} cast to {@code E}, if it actually is a nonnull E. Otherwise, throws
either a NullPointerException or a ClassCastException as appropriate.