code: public static void checkSchema(Schema schema) {
      Preconditions.checkNotNull(schema, "Schema cannot be null");
      List<String> incompatible = getIncompatibleNames(schema);
      ValidationException.check(incompatible.isEmpty(),
          "Field names are not alphanumeric (plus '_'): %s",
          Joiner.on(", ").join(incompatible));
    }
docs: Precondition-style validation that a {@link Schema} is compatible.
  
  @param schema an avro {@code Schema}