private static void checkCanSet(String fieldName, Object fieldVal, Object val) {
        if (fieldVal != null && fieldVal != val)
            throw new SchemaBuilderException("Invalid SchemaBuilder call: " + fieldName + " has already been set.");
    }
Return a concrete instance of the {@link Schema} specified by this builder
@return the {@link Schema}
connect/api/src/main/java/org/apache/kafka/connect/data/SchemaBuilder.java