public Map<String, Object> defaultValues() {
        Map<String, Object> defaultValues = new HashMap<>();
        for (ConfigKey key : configKeys.values()) {
            if (key.defaultValue != NO_DEFAULT_VALUE)
                defaultValues.put(key.name, key.defaultValue);
        }
        return defaultValues;
    }
Returns unmodifiable set of properties names defined in this {@linkplain ConfigDef}

@return new unmodifiable {@link Set} instance containing the keys
clients/src/main/java/org/apache/kafka/common/config/ConfigDef.java