@Override
    public String toString() {
        List<String> features = new ArrayList<>();
        localSupportedFeatures.forEach((key, value) -> features.add(key + ": " + value));
        features.sort(String::compareTo);
        List<String> nodeIds = new ArrayList<>();
        quorumNodeIds.forEach(id -> nodeIds.add("" + id));
        nodeIds.sort(String::compareTo);
        return "QuorumFeatures" +
            "(nodeId=" + nodeId +
            ", localSupportedFeatures={" + features + "}" +
            ", quorumNodeIds=[" + nodeIds + "]" +
            ")";
    }
A holder class of the local node's supported feature flags as well as the quorum node IDs.
metadata/src/main/java/org/apache/kafka/controller/QuorumFeatures.java