public interface ConsumerGroupMetadata {

    String groupId();

    int generationId();

    String memberId();

    Optional<String> groupInstanceId();
}

class DefaultConsumerGroupMetadata implements ConsumerGroupMetadata {
    private final String groupId;
    private final int generationId;
    private final String memberId;
    private final Optional<String> groupInstanceId;

    DefaultConsumerGroupMetadata(String groupId,
                                        int generationId,
                                        String memberId,
                                        Optional<String> groupInstanceId) {
        this.groupId = Objects.requireNonNull(groupId, "group.id can't be null");
        this.generationId = generationId;
        this.memberId = Objects.requireNonNull(memberId, "member.id can't be null");
        this.groupInstanceId = Objects.requireNonNull(groupInstanceId, "group.instance.id can't be null");
    }

    public String groupId() {
        return groupId;
    }

    public int generationId() {
        return generationId;
    }

    public String memberId() {
        return memberId;
    }

    public Optional<String> groupInstanceId() {
        return groupInstanceId;
    }

    @Override
    public String toString() {
        return String.format("GroupMetadata(groupId = %s, generationId = %d, memberId = %s, groupInstanceId = %s)",
            groupId,
            generationId,
            memberId,
            groupInstanceId.orElse(""));
    }

    @Override
    public boolean equals(final Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        final DefaultConsumerGroupMetadata that = (DefaultConsumerGroupMetadata) o;
        return generationId == that.generationId &&
            Objects.equals(groupId, that.groupId) &&
            Objects.equals(memberId, that.memberId) &&
            Objects.equals(groupInstanceId, that.groupInstanceId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(groupId, generationId, memberId, groupInstanceId);
    }

}