pingpong.dart

import 'dart:convert';

void main() {
  final obj = jsonDecode(jsonString) as List<dynamic>;

  final pingpong = obj //
      .map((e) => PingResponse.fromJson(e as Map<String, dynamic>))
      .toList();

  for (final p in pingpong) {
    print(p.seq);
    print(p.ttl);
    print(p.time);
    print(p.ip);
    print('');
  }
}

String jsonString = '''
[{"seq":1,"ttl":null,"time":null,"ip":null}, {"seq":2,"ttl":null,"time":null,"ip":null}]
''';

class PingResponse {
  PingResponse({
    required this.seq,
    this.ttl,
    this.time,
    this.ip,
  });

  factory PingResponse.fromJson(Map<String, dynamic> json) {
    final timeObj = (json['time'] as num?)?.toInt();

    return PingResponse(
      seq: (json['seq'] as num).toInt(),
      ttl: (json['ttl'] as num?)?.toInt(),
      time: timeObj != null ? Duration(milliseconds: timeObj) : null,
      ip: (json['ip'] as num?)?.toInt(),
    );
  }

  final int seq;
  final int? ttl;
  final Duration? time;
  final int? ip;
}