how to implement json array nested in ui flutter

Just check out this example where I have parsed you JSON locally :

{
    "responseCode": "0000",
    "responseMessage": "Success",
    "date": "20200227",
    "time": "115221",
    "content": [
      {
        "postingDate": "20191203",
        "valueDate": "20191203",
        "inputDate": "20191203",
        "inputTime": "214808",
        "desc": "BUNGA JATUH TEMPO",
        "noReff": "B2100000000026",
        "amount": "+20712,33",
        "balance": "+6971357445,15"
      },
      {
        "postingDate": "20191203",
        "valueDate": "20191203",
        "inputDate": "20191203",
        "inputTime": "214809",
        "desc": "BUNGA JATUH TEMPO",
        "noReff": "B2100000000033",
        "amount": "+13808,22",
        "balance": "+6971371253,37"
      }
    ]
  }

below is the model class for the json that you provided.

// To parse this JSON data, do
//
//     final myModel = myModelFromJson(jsonString);

import 'dart:convert';

MyModel myModelFromJson(String str) => MyModel.fromJson(json.decode(str));

String myModelToJson(MyModel data) => json.encode(data.toJson());

class MyModel {
    String responseCode;
    String responseMessage;
    String date;
    String time;
    List<Content> content;

    MyModel({
        this.responseCode,
        this.responseMessage,
        this.date,
        this.time,
        this.content,
    });

    factory MyModel.fromJson(Map<String, dynamic> json) => MyModel(
        responseCode: json["responseCode"],
        responseMessage: json["responseMessage"],
        date: json["date"],
        time: json["time"],
        content: List<Content>.from(json["content"].map((x) => Content.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "responseCode": responseCode,
        "responseMessage": responseMessage,
        "date": date,
        "time": time,
        "content": List<dynamic>.from(content.map((x) => x.toJson())),
    };
}

class Content {
    String postingDate;
    String valueDate;
    String inputDate;
    String inputTime;
    String desc;
    String noReff;
    String amount;
    String balance;

    Content({
        this.postingDate,
        this.valueDate,
        this.inputDate,
        this.inputTime,
        this.desc,
        this.noReff,
        this.amount,
        this.balance,
    });

    factory Content.fromJson(Map<String, dynamic> json) => Content(
        postingDate: json["postingDate"],
        valueDate: json["valueDate"],
        inputDate: json["inputDate"],
        inputTime: json["inputTime"],
        desc: json["desc"],
        noReff: json["noReff"],
        amount: json["amount"],
        balance: json["balance"],
    );

    Map<String, dynamic> toJson() => {
        "postingDate": postingDate,
        "valueDate": valueDate,
        "inputDate": inputDate,
        "inputTime": inputTime,
        "desc": desc,
        "noReff": noReff,
        "amount": amount,
        "balance": balance,
    };

// remove this method from the model
static Future<MyModel> getMutasiDetails(String accNumber, String startDate, String endDate) async {
  String apiURL = urlAPI";
    String username = "username";
    String password = "password";
    var bytes = utf8.encode("$username:$password");
    var credentials = base64.encode(bytes);
    var headers = {
      "Content-Type": "application/json",
      "Authorization": "Basic $credentials"
    };

  var params = Map<String, dynamic>();
  params['accNumber'] = accNumber;
  params['startDate'] = startDate;
  params['endDate'] = endDate;

  http.Response apiResult =
      await http.post(apiURL, body: params, headers: headers);
  if (apiResult.statusCode == 200) {
    return MyModel.fromJson(json.decode(apiResult.body));
  } else {
    throw Exception('failed to load data');
  }
}
}

And below is the main file where the listview gets rendered:

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:sample_project_for_api/model.dart';
import 'package:http/http.dart' as http;

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  MyModel youModel = new MyModel();
  bool _isLoading = false;

  @override
  void initState() {
    super.initState();
    getMutasiDetails("0002100000291", "", "").then((value) {
      youModel = value;
      setState(() {
        _isLoading = false;
      });
    });
  }

  Future<MyModel> getMutasiDetails(
      String accNumber, String startDate, String endDate) async {
    setState(() {
      _isLoading = true;
    });
    String apiURL = "urlAPI";
    String username = "username";
    String password = "password";
    var bytes = utf8.encode("$username:$password");
    var credentials = base64.encode(bytes);
    var headers = {
      "Content-Type": "application/json",
      "Authorization": "Basic $credentials"
    };

    var params = Map<String, dynamic>();
    params['accNumber'] = accNumber;
    params['startDate'] = startDate;
    params['endDate'] = endDate;

    http.Response apiResult =
        await http.post(apiURL, body: params, headers: headers);
    if (apiResult.statusCode == 200) {
      return myModelFromJson(apiResult.body);
    } else {
      throw Exception('failed to load data');
    }
  }


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _isLoading
          ? CircularProgressIndicator()
          : Container(
              child: ListView.builder(
                  itemCount: youModel.content.length,
                  itemBuilder: (context, i) {
                    return Card(
                      child: Column(
                        children: <Widget>[
                          Text(youModel.content[i].amount),
                          Text(youModel.content[i].balance),
                          Text(youModel.content[i].inputDate),
                          Text(youModel.content[i].desc),
                        ],
                      ),
                    );
                  })),
    );
  }
}