accessing data from json in flutter include both local and online method. usually local json consider as a demo data model for project.

json.dart

// demo json file. --------Note the image is from local assest file. so use Image.assest method in dart to access. for network use Image.network in flutter
// place json file outside of lib folder
[
    {
        "id": 0,
        "name": "NexMart - Casual Sneakers",
        "category": "NOP",
        "imageUrl": "img/header_onboarding/first_header.png",
        "oldPrice": "1899",
        "price": "799"
    },
    {
        "id": 1,
        "name": "Asian - Men Bouncer",
        "category": "Men",
        "oldPrice": "999",
        "imageUrl": "img/header_onboarding/headerTwo.jpg",
        "price": "719"
    },
    {
        "id": 2,
        "name": "Sparx",
        "category": "Men",
        "imageUrl": "img/header_onboarding/headerThree.jpg",
        "oldPrice": "1349",
        "price": "944"
    },
    {
        "id": 3,
        "name": "Ethics",
        "category": "Men",
        "imageUrl": "img/header_onboarding/headerFour.jpg",
        "oldPrice": "999",
        "price": "469"
    }
]


// creating model for our local json. -------Note   creating model is essential both for local as well as online josn file


//     --- some useful website to create models

// https://javiercbk.github.io/json_to_dart/ 
// https://quicktype.io/


class Product {
  Product({
    this.id,
    this.name,
    this.category,
    this.imageUrl,
    this.oldPrice,
    this.price,
  });

  int? id;
  String? name;
  String? category;
  String? imageUrl;
  String? oldPrice;
  String? price;

  factory Product.fromJson(Map<String, dynamic> json) => Product(
        id: json["id"],
        name: json["name"],
        category: json["category"],
        imageUrl: json["imageUrl"],
        oldPrice: json["oldPrice"],
        price: json["price"],
      );

  Map<String, dynamic> toJson() => {
        "id": id,
        "name": name,
        "category": category,
        "imageUrl": imageUrl,
        "oldPrice": oldPrice,
        "price": price,
      };
}


// parsing method  -- with futureBuilder
  Future<List<Product>> readJsonData() async {
    // database/produc.json  ----------- this is location of the json file also add into pubspec.yaml file location
    //  assets:
   // - database/produc.json
    
    
    final jsonData = await rootBundle.loadString('database/produc.json');
    final list = json.decode(jsonData) as List<dynamic>;

    return list.map((e) => Product.fromJson(e)).toList();
    
    
 //   -------- without future builder access it as in Column
    
    List<Product> product;
    
// Flutter Widget access method
    
FutureBuilder(
          future: readJsonData(),
          builder: (BuildContext context, AsyncSnapshot snapshot) {
            if (snapshot.hasError) {
              return Center(child: Text('${snapshot.error}'));
            } else if (snapshot.hasData) {
              var product = snapshot.data as List<Product>;
              return ListView.builder(
                  itemCount: product.length,
                  itemBuilder: (context, index) {
                    return Container(
                        width: 100,
                        height: 100,
                      // use network if thee= image inside json is from internet
                        child: Image.asset(product[index].imageUrl.toString()));
                  });
            }
            return const Center(child: CircularProgressIndicator());
          }),
      
      
      
      
      
      
      
      
      
      
      
      
      
      
      
      
      // =====================================================//
      
      // list array of Objects on JSON using stateful widget
      
      // example JSON
      {
  "items": [
    {
      "id": "p1",
      "name": "Item 1",
      "description": "Description 1"
    },
    {
      "id": "p2",
      "name": "Item 2",
      "description": "Description 2"
    },
    {
      "id": "p3",
      "name": "Item 3",
      "description": "Description 3"
    }
  ]
}
      
      
      // add JSon file in assests folder
      flutter:
  assets:
    - assets/sample.json
        
        // in Widget
        
        class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

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

class _HomePageState extends State<HomePage> {
  List _items = [];

  // Fetch content from the json file
  Future<void> readJson() async {
    final String response = await rootBundle.loadString('assets/sample.json');
    final data = await json.decode(response);
    setState(() {
      _items = data["items"]; // array name in JSON file
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: FutureBuilder(
          future: loadJson(),
          builder: (BuildContext context, AsyncSnapshot snapshot) {
            if (snapshot.hasError) {
              return Center(child: Text("${snapshot.error}"));
            } else if (snapshot.hasData) {
      
              return ListView.builder(
                  itemCount: items.length,
                  itemBuilder: (context, index) {
        
                    return Column(
                      children: [
                        
                         Text(items[index]["id"].toString()), // "id" should be samee as in JSON file
                        
                        
                          Text(items[index]["name"].toString()), //  "name" should be samee as in JSON file
                          Text(items[index]["description"].toString()), //   "description" should be samee as in JSON file
                      ],
                    );
                  });
            }
            return Center(
              child: CircularProgressIndicator(),
            );
          }),
  }
}