CODEWITHSUNDEEP
flutterdart
Frans
Frans

Reputation: 43

The argument type 'Object?' can't be assigned to the parameter type 'List<Siswa>'

I have a two error for this Flutter apps.

The argument type 'Object?' can't be assigned to the parameter type 'List'.

The property 'length' can't be unconditionally accessed because the receiver can be 'null'. Try making the access conditional (using '?.') or adding a null check to the target ('!').

When I add ('!') or null check, new error showed up.

The getter 'length' isn't defined for the type 'Object'. Try importing the library that defines 'length', correcting the name to the name of an existing getter, or defining a getter or field named 'length'.

Here's the code :

list() {
return Expanded(
  child: FutureBuilder(
    future: siswa,
    builder: (context, snapshot) {
      if (snapshot.hasData) {
        return dataTable(snapshot.data);
      }
      if (null == snapshot.data || snapshot.data!.length == 0) {
        return const Text("");
      }
      return const CircularProgressIndicator();
    },
  ),
);

}

And here's the full code on my GitHub : https://github.com/kisekifrans/pencatat_nama_mahasiswa

Thank you for the attention and have a wonderful day everyone.

Upvotes: 0

Views: 2987

Answers (1)

Electron
Electron

Reputation: 91

Your snapshot.data type is not defined, so it is taken as an Object?. This causes the errors.

What can you do:

  • Use cast like (snapshot.data as List<Siswa>).length
  • And better option - add cast directly to your builder. Just add at <type> the end of your FutureBuilder. In your case it will be like FutureBuilder<List<Siswa>> and so your snapshot.data will be identified as List

You can use such cast in many different cases, like casting types of your iterable:

  • List<String> can contain only String values
  • Map<String, dynamic> can only get String values as key, but the value can implement any type (that is what dynamic means)

More about casting you can read here https://itnext.io/flutter-methodchannel-dart-generic-and-type-casting-54ca48e6d3ad

Okay, lets look firstly on src/siswa.dart:

class Siswa {
  late int id;
  late String nama;
  late int nim;

  Siswa(this.id, this.nama, this.nim);

  Map<String, dynamic> toMap() {
    var map = <String, dynamic>{
      'id': id,
      'nama': nama,
      'nim': nim,
    };
    return map;
  }

  Siswa.fromMap(Map<String, dynamic> map) {
    id = map['id'];
    nama = map['nama'];
    nim = map['nim'];
  }
}

All it's parameters has type and can not be null. Look at main.dart at line 64:

Siswa e = Siswa(null, nama, nim);

You apply null to int type. You can:

  • make id field nullable (late int? id;)
  • Send int instead of null (Siswa e = Siswa(0, nama, nim);)
Text(siswa.nim),

Here you trying to print int through Text widget. But Text widget applies only String type, so you should do like this:

Text(siswa.nim.toString()),

Upvotes: 1

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