Pass null to named constructer parameter with default value

Question

I made a class HelloWorld with a named constructer which accepts a single parameter named test which also has default value of Default Value. Here is my code:

import 'dart:core';

class HelloWorld {
  final String test;
  
  const HelloWorld({
    this.test = 'Default Value',
  });
}

void main() {
  const Map<String, dynamic> _json = {'hello': 'world'};
  const _helloWorld = HelloWorld(test: _json['test']);
  print(_helloWorld.test);
}

Then I made a map named _json which has only one key hello. Finally I tried to get the value of key test which obviously doesn't exist in _json which will return null. Despite having the default value for the test parameter in the named constructor, I still get this compilation error:

lib/main.dart:13:23:
Error: Constant evaluation error:
  const _helloWorld = HelloWorld(test: _json['test']);
                      ^
lib/main.dart:13:45:
Info: The method '[]' can't be invoked on '<String, dynamic>{"hello": "world"}' in a constant expression.
  const _helloWorld = HelloWorld(test: _json['test']);
                                            ^
lib/main.dart:13:9:
Info: While analyzing:
  const _helloWorld = HelloWorld(test: _json['test']);
        ^
Error: Compilation failed.

Dart version: Dart SDK version: 2.14.4 (stable) (Wed Oct 13 11:11:32 2021 +0200) on "macos_x64"

Md. Yeasin Sheikh · Answer 1 · 2021-11-08T19:43:50.627

const means that the object's entire deep state can be determined entirely at compile time and that the object will be frozen and completely immutable.

const Map<String, dynamic> _json = {'hello': 'world'}; means this _json is compile-time constant.

But for _json['test'] it doesn't know what will be the value of it in compile time. It needs to go through the _json Map and find the value, and this will happen while running the code. For this reason _helloWorld can not be a const.

But if you do HelloWorld(test: "others"); or use default value HelloWorld(), the _helloWorld can be const type.


void main() {
  const Map<String, dynamic> _json = {'hello': 'world'};

  final _helloWorld = HelloWorld(test: _json['test']); // or can be use String
  print(_helloWorld.test);

  const _helloWorld1 = HelloWorld();
  print(_helloWorld1.test);
  const _helloWorld2 = HelloWorld(test: "on _helloWorld2");
  print(_helloWorld2.test);
}

Default value will be available only when you don't pass any value using test, it will be const _helloWorld1 = HelloWorld();.

But for handling null-Value from _json you can use HelloWorld(test: _json['test'] ?? "Got Null");. Passing Got Null because test require a non-nullable String. If you wish to have constructor default value in this case, check _json['x'] provide null or not, then assign on _helloWorld.

  final _helloWorld = _json['test'] == null
      ? HelloWorld()
      : HelloWorld(test: _json['test']);

I will recommend const and final SO answerto learn more it. Also, you can find on dart.dev

Yes because `_json['test']` doesn't exist on map. Does update answer clarify your doubt ? — Md. Yeasin Sheikh, Nov 08 '21 at 19:45

score 1 · Answer 2 · answered Nov 08 '21 at 19:47

1

I ended up using this way:

import 'dart:core';

class HelloWorld {
  final String test;
  
  const HelloWorld({
    String? test,
  }) : test = test ?? 'Default Value';
}

void main() {
  final Map<String, dynamic> _json = {'hello': 'world'};
  final _helloWorld = HelloWorld(test: _json['test']);
  print(_helloWorld.test);
}

answered Nov 08 '21 at 19:47

Alireza Beitari

436
4
15

Can you please help us understand the code? Some comments(In code) would be helpful enough. – Darsh Shah Nov 08 '21 at 19:55
As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Nov 08 '21 at 19:56
Because I didn't know this syntax was allowed until now, I found this post useful. – IcyIcicle Feb 12 '23 at 04:38

score -2 · Answer 3 · answered Nov 08 '21 at 19:14

-2

You simply need to change your test variable to this, to make it nullable:

final String? test;

answered Nov 08 '21 at 19:14

Lars

1,250
9
25

1

I don't want to make `test` nullable. That's the entire point. I set default value and I expect if `null` is passed to this parameter, the default value would be set! – Alireza Beitari Nov 08 '21 at 19:17

Pass null to named constructer parameter with default value

3 Answers3