CODEWITHSUNDEEP
dartflutter
JustLearningAgain
JustLearningAgain

Reputation: 2287

Is there a better way to test for null in Dart constructor

I'm constructing an object using the following constructor:

 class A {
   int col;
   int row;

   A.fromMap(Map<dynamic, dynamic> data)
       : col = data['col'],
         row = data['row'];
 }

 class B {
   A aObj;
   int objType;
   int count;

   B.fromMap(Map<dynamic, dynamic> data)
       : objType = data['objType'],
         count = data['count'],
         aObj = A.fromMap(data['A']);
 }

The problem is that if the map I'm passing in doesn't have a mapping for aObj, it crashes. I have tried moving the assignment into the curly brackets and testing for null:

 if(data['A'] != null) {
    aObj = A.fromMap(data['A']);
 }

This works. But I'd like to test as part of the short cut constructor like in the other data members.

Is that possible?

Upvotes: 4

Views: 7626

Answers (3)

CopsOnRoad
CopsOnRoad

Reputation: 267434

With NNBD introduced in Dart 2.8, you can use:

class Foo {
  final int value;

  // 0 is the default value in case the expression data['key'] returns null
  Foo.fromNonNullable(Map data) : value = data['key'] ?? 0;

  // 0 is the default value in case `data` is `null`
  Foo.fromNullable(Map? data) : value = data?['key'] ?? 0;
}

Upvotes: 0

creativecreatorormaybenot
creativecreatorormaybenot

Reputation: 126634

I think how you can prevent crashes in a neat way is this way:

aObj = A.fromMap(data['A'] ?? Map())

This will return an empty Map to your custom A.fromMap constructor when data['A'] is null, which will result in col and row being null afterwards (if data['A'] is null) because the fields do not exist in the Map.

Upvotes: 5

vbandrade
vbandrade

Reputation: 2662

what about a ternary operator?

aObj = data['A'] ? A.fromMap(data['A']) : null;

Upvotes: 4

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