Why is the value in this map NULL?-CodePudding

Error: "NoSuchMethodError: 'length' method not found. Receiver: null" when updating map values

List<ImageDetails> _images = [
  ImageDetails(
    imagePath: 'assets/images/meal1.jpg',
    date: '2021-11-30',
    details:
        '',
  ),
  ImageDetails(
    imagePath: 'assets/images/meal2.jpg',
    date: '2021-11-30',
    details:
        '',
  ),
];

    var dateToImages = new Map();
    _images.sort((a,b) => a.date.compareTo(b.date));
    //group images by date
    for (int i = 0; i < _images.length; i  ) {
      var d = _images[i].date; //convert string to Datetime
      print("printing datetime in for loop");
      print(d);
      if (dateToImages.containsKey(d)) {
        print("second element");
        var list = dateToImages[d].add(_images[i]);
        dateToImages[d] = list;
      } else {
        print("first element");
        dateToImages[d] = [_images[i]];
      }
    }
    var sortedKeys = dateToImages.keys.toList()..sort((a, b) => a.compareTo(b));
    List<Widget> children = [];
    print("=====printing datetoImages");
    print(dateToImages);
    print("======== printing sortedKeys");
    print(sortedKeys);
    int len = dateToImages['2021-11-30'].length;

Below is result of running above code

printing datetime in for loop
2021-11-30
first element
printing datetime in for loop
2021-11-30
second element
=====printing datetoImages
{2021-11-30: null}
======== printing sortedKeys
[2021-11-30]

After printing some variables, it seems like the issue is with the value for key "2021-11-30" in dateToImages being null... I don't understand why I keep getting null since it seems like the map building process in the for loop seems to be going well? Can anyone shed some light on this bug?

Thanks!

CodePudding user response：

try this :

List<ImageDetails> _images = [
 ImageDetails(
  imagePath: 'assets/images/meal1.jpg',
  date: '2021-12-30',
  details:
      '',
 ),
 ImageDetails(
  imagePath: 'assets/images/meal2.jpg',
  date: '2021-11-30',
  details:
     '',
 ),
]; 

var dateToImages = new Map();

_images.forEach((img){

 if(dateToImages.containsKey(img.date)){
  dateToImages[img.date].add(img);
 }else{
  dateToImages[img.date] = [img];
 }
});

var sortedKeys = dateToImages.keys.toList()..sort((a, b) => 
a.compareTo(b));
print("=====printing datetoImages");
print(dateToImages);
print("======== printing sortedKeys");
print(sortedKeys);

int len = dateToImages['2021-11-30'].length;

Output:

=====printing datetoImages
{2021-12-30: [Instance of 'ImageDetails'], 2021-11-30: [Instance of 
'ImageDetails']}
======== printing sortedKeys
[2021-11-30, 2021-12-30]

CodePudding user response：

The error message suggests you are accessing length method which is not available. since null doesn't have length method but list have it.

So, you may have a logical error, where you think you are returning list but the code is returning null

In your case:

Your Code is fine except for this part:

if (dateToImages.containsKey(d)) {
        print("second element");
        var list = dateToImages[d].add(_images[i]);
        dateToImages[d] = list;
}

add() method returns void which will return null to var list and null will be sent to the map

Update it directly, replace above code with this:

if (dateToImages.containsKey(d)) {
        print("second element");
        dateToImages[d].add(_images[i]);  
}