body: Center(
        child: DropdownButton(
          items: ["bad ", "good ", "ok", "sad", "happy"]
              .map((e) => DropdownMenuItem(
                    child: Text("$e"),
                    value: e,
                  ))
              .toList(),
              onChanged: (val){},
          value: selectedfeeling,
        ),
      ),

This is the error:

Exception has occurred.
_AssertionError ('package:flutter/src/material/dropdown.dart': Failed assertion: line 882 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1': There should be exactly one item with [DropdownButton]'s value: good. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value)

CodePudding user response：

It is because, perhaps you are not giving initial value to selectedfeeling, which is exist in ["bad ", "good ", "ok", "sad", "happy"] Before calling build method, for example in initState or when declaring selectedfeeling just give value to selectedfeeling, which exist here ["bad ", "good ", "ok", "sad", "happy"]

CodePudding user response：

@override
  void initState() {
    selectedfeeling="bad";
  }

body: Center(
        child: DropdownButton(
          items: ["bad ", "good ", "ok", "sad", "happy"]
              .map((e) => DropdownMenuItem(
                    child: Text("$e"),
                    value: e,
                  ))
              .toList(),
              onChanged: (val) {
                selectedfeeling=val;
              },
          value: selectedfeeling,
        ),
      ),

CodePudding user response：

You can create nullabe string like selected value like

  String? selectedfeeling;

Now to provide initial value, you must have to provide a value that exists on items

final items = ["bad ", "good ", "ok", "sad", "happy"];
  late String? selectedfeeling = items[1]; //default is good
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: DropdownButton(
          items: items
              .map((e) => DropdownMenuItem(
                    child: Text("$e"),
                    value: e,
                  ))
              .toList(),
          onChanged: (val) {},
          value: selectedfeeling,
        ),
      ),
    );
  }