Question 202573 in Flutter Question posted in
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Flutter: The argument type 'String?' can't be assigned to the parameter type 'String' in SharedPreference

How to pass the parameter getString need String? but read() used String

class SharedPref {
  read(String key) async {
    final prefs = await SharedPreferences.getInstance();
    return json.decode(prefs.getString(key)); // the error in this line
  }

this is for sharedPreferences of flutter. how to solve this?

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4

Answers


  1. 0

    The line prefs.getString(key) returns a String?. It means that can return a String or a null value. You have to ensure that the value will not be null when passed to json.decode().

    Check out the documentation for more info.

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  2. 0

    Try this:

    return json.decode(prefs.getString(key) ?? '');
// or
return prefs.getString(key) != null ? json.decode(prefs.getString(key)) : '';
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  3. 0

    Update your code

    read(String key) async {
    final prefs = await SharedPreferences.getInstance();
    String? str = prefs.getString(key) ?? '';
    return json.decode(str); 
}
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  4. 0

    It’s cause of getString method have return type String?

      /// Reads a value from persistent storage, throwing an exception if it's not a
  /// String.
  String? getString(String key) => _preferenceCache[key] as String?;

    But json.decode takes String as parameter

      /// Parses the string and returns the resulting Json object.
  ///
  /// The optional [reviver] function is called once for each object or list
  /// property that has been parsed during decoding. The `key` argument is either
  /// the integer list index for a list property, the string map key for object
  /// properties, or `null` for the final result.
  ///
  /// The default [reviver] (when not provided) is the identity function.
  dynamic decode(String source,
      {Object? reviver(Object? key, Object? value)?}) {
    reviver ??= _reviver;
    if (reviver == null) return decoder.convert(source);
    return JsonDecoder(reviver).convert(source);
  }

    So you must pass String into json.decode method. There are many ways to do this

    // prefs.getString(key) return null, use '' as parameter
return json.decode(prefs.getString(key) ?? ''); 
// or
// throw exception when prefs.getString(key) return null. 
// Wrap your method with `try ... catch ... `
return json.decode(prefs.getString(key)!);
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