Functions must have an explicit list of parameters - flutter

Question

I have created this class.But I am getting this error
Functions must have an explicit list of parameters. Try adding a parameter list
class DatabaseHelper{
static late DatabaseHelper? _databaseHelper;
factory DatabaseHelper{
if(_databaseHelper==null ){
_databaseHelper=DatabaseHelper.internal();
return _databaseHelper!;
}else{
return _databaseHelper!;
}
}
DatabaseHelper.internal();
}
What I tried
Upgrade Flutter
Upgrade Android Studio
Invalidate Cache/Restart
flutter clean
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Flutter Doctor output
[√] Flutter (Channel beta, 2.5.0-5.2.pre, on Microsoft Windows [Version 10.0.19042.1110], locale tr-TR)
• Flutter version 2.5.0-5.2.pre at C:\src\flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 19c61fed0d (4 days ago), 2021-08-18 17:10:31 -0700
• Engine revision 7a4c4505f6
• Dart version 2.14.0 (build 2.14.0-377.7.beta)
[√] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
• Android SDK at C:\Users\alige\AppData\Local\Android\sdk
• Platform android-31, build-tools 31.0.0
• Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)
• All Android licenses accepted.
[√] Chrome - develop for the web
• Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe
[√] Android Studio (version 2020.3)
• Android Studio at C:\Program Files\Android\Android Studio
• Flutter plugin can be installed from:
https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
https://plugins.jetbrains.com/plugin/6351-dart
• Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)
[√] VS Code (version 1.59.1)
• VS Code at C:\Users\alige\AppData\Local\Programs\Microsoft VS Code
• Flutter extension version 3.25.0
[√] Connected device (3 available)
• SM G532F (mobile) • 4200c1b8c4d94413 • android-arm • Android 6.0.1 (API 23)
• Chrome (web) • chrome • web-javascript • Google Chrome 92.0.4515.159
• Edge (web) • edge • web-javascript • Microsoft Edge 92.0.902.78
• No issues found!

Answer 1

No need to update your IDE or Flutter.
You're defining a Factory constructor, but it's missing the parenthesis at the end of it. It's defined as a class, which it isn't. just add the missing parenthesis like this, so it behaves as a function, and the error will be gone:
class DatabaseHelper {
static late DatabaseHelper? _databaseHelper;
factory DatabaseHelper() {
if (_databaseHelper == null) {
_databaseHelper = DatabaseHelper.internal();
return _databaseHelper!;
} else {
return _databaseHelper!;
}
}
DatabaseHelper.internal();
}

Answer 2

No need to update your IDE or Flutter.
Error Code:
void answerQuestion {
print('Answer Chosen!');
}
Correct Code:
void answerQuestion() {
print('Answer Chosen!');
}