Exception hierarchy
- Dart course
- Intermediate
- Lesson 100/116
- 5 min
Learn the exception hierarchy and custom exceptions.
Exception hierarchy
In Dart all problems derive from Object. Exception is an abstract interface; you define custom exceptions with class MyError extends Exception.
You build a hierarchy by extending Exception: class AppException extends Exception, class NetworkException extends AppException. Then on AppException catches AppException and its subclasses.
In a catch block on SpecificException catch (e) catches a specific type. This lets you handle different error types differently.
A custom exception example
A custom exception usually carries a message field: class AppException extends Exception { final String message; AppException(this.message); }.
To throw it write throw AppException('connection failed');. To catch it use try { ... } on AppException catch (e) { print(e.message); }.
on AppException catch (e) catches only AppException and its subclasses. A general catch (e) catches everything.
Practice A custom exception class
Define an AppException class that extends Exception with a final String message field and a constructor. In main throw AppException('connection failed') and catch it with on AppException catch (e), then print e.message.
Starter code
// Define the AppException class, extend Exception
void main() {
// throw AppException, catch it and print the message
}Sample solution
class AppException extends Exception {
final String message;
AppException(this.message);
}
void main() {
try {
throw AppException('connection failed');
} on AppException catch (e) {
print(e.message);
}
}Expected output
connection failed
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which is the correct way to define a custom exception?
- class MyError : Exception
- Exception class MyError
- class MyError extends Exception Correct answer
- MyError implements new Exception
Explanation: in Dart you extend Exception with extends.
2 Extend Exception
class AppException extends ____ { final String message; AppException(this.message); }class AppException extends Exception { final String message; AppException(this.message); }Explanation: Which base class should you extend?
3 Exception hierarchy parts
| Term | Meaning |
|---|---|
Exception |
Base exception |
extends |
Subclass keyword |
throw |
Throwing |
on ... catch |
Catching |