Dart
Exception hierarchy
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.
Exercise 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.
Show example 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);
}
}
Test yourself
-
Which is the correct way to define a custom exception?
- class MyError : Exception
- Exception class MyError
- class MyError extends Exception
- MyError implements new Exception
Show answer
class MyError extends Exception — in Dart you extend Exception with extends.
-
Extend Exception
class AppException extends ____ { final String message; AppException(this.message); }
Show answer
class AppException extends Exception { final String message; AppException(this.message); }
-
Exception hierarchy parts
- Exception
- Base exception
- extends
- Subclass keyword
- throw
- Throwing
- on ... catch
- Catching
Next lesson: Error vs Exception
