The Exception class

Learn the Exception class and custom exceptions.

The Exception class

In Dart all exceptions implement the Exception interface. You can write your own exception class; this lets you name the special case clearly. class BalanceError implements Exception { final String message; BalanceError(this.message); String toString() => message; }

You throw it with throw BalanceError("insufficient balance"). In a catch block you can match by type: catch (e) { if (e is BalanceError) ... }. Your own exception class carries the reason explicitly; the caller can act on it. Overriding toString makes print(e) output meaningful.

Overriding toString

When you override toString in an exception class, print(e) shows the text you want. The default toString prints the class name and a complex identifier, making logs unreadable.

Example: String toString() => "BalanceError: $message";. So print(e) outputs BalanceError: insufficient balance. Very useful when debugging. The final String message field carries the reason; in a catch block you access e.message after checking the type with is.

Exercise Your turn: a custom exception class

Define a class BalanceError that implements Exception. Add final String message and a BalanceError(this.message) constructor. The toString method returns message. In main throw BalanceError("insufficient balance") and print it with catch (e).

Show example solution

class BalanceError implements Exception {
  final String message;
  BalanceError(this.message);
  String toString() => message;
}

void main() {
  try {
    throw BalanceError("insufficient balance");
  } catch (e) {
    print(e);
  }
}

Test yourself

Match Exception

Exception

the general interface
implements

binds the class
toString

gives the text
final String

immutable message
Which interface does our exception class implement?
```
class BalanceError ____ Exception {
  final String message;
  BalanceError(this.message);
}
```
- implements
- extends
- with
- abstract
Show answer

abstract — implements Exception applies the interface.

toString override

String ____() => message;

Show answer

String toString() => message;

Next lesson: on clause

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