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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.

Practice 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).

Starter code

main.dart
void main() {
  // Define the BalanceError class
  // try throws BalanceError
  // handle the error and print
}

Sample solution

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

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

Expected output

insufficient balance

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match Exception

Term Meaning
Exception the general interface
implements binds the class
toString gives the text
final String immutable message

2 Which interface does our exception class implement?

main.dart
class BalanceError ____ Exception {
  final String message;
  BalanceError(this.message);
}
  • implements
  • extends
  • with
  • abstract Correct answer

Explanation: implements Exception applies the interface.

3 toString override

Question 
String ____() => message;
Answer 
String toString() => message;

Explanation: toString gives the text representation.

All lessons in Beginner

1 Dart basics

  1. What is Dart?
  2. Your first program
  3. dart run
  4. Comments
  5. String interpolation
  6. Summary - Dart basics

2 Variables

  1. var and type
  2. final
  3. const
  4. late
  5. Null safety
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and String
  4. Type inference
  5. dynamic
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Null-aware operators
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String interpolation
  3. length and methods
  4. substring
  5. contains and indexOf
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Pattern matching
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. for-in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Lists

  1. Creating lists
  2. List access
  3. add and remove
  4. length and contains
  5. List methods
  6. Summary - Lists

11 Sets and Maps

  1. Set
  2. Set operations
  3. Map
  4. Map operations
  5. Map iteration
  6. Summary - Sets and Maps

12 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Optional parameters
  5. First-class functions
  6. Summary - Functions

13 Null safety in depth

  1. Nullable types
  2. Null-aware operators
  3. The ! operator
  4. late
  5. Null check
  6. Summary - Null safety

14 Class basics

  1. class
  2. Constructor
  3. Instance variables
  4. Methods
  5. this
  6. Summary - Class basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. split and join
  2. replace
  3. trim and pad
  4. RegExp
  5. StringBuffer
  6. Summary - Advanced strings

17 Enums

  1. Defining enums
  2. Enum values
  3. Enhanced enums
  4. switch with enum
  5. Enum and patterns
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. The Exception class
  5. on clause
  6. Summary - Exceptions

19 Async basics

  1. Future
  2. async/await
  3. then
  4. Future.error
  5. Future.wait
  6. Summary - Async basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together