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on clause

Learn to catch specific exception types with on.

The on clause

catch catches every error. Sometimes you want only a specific type. For that you use on. on FormatException catch (e) { ... } catches only a FormatException; others propagate up.

Typical use: try { int.parse(input); } on FormatException { print("not a number"); } on Exception catch (e) { print("other error: $e"); }. Each on catches a different type and gives a different message. on can be used without catch (e); then you cannot access the object. With on you pick the type, with catch (e) you get the object.

Practice Your turn: catch by type

Inside try call int.parse on a non-numeric string. Use on FormatException to print "not a number". Then use on Exception catch (e) to print e for general errors. Different types give different messages.

Starter code

main.dart
void main() {
  // try parsing a non-numeric string
  // for parse errors print "not a number"
  // for general errors print the error
}

Sample solution

main.dart
void main() {
  try {
    int.parse("apple");
  } on FormatException {
    print("not a number");
  } on Exception catch (e) {
    print(e);
  }
}

Expected output

not a number

on vs catch

on FormatException catches only FormatException. catch (e) catches everything. You can combine them: on FormatException catch (e). You pick the type and get the object as e. Writing only on FormatException gives no access to the object.

Multiple on clauses can be written side by side; they are tried in order. The first match runs. The order on FormatException { ... } on Exception { ... } matters; since FormatException is an Exception, the more specific one must come first. A finally block can also be added at the end.

Questions and answers

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

1 on FormatException

Question 
try {
  int.parse("apple");
} ____ FormatException {
  print("not a number");
}
Answer 
try {
  int.parse("apple");
} on FormatException {
  print("not a number");
}

Explanation: on catches a specific type.

2 What happens when multiple on clauses are written together?

main.dart
try {
  process();
} on FormatException {
  ...
} on Exception {
  ...
}
  • The first match runs Correct answer
  • Every block runs
  • Only the last runs
  • It throws

Explanation: The first matching on block runs.

3 Match on/catch

Term Meaning
on FormatException only this type
catch (e) all types
on ... catch (e) type + object
finally always

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