Dart Future.error()
Syntax & Examples

Future.error constructor

The `Future.error` constructor in Dart creates a future that completes with an error.

Syntax of Future.error

The syntax of Future.Future.error constructor is:

Future.error(Object error, [ StackTrace stackTrace ])

This Future.error constructor of Future creates a future that completes with an error.

Parameters

ParameterOptional/RequiredDescription
errorrequiredAn object representing the error that the future will complete with.
stackTraceoptionalA stack trace associated with the error, used for debugging purposes.

✐ Examples

1 Creating a future with a simple error

In this example,

  1. We create a future using the `Future.error` constructor and pass a string as the error object.
  2. We handle the error using the `catchError` method and print the error message.

Dart Program

void main() {
  Future<void>.error('Something went wrong!')
      .catchError((error) {
    print('Error: $error');
  });
}

Output

Error: Something went wrong!

2 Creating a future with a custom exception and stack trace

In this example,

  1. We create a future using the `Future.error` constructor and pass an exception and current stack trace as arguments.
  2. We handle the error using the `catchError` method and print both the error and the stack trace.

Dart Program

void main() {
  Future<int>.error(Exception('Custom Exception'), StackTrace.current)
      .catchError((error, stackTrace) {
    print('Error: $error\nStackTrace: $stackTrace');
  });
}

Output

Error: Exception: Custom Exception
StackTrace: Error
    at get current (https://storage.googleapis.com/nnbd_artifacts/3.3.4
dart_sdk.js:140009:30)
    at Object.main$0 [as main] (blob:null
5dcf70dd-e929-4d4e-b70c-38dd1346c258:55:83)
    at Object.main$ [as main] (blob:null
5dcf70dd-e929-4d4e-b70c-38dd1346c258:52:10)
    at blob:null/5dcf70dd-e929-4d4e-b70c-38dd1346c258:95:26
    at Object.execCb (https://dartpad.dev/require.js:5:16727)
    at e.check (https://dartpad.dev/require.js:5:10499)
    at e.<anonymous> (https://dartpad.dev/require.js:5:12915)
    at https://dartpad.dev/require.js:5:1542
    at https://dartpad.dev/require.js:5:13376
    at each (https://dartpad.dev/require.js:5:1020)
    at e.emit (https://dartpad.dev/require.js:5:13344)
    at e.check (https://dartpad.dev/require.js:5:11058)
    at e.enable (https://dartpad.dev/require.js:5:13242)
    at e.init (https://dartpad.dev/require.js:5:9605)
    at a (https://dartpad.dev/require.js:5:8305)
    at Object.completeLoad (https://dartpad.dev/require.js:5:15962)
    at HTMLScriptElement.onScriptLoad (https://dartpad.dev
require.js:5:16882)

Summary

In this Dart tutorial, we learned about Future.error constructor of Future: the syntax and few working examples with output and detailed explanation for each example.