what's mean of "?" in method "String substring(int startIndex, [int? endIndex]);" in Dart;

Asked Aug 27 '20 at 15:16

Active Aug 27 '20 at 15:16

Viewed 35 times

Here is Dart "String"'s Method :

I don't know what's "?" mean? And I try this in DartPad like this:

But it throw a error.

I'll be grateful to your anwser.

asked Aug 27 '20 at 15:16

Oo_oO

2

Did you try reading the error? It says "This requires the 'non-nullable' language feature to be enabled." You can then search for what that is, such as the following... [What is Null Safety in Dart?](https://stackoverflow.com/questions/60068435/what-is-null-safety-in-dart); look for point 2.2 of the answer. In short: `?` tells Dart the variable can be nullable, but unless you're in non-nullable-by-default mode, it's redundant, as everything can be null and initially is. – underscore_d Aug 27 '20 at 15:24
@underscore_d Thanks, I should thinking and inquiring more – Oo_oO Aug 27 '20 at 16:24
@Mobina Yes,thank you. – Oo_oO Aug 27 '20 at 16:27

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