I am completely new to the new Dart Null Safety and am trying to convert one of my projects and learn it. I'm getting a little confused with one error I received on a function, where it is returning a type. Here is the code:

Exercise getExerciseByID(String exerciseId) {
for (var exercise in _exercises) {
  if (exercise.id == exerciseId) {
    return exercise;
  } 
}
}

The error I am receiving is as follows:

The body might complete normally, causing 'null' to be returned, but the return type, 'Exercise', is a potentially non-nullable type. (Documentation) Try adding either a return or a throw statement at the end.

I am wondering what should I be doing/ returning in this case? Any advice on this would be really helpful. Thanks so much.

CodePudding user response：

It is because you have implicit return null here. If none of if statements will be fulfilled, excersise will not be returned, therefore ther result will be null.

Exercise getExerciseByID(String exerciseId) {
for (var exercise in _exercises) {
  if (exercise.id == exerciseId) {
    return exercise;
  } 
}
 return null; //this is what it complains, that the result might be null while you declare non null response
}

Options (alternatives):

1. Change return declaration to nullable type (jamesdlin) Exercise?
2. Throw exception at the end instead of returning null
3. Always return something - eg default value or 'nothing found value'

CodePudding user response：

You are getting that error because your return statement is inside of if condition, so it assumes that it can possibly never return a value (if all conditions fail). So this could be solution for you:

Exercise getExerciseByID(String exerciseId) {
    // initialize some default value to return if all conditions fail
    Exercise returnValue = Exercise();
    for (var exercise in _exercises) {
      if (exercise.id == exerciseId) {
        // if found update your initial value with found one
        returnValue = exercise;
        // stop for loop after finding right value
        break;
      }
    }
    return returnValue;
  }

CodePudding user response：

add return at the end of the method in case the if condition is not true.

or you can use simple firstWhere method like:

return _exercises.firstWhere((exercise) => exercise.id == exerciseId);