Dart Null Safety

As we knew that all variables are non-null by default, we can use either the ? operator or the late keyword.

Null safety in simple words means a variable cannot contain a null value unless you initialized with null to that variable.

with null safety, all runtime null-dereference errors will now be shown in compile time.

String name = null ; // This means the variable name has a null valu

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Null-Nullable Types

When we use null safety, all the types are non-nullable by default.

When we declare a variable of type int, the variable will contain some integer value.

void main() {
  int marks;

  // The value of type `null` cannot be
  // assigned to a variable of type 'int'
  marks = null; 
}

Non-nullable variables must always be initialized with non-null values.

Nullable Types

To specify if the variable can be null, then you can use nullable type ? operator.

String? carName;  // initialized to null by default
int? marks = 36;  // initialized to non-null
marks = null; // can be re-assigned to null

The Assertion Operator (!)

Use the null assertion operator ( ! ) to make Dart treat a nullable expression as non-nullable if you’re certain it isn’t null.

int? someValue = 30;
int data = someValue!; // This is valid as value is non-nullable

In the above example, we are telling Dart that the variable someValue is null, and it is safe to assign it to a non-nullable variable i.e. data.

Type Promotion

Dart's analyzer, which tells you what compile-time errors and warnings, is intelligent enough to determine whether a nullable variable is guaranteed to have values that are not null.

Dart uses Flow Analysis at runtime for type promotion.

int checkValue(int? someValue) {
  if (someValue == null) {
    return 0;
  }
  // At this point the value is not null.
  return someValue.abs();
}

void main(){
  print(checkValue(5));
  print(checkValue(null));
}

In the above code, if statement checks if the value is null or not.

After the if statement value cannot be null and is treated as non-nullable value.

This allows us to safely use someValue.abs().

Here, .abs() function will return an absolute value.

Late Keyword

As we knew that all variables are non-null by default, we can use either the ? operator or the late keyword.

String? carName;       // using ? operator
late String bikeName;  // using "late" keyword