Dart nullable, if-null, and assertion operator

Not null-able

If we annotate a type to be say, int, it means it can only be an integer. It cannot be null.

1	int a = null // Error, a can't be null

We can’t add two numbers if one of them is null:

int a; // null

int b = 2;

print(a + b); // Without Nullable, we get run time error!

Nullable (?)

If you want that int to be null, use the question mark to denote that.

1	int? a = null; // Can be null

int? a; // null

int b = 2;

print(a+b); // <-- compile time error here!

With nullable, we get Compile Time Error.
This makes us write safer programs.

Assertion Operator (!)

If you’re sure that a nullable variable will ALWAYS have non-nullable values,
it’s safe to assign it to a non-nullable variable with the ! operator.

1 2	int? aNullableInt = 8; print(aNullableInt!.isEven); // 8

However, if you’re wrong, you’ll get a runtime error!

int? aNullableInt = null;

print(aNullableInt!.isEven); // run time error!

// Unhandled exception:

// Null check operator used on a null value

if-null operator (??)

In the example, x is -1, and thus maybeValue will be null;

int x = -1;

int? maybeValue; // null, due to x is -1

if ( x > 0) {

print('larger than 0');

maybeValue = x;

}

Usually, we’d write it like this:

1	int value = maybeValue == null ? 0 : maybeValue;

Now, we can use if-null operator like so:

// if not null, assign value to maybeValue

// if null, assign 0 to value

int value = maybeValue ?? 0;

Null Safety with type safety

So if x is -1, then maybeValue is left with null.
That’s not very good for future usage and printing.

So we can use a Type safety where if maybeValue is null, then we assign it to a default number.
Its a shorthand for if-null

int x = -1;

int? maybeValue;

if ( x > 0) {

print('larger than 0');

maybeValue = x;

}

// if maybeValue is null, then take on 0.

maybeValue ??= 0;

print(maybeValue);

Conditional Access Operator

Say we declare an array, and its of type String?.

const cities = <String?>['Shenzhen', 'Beijing', null];

for (var city in cities) {

print(city.toUppercase()); // run time crash at index 2

// city is null, and null does not have toUppercase method.

}

in order to fix this, we use conditional access operator:

String? haha;

print(haha?.toUpperCase()); // if null, would display null

// then

const cities = <String?>['Shenzhen', 'Beijing', null];

for (var city in cities) {

print(city?.toUpperCase()); // when city is null, it'll simply print it

}