Null-aware operators was introduced in Dart 1.12.0. This feature helps us reduce the amount of code when working with potential nullable objects. It includes 4 operators: if null operator ( ?? ), null-aware assignment ( ??= ), null-aware access ( x?.p ) and null-aware method invocation ( x?.m() ).
1. if null operator
expr ?? otherExpr
Meaning:
– if expr is not null, use it.
– otherwise use otherExpr.
void testIfNullOperator() {
int number;
// number is null, use 10.
final a = number ?? 10;
print(a);
// a is not null, use a.
final b = a ?? 0;
print(b);
}
// Output
10
10
2. null-aware assignment
x ??= expr
Meaning: assign expr to x only if x is null.
void testNullAwareAssignment() {
int number;
// number is null, assign 100 to number.
number ??= 100;
print(number);
// number is not null, keep number as is, don't assign 200 to it.
number ??= 200;
print(number);
}
// Output
100
100
3. null-aware access
x?.p
Meaning:
– if x is not null, use it.
– otherwise use null.
void testNullAwareAccess() {
String s;
// s is null, use null.
print(s?.length);
s = 'Coflutter';
// s is not null, use s.
print(s?.length);
}
// Output
null
9
4. null-aware method invocation
x?.m()
Meaning: invoke the method m only if x is not null
void testNullAwareMethodInvocation() {
String s;
// s is null, don't call toUpperCase()
print(s?.toUpperCase());
s = 'Coflutter';
// s is not null, call toUpperCase()
print(s?.toUpperCase());
}
// Output
null
COFLUTTER
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