Using constructors

Using constructors

You can create an object using a constructor.Constructor names can be either ClassName orClassName.identifier. For example,the following code creates Point objects using thePoint() and Point.fromJson() constructors:

var p1 = Point(2, 2);
var p2 = Point.fromJson({'x': 1, 'y': 2});

The following code has the same effect, butuses the optional new keyword before the constructor name:

var p1 = new Point(2, 2);
var p2 = new Point.fromJson({'x': 1, 'y': 2});

Version note: The new keyword became optional in Dart 2.

Some classes provide constant constructors.To create a compile-time constant using a constant constructor,put the const keyword before the constructor name:

var p = const ImmutablePoint(2, 2);

Constructing two identical compile-time constants results in a single,canonical instance:

var a = const ImmutablePoint(1, 1);
var b = const ImmutablePoint(1, 1);
assert(identical(a, b)); // They are the same instance!

Within a constant context, you can omit the const before a constructoror literal. For example, look at this code, which creates a const map:

// Lots of const keywords here.
const pointAndLine = const {
  'point': const [const ImmutablePoint(0, 0)],
  'line': const [const ImmutablePoint(1, 10), const ImmutablePoint(-2, 11)],
};

You can omit all but the first use of the const keyword:

// Only one const, which establishes the constant context.
const pointAndLine = {
  'point': [ImmutablePoint(0, 0)],
  'line': [ImmutablePoint(1, 10), ImmutablePoint(-2, 11)],
};

If a constant constructor is outside of a constant contextand is invoked without const,it creates a non-constant object:

var a = const ImmutablePoint(1, 1); // Creates a constant
var b = ImmutablePoint(1, 1); // Does NOT create a constant
assert(!identical(a, b)); // NOT the same instance!

Version note: The const keyword became optional within a constant context in Dart 2.