Why doesn't the Set contain the Point - dart

Question

I have a Set<Point<double>> and I want to test if the set contains a particular Point<double>.
My understanding is that if two Point<double> instances have the same hashCode and are equal according to ==, then the default Set implementation considers them identical and will return true when one of the points is in the set and we test for the other using contains.
But in the following example this doesn't seem to be the case:
import 'dart:math';
void main() {
final points = Set<Point<double>>.identity();
final a = Point<double>(5, 2);
final b = Point<double>(5, 2);
points.add(a);
print("a == b ? ${a == b}");
print("a.hashCode == b.hashCode ? ${a.hashCode == b.hashCode}");
print("points.contains(a) ? ${points.contains(a)}");
print("points.contains(b) ? ${points.contains(b)}");
}
According to DartPad the output is:
a == b ? true
a.hashCode == b.hashCode ? true
points.contains(a) ? true
points.contains(b) ? false
Where have I gone wrong here?

Answer 1

Set<Point<double>>.identity() uses idenity hash for comparing. But Point has overriden comparsion by equality of x and y (See source code)
So in this case a.idenityHash != a.hashcode and a.idenityHash != b.idenityHash even if a.hashCode == b.hashCode. This all happend because == and hashCode were overriden for object compararsion by coordinates.
So if you create normal set, which is using hashCode methods for comparsion, you get expected result
final points = Set<Point<double>>();
points.add(a);
print("points.contains(b) ? ${points.contains(b)}"); // true

Answer 2

This happens because thay are not identical but you specified to compare them by identity instead of equilvalence.
import 'dart:math';
void main() {
//final points = Set<Point<double>>.identity();
final points = Set<Point<double>>();
final a = Point<double>(5, 2);
final b = Point<double>(5, 2);
points.add(a);
print("a == b ? ${a == b}");
print("a.hashCode == b.hashCode ? ${a.hashCode == b.hashCode}");
print("points.contains(a) ? ${points.contains(a)}");
print("points.contains(b) ? ${points.contains(b)}");
}
You disabled this operator.
/**
* A `Point` is only equal to another `Point` with the same coordinates.
*
* This point is equal to `other` if, and only if,
* `other` is a `Point` with
* [x] equal to `other.x` and [y] equal to `other.y`.
*/
bool operator ==(dynamic other) =>
// Cannot change parameter type to `Object` in case some class
// inherits the type and uses their argument dynamically.
other is Point && x == other.x && y == other.y;
This is explains this in details (use equals: identical).
/**
* Creates an insertion-ordered identity-based set.
*
* Effectively a shorthand for:
*
* new LinkedHashSet<E>(equals: identical,
* hashCode: identityHashCode)
*/
external factory LinkedHashSet.identity();