hashCode for non-const objects in Dart

non_const_set.dart

class MyObject {
  const MyObject(this.field);
  final String field;
  // Since this does not implement hashCode, every MyObject instance will
  // have a unique hashCode.  Meaning you can end up with multiple of the
  // "same" object in a Set.
  // If they're constructed with const, obviously there will only be one
  // "const" instance, so those will all be the same.
}

void main() {
  final one = const MyObject("a");
  final two = const MyObject("b");
  final three = const MyObject("b");
  final set1 = {one, two, three};
  // two and three end up being the same object.
  print(set1.length); // 2

  // Note that we're explicitly not using `const` here
  // so this ends up being a unique object.
  // Since MyObject does not implement hashCode it has no
  // "deep equals" and all instances are unique.
  final four = MyObject("b");
  final set2 = {one, two, three, four};
  print(set2.length); // 3
}