dart - 为什么Set不包含Point

Question

我有一个Set<Point<double>>，我想测试该集合是否包含特定的Point<double>。

我的理解是，如果两个Point<double>实例具有相同的hashCode并根据==相等，则默认Set实现会认为它们相同，并且当其中一个点位于集合中时将返回true，我们将使用contains测试其他点。

但是在以下示例中，情况似乎并非如此:

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的输出是:

a == b ? true
a.hashCode == b.hashCode ? true
points.contains(a) ? true
points.contains(b) ? false

我在哪里错了？

Answer 1

发生这种情况是因为thay不相同，但是您指定了通过身份而不是等效性来比较它们。

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)}");
}

您禁用了此运算符。

 /**
   * 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;

对此进行了详细说明(使用等于:相同)。

/**
   * Creates an insertion-ordered identity-based set.
   *
   * Effectively a shorthand for:
   *
   *     new LinkedHashSet<E>(equals: identical,
   *                          hashCode: identityHashCode)
   */
  external factory LinkedHashSet.identity();