另一种违反合同的比较方法

Question

我有一个类：

class Column implements Comparable<Column> {
  private final float startX;
  private final float endX;

  public Column(float startX, float endX) {
    this.startX = startX;
    this.endX = endX;
  }

  public boolean isSameColumn(Column c) {
    return c.startX <= this.startX && this.startX < c.endX || this.startX <= c.startX && c.startX < this.endX;
  }

  @Override
  public int hashCode() {
    return Objects.hash(startX, endX);
  }

  @Override
  public boolean equals(Object obj) {
    if (obj == null) return false;
    if (getClass() != obj.getClass()) return false;
    final Column other = (Column) obj;
    return this.startX == other.startX && this.endX == other.endX;
  }

  @Override
  public int compareTo(Column o) {
    return isSameColumn(o) ? 0 : Float.compare(this.startX, o.startX);
  }
}

我认为，compareTo 方法符合比较器契约，即使它与 equals 不一致（故意）- 根据 javadoc，这应该不是问题：

通常情况下，但并非严格要求，(compare（x，y）== 0）==（x.equals（y））

然而，有时我会得到以下错误：

java.lang.IllegalArgumentException: Comparison method violates its general contract!

例如，下面的代码会抛出异常（在ideone上也是如此——它有点长，但大部分是测试数据）。

还要注意，在流中运行distinct()之前运行sorted()可以解决问题。

public static void main (String[] args) {
  String points = "54.199997, 88.399216, 135.2, 250.09616, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 120.79874, 135.2, 168.19669, 178.7, 207.49712, " +
          "135.2, 168.19669, 370.1, 391.69785, 108.2, 120.79874, 135.2, 189.49884, 135.2, 180.7966, 370.1, 391.69785, 108.2, 115.39928, 135.2, 215.59856, " +
          "135.2, 172.69742, 377.9, 391.6986, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 120.79874, 135.2, 212.59627, 135.2, 172.69742, 370.1, 391.69785, " +
          "108.2, 120.79874, 135.2, 212.59627, 135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, 212.59627, 135.2, 172.69742, 374.0, 391.69824, " +
          "108.2, 115.39928, 135.2, 215.59856, 135.2, 172.69742, 377.9, 391.6986, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 120.79874, 135.2, 189.49884, " +
          "135.2, 180.7966, 374.0, 391.69824, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 120.79874, 135.2, 212.59627, 135.2, 172.69742, 374.0, 391.69824, " +
          "108.2, 120.79874, 135.2, 217.09616, 135.2, 170.59763, 377.9, 391.6986, 108.2, 120.79874, 135.2, 208.99689, 135.2, 163.39717, 374.0, 391.69824, " +
          "108.2, 120.79874, 135.2, 214.6982, 135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, 189.49884, 135.2, 180.7966, 374.0, 391.69824, " +
          "135.2, 250.09616, 517.1, 544.6972, 108.2, 120.79874, 135.2, 198.79779, 135.2, 187.69778, 377.9, 391.6986, 108.2, 115.39928, 135.2, 215.59856, " +
          "135.2, 172.69742, 377.9, 391.6986, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 121.69865, 135.2, 191.89688, 135.2, 214.39604, 370.1, 391.69785, " +
          "108.2, 120.79874, 135.2, 217.39865, 135.2, 175.69711, 374.0, 391.69824, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 120.79874, 135.2, 219.49155, " +
          "135.2, 163.39717, 374.0, 391.69824, 108.2, 120.79874, 135.2, 201.19598, 135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, 218.59897, " +
          "135.2, 189.49573, 370.1, 391.69785, 108.2, 120.79874, 135.2, 218.59897, 135.2, 189.49573, 370.1, 391.69785, 108.2, 120.79874, 135.2, 209.5979, " +
          "135.2, 163.39717, 370.1, 391.69785, 108.2, 120.79874, 135.2, 174.19609, 135.2, 172.69742, 370.1, 391.69785, 108.2, 120.79874, 135.2, 174.19609, " +
          "135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, 189.49884, 135.2, 180.7966, 374.0, 391.69824, 108.2, 120.79874, 135.2, 214.39862, " +
          "135.2, 178.9956, 374.0, 391.69824, 108.2, 120.79874, 135.2, 161.59735, 135.2, 196.39772, 374.0, 391.69824, 517.1, 544.6972, 54.199997, 88.399216, " +
          "108.2, 115.39928, 135.2, 215.59856, 135.2, 172.69742, 377.9, 391.6986, 108.2, 115.39928, 135.2, 215.59856, 135.2, 172.69742, 377.9, 391.6986, " +
          "108.2, 120.79874, 135.2, 195.1961, 135.2, 208.6958, 135.2, 178.0957, 374.0, 391.69824, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 118.99892, " +
          "135.2, 144.1991, 374.0, 391.69824, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 118.99892, 135.2, 193.09724, 374.0, 391.69824, 108.2, 120.79874, " +
          "135.2, 185.59799, 135.2, 189.19801, 377.9, 391.6986, 108.2, 120.79874, 135.2, 202.997, 135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, " +
          "135.2, 217.69742, 135.2, 163.39717, 374.0, 391.69824, 135.2, 250.09616, 517.1, 544.6972, 108.2, 120.79874, 135.2, 187.69786, 135.2, 172.69742, " +
          "370.1, 391.69785, 108.2, 115.39928, 135.2, 215.59856, 135.2, 172.69742, 377.9, 391.6986, 108.2, 120.79874, 135.2, 208.99646, 135.2, 182.8964, " +
          "374.0, 391.69824, 108.2, 120.79874, 135.2, 214.6982, 135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, 178.69823, 135.2, 163.39717, " +
          "374.0, 391.69824, 108.2, 120.79874, 135.2, 178.69844, 135.2, 172.69742, 370.1, 391.69785, 108.2, 120.79874, 135.2, 178.69844, 135.2, 172.69742, " +
          "377.9, 391.6986, 108.2, 120.79874, 135.2, 221.29475, 135.2, 163.39717, 374.0, 391.69824, 108.2, 120.79874, 135.2, 221.29475, 135.2, 163.39717, " +
          "370.1, 391.69785, 524.7, 553.7995, 54.199997, 88.399216, 108.2, 121.69865, 135.2, 191.89688, 135.2, 214.39604, 440.6, 468.19724, 108.2, 125.59826, " +
          "135.2, 179.2979, 187.7, 210.49771, 135.2, 208.69688, 370.1, 391.69785, 108.2, 120.79874, 135.2, 178.69844, 135.2, 172.69742, 374.0, 391.69824, " +
          "517.1, 544.6972, 54.199997, 88.399216, 108.2, 121.69865, 135.2, 191.89688, 135.2, 214.39604, 436.7, 468.19687, 108.2, 118.698944, 135.2, " +
          "196.99606, 135.2, 172.99727, 360.2, 391.69687, 108.2, 120.79874, 135.2, 201.19598, 135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, " +
          "209.5979, 135.2, 163.39717, 370.1, 391.69785, 108.2, 120.79874, 135.2, 189.49884, 135.2, 180.7966, 374.0, 391.69824, 108.2, 120.79874, 135.2, " +
          "208.69637, 135.2, 175.39758, 377.9, 391.6986, 108.2, 115.39928, 135.2, 215.59856, 135.2, 172.69742, 377.9, 391.6986, 108.2, 115.39928, 135.2, " +
          "215.59856, 135.2, 172.69742, 377.9, 391.6986, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 121.69865, 135.2, 191.89688, 135.2, 214.39604, 436.7, " +
          "468.19687, 108.2, 118.698944, 135.2, 208.09802, 135.2, 162.79724, 364.1, 391.69724, 135.2, 250.09616, 517.1, 544.6972, 108.2, 120.79874, 135.2, " +
          "178.0957, 135.2, 163.39717, 374.0, 391.69824, 108.2, 120.79874, 135.2, 185.59691, 135.2, 163.39717, 370.1, 391.69785, 108.2, 115.39928, 135.2, " +
          "178.69844, 135.2, 172.69742, 374.0, 391.69824, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 125.59826, 135.2, 179.2979, 187.7, 210.49771, 135.2, " +
          "213.79517, 370.1, 391.69785, 517.1, 544.6972, 54.199997, 88.399216, 108.2, 120.79874, 135.2, 214.6982, 135.2, 172.69742, 370.1, 391.69785, 108.2, " +
          "120.79874, 135.2, 212.59627, 135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, 180.79544, 135.2, 167.29678, 374.0, 391.69824, 108.2, " +
          "120.79874, 135.2, 165.49696, 135.2, 167.29678, 374.0, 391.69824, 108.2, 120.79874, 135.2, 165.49696, 135.2, 167.29678, 374.0, 391.69824, 108.2, " +
          "115.39928, 135.2, 215.59856, 135.2, 172.69742, 377.9, 391.6986, 108.2, 115.39928, 135.2, 215.59856, 135.2, 172.69742, 377.9, 391.6986, 108.2, " +
          "115.39928, 135.2, 190.69446, 135.2, 190.69894, 377.9, 391.6986, 108.2, 115.39928, 135.2, 165.49861, 135.2, 195.19905, 377.9, 391.6986, 517.1, " +
          "544.6972, 54.199997, 88.399216, 108.2, 118.99892, 135.2, 205.99895, 135.2, 195.79625, 374.0, 391.69824, 517.1, 544.6972, 54.199997, 88.399216, " +
          "108.2, 118.99892, 135.2, 201.49861, 374.0, 391.69824, 108.2, 118.698944, 135.2, 201.79861, 135.2, 183.19757, 364.1, 391.69724, 517.1, 544.6972, " +
          "54.199997, 87.499214, 108.2, 125.59826, 135.2, 182.2976, 190.7, 214.39763, 135.2, 215.897, 370.1, 391.69785, 517.1, 544.6972, 54.199997, " +
          "87.499214, 108.2, 118.99892, 135.2, 211.99535, 370.1, 391.69785, 108.2, 121.69865, 135.2, 191.89688, 135.2, 214.39604, 364.1, 391.69724, 108.2, " +
          "121.69865, 135.2, 191.89688, 135.2, 214.39604, 370.1, 391.69785, 108.2, 125.59826, 135.2, 222.19763, 135.2, 215.2975, 370.1, 391.69785, 108.2, " +
          "115.39928, 135.2, 199.99884, 135.2, 193.99792, 377.9, 391.6986, 108.2, 115.39928, 135.2, 175.69751, 135.2, 166.39688, 374.0, 391.69824, 135.2, " +
          "250.09616, 523.1, 544.6978, 108.2, 115.39928, 135.2, 198.797, 135.2, 171.1964, 374.0, 391.69824, 108.2, 115.39928, 135.2, 198.797, 135.2, " +
          "171.1964, 377.9, 391.6986, 108.2, 115.39928, 135.2, 198.797, 135.2, 171.1964, 377.9, 391.6986, 523.1, 544.6978, 54.199997, 87.499214, 108.2, " +
          "118.698944, 135.2, 174.19894, 135.2, 161.59735, 370.1, 391.69785, 108.2, 120.79874, 135.2, 178.69844, 135.2, 180.7966, 377.9, 391.6986, 108.2, " +
          "120.79874, 135.2, 189.49884, 135.2, 180.7966, 370.1, 391.69785, 108.2, 115.39928, 135.2, 160.99742, 135.2, 195.79776, 377.9, 391.6986, 523.1, " +
          "544.6978, 54.199997, 87.499214, 108.2, 120.79874, 135.2, 217.69742, 135.2, 163.39717, 370.1, 391.69785, 108.2, 120.79874, 135.2, 212.59627, " +
          "135.2, 172.69742, 374.0, 391.69824, 108.2, 120.79874, 135.2, 187.09769, 135.2, 163.39717, 377.9, 391.6986, 108.2, 115.39928, 135.2, 215.59856, " +
          "135.2, 172.69742, 377.9, 391.6986, 108.2, 120.79874, 135.2, 195.1961, 135.2, 209.29948, 135.2, 166.09691, 135.2, 173.89784, 135.2, 200.29836, " +
          "370.1, 391.69785, 108.2, 118.698944, 135.2, 181.39537, 135.2, 193.09871, 377.9, 391.6986, 527.0, 544.69824, 54.199997, 87.499214, 108.2, " +
          "125.59826, 135.2, 179.2979, 187.7, 211.39763, 135.2, 208.69688, 370.1, 391.69785, 108.2, 120.79874, 135.2, 212.59627, 135.2, 172.69742, 374.0, " +
          "391.69824, 524.7, 553.7995, 54.199997, 87.499214, 108.2, 120.79874, 135.2, 216.79735, 135.2, 163.39717, 374.0, 391.69824, 108.2, 121.69865, " +
          "135.2, 191.89688, 135.2, 214.39604, 440.6, 468.19724, 517.1, 544.6972";
  String[] pointss = points.split(", ");
  List<Column> columns = new ArrayList<> ();
  for (int i = 0; i < pointss.length; i += 2) {
    columns.add(new Column(Float.parseFloat(pointss[i]), Float.parseFloat(pointss[i+1])));
  }
  List<Column> columnsWithOverlap = columns.stream()
          .distinct()
          .sorted()
          .collect(toList());
  System.out.println(columnsWithOverlap);
}

- assylias

---

你确定这与等于和比较有关吗？https://dev59.com/9Wsy5IYBdhLWcg3wxQ0k。逐步执行代码并找到导致它抛出异常的两个点。 - Falmarri

有趣的是，如果在对列进行流操作之前调用 Collections.sort(columns);，则不会从 TimSort 抛出异常。 - beresfordt

另一个有趣的事情是，如果你只执行去重操作，然后使用Collections.sort对结果列表进行排序，你仍然会遇到这个问题。 - reegnz

我认为你的比较器不具有传递性，即 columnA(1, 4), columnB(3, 7), columnC(6, 9)。在这种情况下，columnA.compareTo(columnB) = 0 和 columnB.compareTo(columnC) = 0，但 columnA.compareTo(columnC) < 0。 - fps

@assylias 如果这是您的用例，您应该按照 startX 和 endX 的中间点对列进行排序，即 (startX + endX) / 2。然后，您可以找到中间点位于最大列数内的列组。 - fps

Answer 1

我认为您的比较器不具有传递性，即columnA(1, 4), columnB(3, 7), columnC(6, 9)。

在这种情况下，columnA.compareTo(columnB) = 0和columnB.compareTo(columnC) = 0，但是columnA.compareTo(columnC) < 0。

至于为什么在sorted()出现在distinct()之前时会失败，当compareTo()不具有传递性时会发生什么，谁知道呢？

- fps

---

如果列表不是不同的，那么可能从未出现过比较失败的情况，因为在示例中导致它失败的两个值在原始列表中永远不会相邻。 - reegnz

@reegnz 这可能是一个解释，但如果流是并行的呢？ - fps

据我所知，并行排序是使用分治法和forkjoin实现的某种确定性排序，因此不应该影响它。但我可能还不是回答这个问题的合适人选。 :P - reegnz

@reegnz：你说得对，分治策略与归并排序变体（又称TimSort）的顺序处理方式非常相似。在任一情况下，数据都会被细分、处理和合并。但是，在拆分位置上可能存在微妙的差异，因此在存在破碎的排序情况下，两种情况的结果都是不确定的。添加或删除一个元素可能会改变整个结果。 - Holger

Answer 2

好的，我认为我可能已经找到了问题所在，但不确定如何解决。

问题在于比较器不是传递性的。其他人也注意到了这一点，我在问题的评论中也提到了。

如果您将Float.compare的值从这个位置交换：

return isSameColumn(o) ? 0 : Float.compare(this.startX, o.startX);

转换为：

return isSameColumn(o) ? 0 : Float.compare(o.startX, this.startX);

排序功能没有问题，但是会给出相反的顺序。

现在原因还不是完全清楚，但我希望这能给你正确方向上的推动。

- reegnz

---

对我来说也很奇怪的是，反转排序顺序会使问题重新出现 sort(Collections.reverseOrder()) - reegnz

考虑到传递性，很容易举出一个例子来证明比较器不具有传递性。请参见Federico的答案。 - reegnz