Another comparison method that violates his contract

Question

I have this class:

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); } } 

It seems to me that the compareTo method conforms to the comparator contract, even if it does not agree with equals (specifically) - this should not be a problem according to javadoc :

In the general case, but it is not strictly required that (compare(x, y)==0) == (x.equals(y))

However, sometimes I get:

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

For example, the code below throws an exception (also on ideone - it's a bit long, but most of them are test data).

Also note that running sorted() before distinct() on the stream solves the problem.

 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); } 

Answer 1

I do not think your comparator is transitive, i.e. columnA(1, 4), columnB(3, 7), columnC(6, 9) .

In this case, columnA.compareTo(columnB) = 0 and columnB.compareTo(columnC) = 0 , however columnA.compareTo(columnC) < 0 .

As for why it doesn't work when sorted() appears before distinct() , who knows what happens when compareTo() not transitive?

Answer 2

Well, I think I could find a problem, but not quite exactly how to solve it. The problem is that the comparator is not transitive. Others also noted this, and I also did this in the comments on this question.

If you replace the values ​​of Float.compare with this:

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

:

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

sorting works without problems, but it gives the reverse order.

Now the reason is still not entirely clear to me, but I hope this will give you a push in the right direction.