After looking at the code of ComparableTimSort I am not quite sure. Let's analyze it. Here is the only method that throws it (there is a similar method that does the same only with exchanged roles, so analyzing one of them is enough).
private void mergeLo(int base1, int len1, int base2, int len2) {
assert len1 > 0 && len2 > 0 && base1 + len1 == base2;
// Copy first run into temp array
Object[] a = this.a; // For performance
Object[] tmp = ensureCapacity(len1);
int cursor1 = tmpBase; // Indexes into tmp array
int cursor2 = base2; // Indexes int a
int dest = base1; // Indexes int a
System.arraycopy(a, base1, tmp, cursor1, len1);
// Move first element of second run and deal with degenerate cases
a[dest++] = a[cursor2++];
if (--len2 == 0) {
System.arraycopy(tmp, cursor1, a, dest, len1);
return;
}
if (len1 == 1) {
System.arraycopy(a, cursor2, a, dest, len2);
a[dest + len2] = tmp[cursor1]; // Last elt of run 1 to end of merge
return;
}
int minGallop = this.minGallop; // Use local variable for performance
outer:
while (true) {
int count1 = 0; // Number of times in a row that first run won
int count2 = 0; // Number of times in a row that second run won
/*
* Do the straightforward thing until (if ever) one run starts
* winning consistently.
*/
// ------------------ USUAL MERGE
do {
assert len1 > 1 && len2 > 0;
if (((Comparable) a[cursor2]).compareTo(tmp[cursor1]) < 0) {
a[dest++] = a[cursor2++];
count2++;
count1 = 0;
if (--len2 == 0)
break outer;
} else {
a[dest++] = tmp[cursor1++];
count1++;
count2 = 0;
if (--len1 == 1)
break outer;
}
} while ((count1 | count2) < minGallop);
// ------------------ GALLOP
/*
* One run is winning so consistently that galloping may be a
* huge win. So try that, and continue galloping until (if ever)
* neither run appears to be winning consistently anymore.
*/
do {
assert len1 > 1 && len2 > 0;
count1 = gallopRight((Comparable) a[cursor2], tmp, cursor1, len1, 0);
if (count1 != 0) {
System.arraycopy(tmp, cursor1, a, dest, count1);
dest += count1;
cursor1 += count1;
len1 -= count1;
// -->>>>>>>> HERE IS WHERE GALLOPPING TOO FAR WILL TRIGGER THE EXCEPTION
if (len1 <= 1) // len1 == 1 || len1 == 0
break outer;
}
a[dest++] = a[cursor2++];
if (--len2 == 0)
break outer;
count2 = gallopLeft((Comparable) tmp[cursor1], a, cursor2, len2, 0);
if (count2 != 0) {
System.arraycopy(a, cursor2, a, dest, count2);
dest += count2;
cursor2 += count2;
len2 -= count2;
if (len2 == 0)
break outer;
}
a[dest++] = tmp[cursor1++];
if (--len1 == 1)
break outer;
minGallop--;
} while (count1 >= MIN_GALLOP | count2 >= MIN_GALLOP);
if (minGallop < 0)
minGallop = 0;
minGallop += 2; // Penalize for leaving gallop mode
} // End of "outer" loop
this.minGallop = minGallop < 1 ? 1 : minGallop; // Write back to field
if (len1 == 1) {
assert len2 > 0;
System.arraycopy(a, cursor2, a, dest, len2);
a[dest + len2] = tmp[cursor1]; // Last elt of run 1 to end of merge
} else if (len1 == 0) {
throw new IllegalArgumentException(
"Comparison method violates its general contract!");
} else {
assert len2 == 0;
assert len1 > 1;
System.arraycopy(tmp, cursor1, a, dest, len1);
}
}
The method performs a merging of two sorted runs. It does a usual merge but starts "gallopping" once it encounters that one side starts "winning" (I.e., being always less than the other) all the time. Gallopping tries to make things faster by looking ahead more elements instead of comparing one element at a time. Since the runs should be sorted, looking ahead is fine.
You see that the exception is only throw when len1 is 0 at the end.
The first observation is the following: During the usual merge, the exception can never be thrown since the loop aborts directly once len this 1. Thus, the exception can only be thrown as result of a gallop.
This already gives a strong hint that the exception behaviour is unreliable: As long as you have small data sets (so small that a generated run may never gallop, as MIN_GALLOP is 7) or the generated runs always coincidentally generate a merge that never gallops, you will never receive the exception. Thus, without further reviewing the gallopRight method, we can come to the conclusion that you cannot rely on the exception: It may never be thrown no matter how wrong your comparator is.
Comparatorthat caused the exception because the comparator was not transitive: https://mcmap.net/q/49264/-quot-comparison-method-violates-its-general-contract-quot – CowbirdComparatordoesn't work in the course of its normal sorting algorithm. If it doesn't happen to do the right set of comparisons to make something break, it won't throw. You shouldn't depend on the sorting algorithm as a test. – Schoonover