IllegalArgumentException in ArrayList.sort () - method

Question

I got the following exception, and I don’t quite understand why:

java.lang.IllegalArgumentException: the comparison method violates the general contract!

in java.util.TimSort.mergeLo (TimSort.java:777) in java.util.TimSort.mergeAt (TimSort.java∗14) at java.util.TimSort.mergeCollapse (TimSort.java:441) at java.util. TimSort.sort (TimSort.java:245) in java.util.Arrays.sort (Arrays.java:1512) in java.util.ArrayList.sort (ArrayList.java:1454)

I wrote the following JUnit test to test the behavior:

@Test
public void testComparator() {
    List<Boolean> item = new ArrayList<>();

    item.add(true);
    for (int i = 0; i < 1000000; i++) {
        item.add(false);
    }

    while(true) {
        System.out.println("Sorting");
        Collections.shuffle(item);

        item.sort((lineItem1, lineItem2) -> {
            if (lineItem1 && lineItem2) {
                return 0;
            } else if (!lineItem1) {
                return 1; 
            } else if (!lineItem2 ) {
                return -1;
            } 

            return 0;
        });
    }
}

If I change return 1 and return -1, it suddenly works without exception. But why? This should only change the sort order and not break the entire comparator.

What am I missing?

Answer 1

, false, 1 - if (!lineItem1) { return 1; }….

, , TimSort , , .

item.sort((lineItem1, lineItem2) -> {
    if (lineItem1.equals(lineItem2)) {
        return 0;
    } else if (!lineItem1) {
        return 1; 
    } else {
        return -1;
    }
});

,

item.sort((lineItem1, lineItem2) -> lineItem1^lineItem2? lineItem1? -1: 1: 0);

item.sort(Comparator.reverseOrder());