Why does this program using Collections.sort only fail for lists of size 32 or more?

Question

The following program throws the following exception:

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

Answer 1

Java 8 uses TimSort algorithm to sort the input. In TimSort there is a merging phase that happens when the length is at least 32. When the length is lower than 32 then a simple sorting algorithm is used that probably doesn't detect the contract violation. Let the source code comments of TimSort.java speak for itself:

class TimSort {
    /**
     * This is the minimum sized sequence that will be merged.  Shorter
     * sequences will be lengthened by calling binarySort.  If the entire
     * array is less than this length, no merges will be performed.
     *
     * This constant should be a power of two.  It was 64 in Tim Peter's C
     * implementation, but 32 was empirically determined to work better in
     * this implementation.  In the unlikely event that you set this constant
     * to be a number that's not a power of two, you'll need to change the
     * {@link #minRunLength} computation.
     *
     * If you decrease this constant, you must change the stackLen
     * computation in the TimSort constructor, or you risk an
     * ArrayOutOfBounds exception.  See listsort.txt for a discussion
     * of the minimum stack length required as a function of the length
     * of the array being sorted and the minimum merge sequence length.
     */
    private static final int MIN_MERGE = 32;