ComparatorCheck.java

   /**
     * Check comparator respects its contract against a given collection.
     *
     * @param elements
     *            collection
     * @param comparator
     *            comparator
     */
    public static <T> void checkComparatorContract(Collection<T> elements, Comparator<? super T> comparator) {

        @SuppressWarnings("unchecked")
        final T[] array = elements.toArray((T[]) new Object[elements.size()]);

        elements.parallelStream().forEach(x -> {
            for (T y : array) {

                // The implementor must ensure that sgn(compare(x, y)) ==
                // -sgn(compare(y, x)) for all x and y. (This implies that compare(x, y)
                // must throw an exception if and only if compare(y, x) throws an
                // exception.)
                final int xy = comparator.compare(x, y);
                final int yx = comparator.compare(y, x);
                if (xy * yx > 0)
                    throw new IllegalStateException("Non symetric signum on " + LINE_SEPARATOR + x + ", "
                                                    + LINE_SEPARATOR + y + LINE_SEPARATOR + "(" + xy + " and " + yx
                                                    + ")");

                if (xy >= 0)
                    for (T z : array) {
                        final int xz = comparator.compare(x, z);
                        final int yz = comparator.compare(y, z);

                        // The implementor must also ensure that the relation is transitive:
                        // ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.
                        if (xy > 0 && yz > 0 && xz < 0)
                            throw new IllegalStateException("Comparator says: " + LINE_SEPARATOR + x + " > " + y
                                                            + LINE_SEPARATOR + y + " > " + z + LINE_SEPARATOR + x
                                                            + " < " + z + LINE_SEPARATOR);

                        // Finally, the implementor must ensure that compare(x, y)==0 implies
                        // that sgn(compare(x, z))==sgn(compare(y, z)) for all z.
                        if (xy == 0 && xz * yz < 0)
                            throw new IllegalStateException("Wrong signum on " + x + ", " + y + ", " + z);
                    }
            }
        });
    }