when I am writing a comparator using lamba, java throws out this exception

         at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)         at java.util.concurrent.FutureTask.run(FutureTask.java:266)         at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)         at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)         at java.lang.Thread.run(Thread.java:748) Caused by: java.lang.IllegalArgumentException: Comparison method violates its general contract!         at java.util.TimSort.mergeLo(TimSort.java:777)         at java.util.TimSort.mergeAt(TimSort.java:514)         at java.util.TimSort.mergeCollapse(TimSort.java:441)         at java.util.TimSort.sort(TimSort.java:245)         at java.util.Arrays.sort(Arrays.java:1512)         at java.util.ArrayList.sort(ArrayList.java:1464)         at java.util.stream.SortedOps$RefSortingSink.end(SortedOps.java:392)         at java.util.stream.Sink$ChainedReference.end(Sink.java:258)         at java.util.stream.Sink$ChainedReference.end(Sink.java:258)         at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:500)         at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:486)         at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)         at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)         at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)         at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)

seems like `TimSort` is having a safety check to ensure the Comparator contract, however, it's not always working.

So this only throws out an issue when running enough times

     @Test     public void test3() {         List<Domain> domains = new ArrayList<>();         for (int i = 0; i < 1_000; i++) { //            domains.clear();             domains.add(Domain.builder().val(0.1).name("1").build());             domains.add(Domain.builder().val(0.5).name("2").build());             domains.add(Domain.builder().val(1.2).name("3").build());             domains.add(Domain.builder().val(2.1).name("4").build());             Collections.shuffle(domains);             log.info("{}th start sorting {}", i, domains);             log.info("check sorted {}", domains.parallelStream().sorted((o1, o2) -> {                         final int diff = (int) (o2.val - o1.val);                         log.info("check {} - {} = {}", o1, o2, diff);                         return diff;                     }).collect(Collectors.toList())             );         }      }     @Test     public void test4() {         List<Domain> domains = new ArrayList<>();         for (int i = 0; i < 1_000_000; i++) { //            domains.clear();             domains.add(Domain.builder().val(0.1).name("1").build());             domains.add(Domain.builder().val(0.5).name("2").build());             domains.add(Domain.builder().val(1.2).name("3").build());             domains.add(Domain.builder().val(2.1).name("4").build());             Collections.shuffle(domains); //            log.info("{}th start sorting {}", i, domains);             log.info("check sorted {}", domains.parallelStream().sorted((o1, o2) -> Double.compare(o2.val, o1.val)).collect(Collectors.toList()).size()             );         }      } }  @Data @Builder class Domain {     double val;     String name; }