Java error Comparison method violates its general contract

Question

I saw many questions about this  and tried to solve the problem  but after one hour of googling and a lots of trial  amp  error  I still can t fix it  I hope some of you catch the problem   This is what I get   java lang IllegalArgumentException  Comparison method violates its general contract      at java util ComparableTimSort mergeHi ComparableTimSort java 835      at java util ComparableTimSort mergeAt ComparableTimSort java 453      at java util ComparableTimSort mergeForceCollapse ComparableTimSort java 392      at java util ComparableTimSort sort ComparableTimSort java 191      at java util ComparableTimSort sort ComparableTimSort java 146      at java util Arrays sort Arrays java 472      at java util Collections sort Collections java 155            And this is my comparator    Override public int compareTo Object o        if this    o           return 0             CollectionItem item    CollectionItem  o       Card card1   CardCache getInstance   getCard cardId       Card card2   CardCache getInstance   getCard item getCardId          if  card1 getSet    lt  card2 getSet              return -1        else           if  card1 getSet      card2 getSet                  if  card1 getRarity    lt  card2 getRarity                      return 1                else                   if  card1 getId      card2 getId                          if  cardType  gt  item getCardType                              return 1                        else                           if  cardType    item getCardType                                  return 0                                                    return -1                                                          return -1                                  return 1            Any idea

User · Answer

I got the same error with a class like the following StockPickBean. Called from this code:

List<StockPickBean> beansListcatMap.getValue();
beansList.sort(StockPickBean.Comparators.VALUE);

public class StockPickBean implements Comparable<StockPickBean> {
    private double value;
    public double getValue() { return value; }
    public void setValue(double value) { this.value = value; }

    @Override
    public int compareTo(StockPickBean view) {
        return Comparators.VALUE.compare(this,view); //return 
        Comparators.SYMBOL.compare(this,view);
    }

    public static class Comparators {
        public static Comparator<StockPickBean> VALUE = (val1, val2) -> 
(int) 
         (val1.value - val2.value);
    }
}

After getting the same error:

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

I changed this line:

public static Comparator<StockPickBean> VALUE = (val1, val2) -> (int) 
         (val1.value - val2.value);

to:

public static Comparator<StockPickBean> VALUE = (StockPickBean spb1, 
StockPickBean spb2) -> Double.compare(spb2.value,spb1.value);

That fixes the error.

User · Answer

The exception message is actually pretty descriptive  The contract it mentions is transitivity  if A  gt  B and B  gt  C then for any A  B and C  A  gt  C  I checked it with paper and pencil and your code seems to have few holes   if  card1 getRarity    lt  card2 getRarity        return 1    you do not return -1 if card1 getRarity    gt  card2 getRarity       if  card1 getId      card2 getId                return -1    You return -1 if ids aren t equal  You should return -1 or 1 depending on which id was bigger     Take a look at this  Apart from being much more readable  I think it should actually work   if  card1 getSet    gt  card2 getSet          return 1    if  card1 getSet    lt  card2 getSet          return -1     if  card1 getRarity    lt  card2 getRarity          return 1    if  card1 getRarity    gt  card2 getRarity          return -1    if  card1 getId    gt  card2 getId          return 1    if  card1 getId    lt  card2 getId          return -1    return cardType - item getCardType       watch out for overflow

User · Answer

You can use the following class to pinpoint transitivity bugs in your Comparators           author Gili Tzabari     public final class Comparators                  Verify that a comparator is transitive                 param  lt T gt         the type being compared         param comparator the comparator to test         param elements   the elements to test against         throws AssertionError if the comparator is not transitive             public static  lt T gt  void verifyTransitivity Comparator lt T gt  comparator  Collection lt T gt  elements                for  T first  elements                        for  T second  elements                                int result1   comparator compare first  second                   int result2   comparator compare second  first                   if  result1    -result2                                           Uncomment the following line to step through the failed case                       comparator compare first  second                       throw new AssertionError  compare     first          second             result1                             but swapping the parameters returns     result2                                                     for  T first  elements                        for  T second  elements                                int firstGreaterThanSecond   comparator compare first  second                   if  firstGreaterThanSecond  lt   0                      continue                  for  T third  elements                                        int secondGreaterThanThird   comparator compare second  third                       if  secondGreaterThanThird  lt   0                          continue                      int firstGreaterThanThird   comparator compare first  third                       if  firstGreaterThanThird  lt   0                                                   Uncomment the following line to step through the failed case                           comparator compare first  third                           throw new AssertionError  compare     first          second       gt  0                                   compare     second          third       gt  0  but compare     first          third                                         firstGreaterThanThird                                                                                         Prevent construction              private Comparators                   Simply invoke Comparators verifyTransitivity myComparator  myCollection  in front of the code that fails

User · Answer

Consider the following case   First  o1 compareTo o2  is called  card1 getSet      card2 getSet   happens to be true and so is card1 getRarity    lt  card2 getRarity    so you return 1   Then  o2 compareTo o1  is called  Again  card1 getSet      card2 getSet   is true  Then  you skip to the following else  then card1 getId      card2 getId   happens to be true  and so is cardType  gt  item getCardType    You return 1 again   From that  o1  gt  o2  and o2  gt  o1  You broke the contract

User · Answer

It also has something to do with the version of JDK  If it does well in JDK6  maybe it will have the problem in JDK 7 described by you  because the implementation method in jdk 7 has been changed   Look at this   Description  The sorting algorithm used by java util Arrays sort and  indirectly  by java util Collections sort has been replaced  The new sort implementation may throw an IllegalArgumentException if it detects a Comparable that violates the Comparable contract  The previous implementation silently ignored such a situation  If the previous behavior is desired  you can use the new system property  java util Arrays useLegacyMergeSort  to restore previous mergesort behaviour   I don t know the exact reason  However  if you add the code before you use sort  It will be OK   System setProperty  java util Arrays useLegacyMergeSort    true

User · Answer

I had to sort on several criterion  date  and  if same date  other things      What was working on Eclipse with an older version of Java  did not worked any more on Android   comparison method violates contract      After reading on StackOverflow  I wrote a separate function that I called from compare   if the dates are the same  This function calculates the priority  according to the criteria  and returns -1  0  or 1 to compare    It seems to work now

User · Answer

It might also be an OpenJDK bug     not in this case but it is the same error   If somebody like me stumbles upon this answer regarding the  java lang IllegalArgumentException  Comparison method violates its general contract    then it might also be a bug in the Java-Version  I have a compareTo running since several years now in some applications  But suddenly it stopped working and throws the error after all compares were done  i compare 6 Attributes before returning  0     Now I just found this Bugreport of OpenJDK    JDK-8210311 Affects Version s  8  11 Fix Version s  12 https   bugs openjdk java net browse JDK-8210311

User · Answer

I had the same symptom  For me it turned out that another thread was modifying the compared objects while the sorting was happening in a Stream  To resolve the issue  I mapped the objects to immutable temporary objects  collected the Stream to a temporary Collection and did the sorting on that

User · Answer

I ran into a similar problem where I was trying to sort a n x 2 2D array named contests which is a 2D array of simple integers  This was working for most of the times but threw a runtime error for one input -  Arrays sort contests   row1  row2  - gt                if  row1 0   lt  row2 0                     return 1                else return -1                Error -  Exception in thread  main  java lang IllegalArgumentException  Comparison method violates its general contract      at java base java util TimSort mergeHi TimSort java 903      at java base java util TimSort mergeAt TimSort java 520      at java base java util TimSort mergeForceCollapse TimSort java 461      at java base java util TimSort sort TimSort java 254      at java base java util Arrays sort Arrays java 1441      at com hackerrank Solution luckBalance Solution java 15      at com hackerrank Solution main Solution java 49    Looking at the answers above I tried adding a condition for equals and I don t know why but it worked  Hopefully we must explicitly specify what should be returned for all cases  greater than  equals and less than            Arrays sort contests   row1  row2  - gt                if  row1 0   lt  row2 0                     return 1                            if row1 0     row2 0   return 0              return -1

User · Answer

if  card1 getRarity    lt  card2 getRarity                  return 1    However  if card2 getRarity   is less than card1 getRarity   you might not return -1   You similarly miss other cases   I would do this  you can change around depending on your intent   public int compareTo Object o            if this    o           return 0             CollectionItem item    CollectionItem  o       Card card1   CardCache getInstance   getCard cardId       Card card2   CardCache getInstance   getCard item getCardId         int comp card1 getSet   - card2 getSet        if  comp  0           return comp            comp card1 getRarity   - card2 getRarity        if  comp  0           return comp            comp card1 getSet   - card2 getSet        if  comp  0           return comp               comp card1 getId   - card2 getId        if  comp  0           return comp               comp card1 getCardType   - card2 getCardType         return comp

[java] Java error: Comparison method violates its general contract

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