java - Comparison method violates its general contract -


i'm trying sort list. used collections.sort(list), , got illegalargumentexception exception.
know works on jdk6 , jdk7 using timsort. rules got below. think got right, doesn't work.

is there can give me situation code doesn't work? lot

rules:

> 1.sgn(compare(x, y)) == -sgn(compare(y, x)) >  > 2.((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0 >  > 3.compare(x, y)==0 implies sgn(compare(x, z))==sgn(compare(y, z)) z      //stringutil.isblank(str) return if str==null||str.trim().length()==0 @override public int compareto(novelchapter o) {     if(o == null){         return 1;     }     else if(o.equals(this) || == o){         return 0;     }      if(!stringutil.isblank(o.getsourceurl())&& !stringutil.isblank(this.getsourceurl())){         if(o.getsourceurl().length() != this.getsourceurl().length()){             return o.getsourceurl().compareto(this.getsourceurl());         }         else{             return o.getsourceurl().compareto(this.getsourceurl()) * (-1);         }     }     else if(!stringutil.isblank(o.getsourceurl())&& !stringutil.isblank(this.getsourceurl())){         return 1;     }     else if(!stringutil.isblank(o.getsourceurl())&& stringutil.isblank(this.getsourceurl())){         return -1;     }     else{         return 0;     } }

btw, system.setproperty("java.util.arrays.uselegacymergesort", "true"); using jdk6, not solve problem itself.

================================================

thanks answers,and changed code,now throw npe .is better ?but still give me same exception.

@override public int compareto(novelchapter o) {      if(o == null){         throw new nullpointerexception();     }else if(o.equals(this) || == o){         return 0;     }      string self = this.getsourceurl();     string item = o.getsourceurl();      if(self == null && item != null){         return -1;     }else if (self == null && item != null){         return 1;     }else if(self == null && item == null){         return 0;     }      if(self.length() == item.length()){         return self.compareto(item);     }else{         return self.compareto(item) * (-1);     }  }

i can't see specific cause of exception, there couple of problems in code have shown us.

  1. the compareto method should not used on null, special handling of null should not there. comparator >>should<< throw npe. (and corollary should not attempting sort arrays / collections contain null values.)

  2. your use of trim looks inconsistent. trim when "blank" urls, not when comparing "non-blank" urls. weird.

i add code verbose , inefficient. rather repeatedly calling isblank utility in chain of if / else statements should convert 2 urls strings (i'm guessing) can compared directly.

finally, fact have have isblank method @ says (to me) rest of code doing poor job of maintaining data consistency. there should 1 representation means "no source url" ... not (at least) three.

there number of possible causes of problem can eliminated examining complete classes involved ... starting novelchapter , whatever getsourceurl() returns.


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