Why is my simple comparator broken?

I have a class, which I have simplified to this:

final class Thing {
    private final int value;
    public Thing(int value) {
        this.value = value;
    }
    public int getValue() {
        return value;
    }
    @Override public String toString() {
        return Integer.toString(value);
    }
}

I want to sort an array of this thing. So I have created a simple copmarator:

private static final Comparator<Thing> reverse = new Comparator<Thing>() {
    public int compare(Thing a, Thing b) {
        return a.getValue() - b.getValue();
    }
};

I then use the two argument form of Arrays.sort.

This works fine for my test cases, but sometimes it goes all wrong with the array ending up in a strange but repeatable order. How can this be?

From stackoverflow

• You cannot use minus to create the comparison. You'll overflow when the absolute difference exceeds Integer.MAX_VALUE.

  Instead, use this algorithm:

  int compareInts( int x, int y ) {
    if ( x < y ) return -1;
    if ( x > y ) return 1;
    return 0;
  }
  

  I like to have this function in a library for such purposes.

  Tom Hawtin - tackline : For a moment I was going to point you to the static method in Integer. But it isn't there...

  Grundlefleck : @Tom: `Integer.valueOf(x).compareTo(y);` is the most succinct way I can think of. Strange how Double has the static `compare()` method and the other number types don't.

  Jason Cohen : @Grundlefleck: True! But of course my method is much faster to execute because it doesn't create a new `Integer` instance.

• What kind of numbers do you throw in there? If your numbers are large enough, you could wrap through the MIN/MAX values for integers and end up in a mess.

• Integer overflow… or more precisely, underflow.

  Instead, do an explicit comparison:

  private static final Comparator<Thing> reverse = new Comparator<Thing>() {
      public int compare(Thing a, Thing b) {
        int av = a.getValue(), bv = b.getValue();
        return (av == bv) ? 0 : ((av < bv) ? -1 : +1);
      }
  };
  

  Using subtraction is fine if you are sure that the difference won't "wrap around". For example, when the values in question are constrained to be non-negative.

• If a's value is very negative and b's value is very positive your answer will be very wrong.

  IIRC, Int overflow silently wraps around in the JVM

  -- MarkusQ

• try

  System.out.println(Integer.MAX_Value - Integer.MIN_VALUE);
  

  This needs to return a positive number as MAX_VALUE > MIN_VALUE but instead prints -1

  Stefan Kendall : +1 for the most non-obvious "wrapping" fail case.

• When comparing Java primitives, it is advisable to convert them to their Object counterparts and rely on their compareTo() methods.

  In this case you can do:

  return Integer.valueOf(a.getValue()).compareTo(b.getValue())
  

  When in doubt, use a well-tested library.

  Tom Hawtin - tackline : Bit of an overhead there (for non-small values).