Arrays.sort (Comparator) - Java 6 vs Java 7

Question

import java.util.Comparator; public class CompareTester { int i; public static class Inside implements Comparator<CompareTester> { @Override public int compare(CompareTester o1, CompareTester o2) { // TODO Auto-generated method stub System.out.println(o1.i - o2.i); return 0; }}} public class CompareTest { public static void main(String[] args) { CompareTester j = new CompareTester(); j.i = 3; CompareTester k = new CompareTester(); k.i = 5; CompareTester l = new CompareTester(); l.i = 7; CompareTester[] numbers = { j, k, l }; Arrays.sort(numbers, new CompareTester.Inside()); }}

These are two classes that are giving two different results in CompareTester class when ran in Java 6 and Java 7.

Is there any different kind of implementation that has been introduced in Java7 for Comparator?

return 0 in compare would mean objects are the same for such purpose so the order depends if the algorithm is stable or not — gtgaxiola
– gtgaxiola, Commented Jul 11, 2014 at 14:50
Your compare tester returns everything is equal. This isn't a good idea, or a good test. — Elliott Frisch
– Elliott Frisch, Commented Jul 11, 2014 at 14:50
Here I am just considering the issue why the "compare" method in "CompareTester" class is taking two different parameters (o1 and o2) for Java 6 and Java7. — Krishna
– Krishna, Commented Jul 11, 2014 at 14:56

JCW · Accepted Answer · 2014-07-11 14:55:06Z

Java 7 uses TimSort, Java 6 used MergeSort. Your comparator always returns 0 which breaks the Comparable Contract and TimSort is picking up on that.

To fix the code you really need to address that. If you really want to use the old sort algorithm then compile with -Djava.util.Arrays.useLegacyMergeSort=true

Yanick Rochon · Accepted Answer · 2014-07-11 15:01:58Z

Apart from a different sorting algorithm, which is only relevant internally, there are no difference between the final result of Array.sort(E, Comparator). So, no, there is not difference "introduced in Java7 for Comparator".

Just change your implementation to

import java.util.Comparator; public class CompareTester { int i; public static class Inside implements Comparator<CompareTester> { @Override public int compare(CompareTester o1, CompareTester o2) { return o1.i - o2.i; } } }

and compare the final resulting array instead and you'll see that there is no difference between Java 6 and 7.

Edit

If you want to avoid using a Comparator, then make your CompareTester class implement Comparable<T> instead (Java 7 doc here);

import java.lang.Comparable; public class CompareTester implements Comparable<CompareTester> { int i; public int compareTo(CompareTester o) { return this.i - o.i; } }

Then, you don't need anything special with Array.sort :

Arrays.sort(numbers);

Thanks for the reply. Here different sorting algorithm is irrelevant because I am just using integers. But in actual application code I am using objects which I have to override compare method to compare the objects. This is causing an issue.
If you're comparing two Objects, as in obj1 == obj2, then take a look at this answer; you should override .equals and .hashCode. For sorting, then implementing the Comparable<T> interface is the correct way to go.

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