Comparator interface

[praneeth ram]

Hi all,


A Comparator must define a compare function which takes two Objects and returns a -1, 0, or 1.

Here what happens internally, when the compare method returns -1 or 0 or 1.


Thanks,
Praneeth.

[Paul Clapham]

praneeth ram wrote:A Comparator must define a compare function which takes two Objects and returns a -1, 0, or 1.

Well, no. It should return a negative number, zero, or a positive number.

Here what happens internally, when the compare method returns -1 or 0 or 1.

What happens "internally"? Some code like this:

return 0;

But obviously you had some other question, since that's trivially obvious. Could you clarify?

[Joseph Arnold]

The compare method returns a int which can be a negative,positive or a 0.
Example: class Example implements comparator {
public int compare(Object A,Object B) {
return ObjA.compareTo(Objb);
}
}

If 0 is returned ObjA = Objb
If -ve is returned ObjA < ObjB
If +ve is returned ObjA > ObjB
So it actually uses a compareTo method internally for sorting.The comparator interface is used if you would want to sort a collection in multiple ways.

Joseph

[praneeth ram]

not satisfied with your reply. we can easily understand obj1<obj2 returns positive, greter than mean +ve, equal mean zero....nothig is great here.

who are going to use these +ve -ve zero values, where these can be useful, and how it works,

thanks,
praneeth.

[Matthew Brown]

It can be useful if you want to sort the objects. Sort algorithms generally work by comparing pairs of objects to see what order they should be in (the clever parts of the algorithm are deciding which objects to compare and what to do then - e.g. a bubble sort compares adjacent pairs and swaps them if they're in the wrong order, until there are no more to swap). So all they need is a method to tell them how to compare two objects - that's what Comparator (and Comparable) supplies.

[Joseph Arnold]

Who will use? Anyone who wants to sort a collection. It is used implicitly if the objects are in a TreeSet where you MUST implement comparable interface for the objects. How does the system compare? For numeric variables then sorting is obvious, if they are characters, they will be sorted by their ASCII codes. Hope I have answered your question clear enough.

Joseph