Does anyone have a useful mnemonic for implementing Comparator?

Question

Every time I need to implement a comparator, I get stuck trying to remember when I should return -1 and when 1, and I have to look it up.

I mean, obviously -1 is less, so it implies that first is less than second. But whenever I say that to myself, I get that nagging "are you sure?" feeling. I suspect part of my confusion comes from implementing it the other way around whenever I need a descending sort.

What do you use to remember which is which?

I predict that now that you've taken the time and effort to write this as a question on this site, you will find out that you no longer have a problem remembering what values you should return. :-D — Tommi, Mar 30 '11 at 14:05
Can you clarify in your quesiton whether you are looking for a ascending or descending result? — djangofan, Dec 22 '20 at 19:00

score 11 · Answer 1 · answered Mar 30 '11 at 14:00

11

I use this simple "substraction" mnemonic:

first - second

So, if first is "less" than second you'll get negative result, otherwise - positive or zero if they are equal.

answered Mar 30 '11 at 14:00

xappymah

1,624
10
14

3

+0: Unless overflow is a problem in which case `Integer.MIN_VALUE - Integer.MAX_VALUE = 1` ;) – Peter Lawrey Mar 30 '11 at 14:05
2

or the ever so golden `(0 - Integer.MIN_VALUE) == Integer.MIN_VALUE`. :-) – Sanjay T. Sharma Mar 30 '11 at 14:18
My mind uses Doubles. That's why there is no equality for me and I do usually forget a lot of details :) – xappymah Mar 30 '11 at 14:21

score 6 · Accepted Answer · edited Mar 30 '11 at 17:33

6

comparator.compare(a, b) < 0 <==> a < b

edited Mar 30 '11 at 17:33

Paŭlo Ebermann

73,284
20
146
210

answered Mar 30 '11 at 14:18

Tadeusz Kopec for Ukraine

12,283
6
56
83

For some reason I like this one the best. – itsadok Mar 30 '11 at 14:46

score 5 · Answer 3 · answered Mar 30 '11 at 14:02

I am not sure what you mean by mnemonic. However, I have had a very similar cognitive dissonance.

I am very visual, so I use the number line (the one I was taught in grade school). I just visualize the negative numbers as "left", 0 as "center" and positive numbers as "right". That the corresponds to the truth: -1 < 0 < 1

score 2 · Answer 4 · answered Mar 30 '11 at 14:02

I remember the base integer case (pseudocode):

int comparator(int a, int b) {
   return a-b;
}

So if we give in a small a and a large b which is the first < last we get a negative result.

I have a more visual memory, so remembering the "structure" of that function is easy and natural for me.

dsharew · Answer 5 · 2020-12-22T19:01:16.533

I used to always check the documentation when implementing Comparator and Comparable interfaces.

Question: Compare a and b
Lets first at look ascending order since the descending order will be just the inverse of whatever we do.
Question can be translated to given two numbers a and b, how would you put them on the number line?
if a < b, then we will put a on the negative side and b on the positive side.
else if a = b then we will put both at the center (at 0)
otherwise b will be on the negative side and a will be on the positive side.

Comparator Implementation:
Here you are comparing a to b.

 @Override
    public int compare(MyClass a, MyClass b) { //always good to rename your variables like this to be consistent 
        return a.f1 - b.f1; 
    }

Comparable Implementation:
Here you are comparing this to other.

@Override
    public int compareTo(MyClass other) { // this is same as compare(this, other)
        return this.f1 - o.f1;
    }

Does anyone have a useful mnemonic for implementing Comparator?

5 Answers5