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# Define a partial order interface help

Craig Warnick
Greenhorn
Posts: 8
I need some help understanding what a partial order interface with one method greaterThan(). I am working on an exercise to compare the volume of three boxes and this statement is the next step. Thank you,

Rob Spoor
Sheriff
Posts: 20709
68
Why not use Comparable? You can use compareTo for the same causes. For a.compareTo(b)
• < 0 means that a is less than b
• > 0 means that a is greater than b
• == 0 means that a is equal to b

• You can also use <= and >= of course.

Jeff Verdegan
Bartender
Posts: 6109
6
Craig Warnick wrote:I need some help understanding what a partial order interface with one method greaterThan().

Can you provide some context? Where did you come across this term? And what specific problems are you having?

Paul Clapham
Sheriff
Posts: 21443
33
Well, here's a link to the Wikipedia article about it: Partially ordered set. I must say, I don't really see how to implement a partial ordering over a domain in Java; perhaps you throw a runtime exception if you try to compare a pair of incomparable elements?

But anyway if you're just going to compare the volume of boxes, then that's really a Total order. In that case it's much easier to implement a greaterThan method.

Craig Warnick
Greenhorn
Posts: 8
class BoxVolSort implements Comparator<Box> {
public int compare(Box b1, Box b2, Box b3) {
if(b1.getVolume() == b2.getVolume()==b3.get volume)
return 0;
else if (b1.getVolume() < b2.getVolume() > b3.getVolume)
return -1;
else
return 1;

This is what I came up with off the top of my head, but the statement that caused my helmet fire is:

box b > box c returns 1 whenever box c fits inside box box b. Note that c fits inside b if there is a way to arrange the dimensions of each box so
the corresponding dimensions of b are each larger than c.

No clue how to do that. Thanks for your earlier reply Saloon Keeper.

Craig Warnick
Greenhorn
Posts: 8
Its just a programming exercise I was given from a coworker. to help me learn Java. I am not in the programming business but trying to learn. Thanks

Winston Gutkowski
Bartender
Posts: 10527
64
Craig Warnick wrote:box b > box c returns 1 whenever box c fits inside box box b. Note that c fits inside b if there is a way to arrange the dimensions of each box so
the corresponding dimensions of b are each larger than c.

No clue how to do that. Thanks for your earlier reply Saloon Keeper.

Well, first off, java.util.Comparator works on two objects, not three, so I'd start off by amending your class accordingly.

Second, as you said yourself:
Note that c fits inside b if there is a way to arrange the dimensions of each box so the corresponding dimensions of b are each larger than c.
So, in terms of a "fits inside" order, I would say that if Box a fits inside Box b, then
compare(a, b)
should return a negative number, and
compare(b, a)
should return a positive one; otherwise, your comparator should return 0 because the two objects have no 'order' with respect to each other in terms of "fitting in".

Then it's just a matter of calling the method for combinations of pairs for larger numbers of boxes.

 Note that while the above would work for providing a rudimentary "fits in" ordering, I suspect there's quite a bit more logic required to actually create a POS for these boxes. In terms of structure, I think I might look at an array of linked lists, but I suspect you're also going to need to come up with a heuristic to ensure that as many boxes are "fitted" as possible.

HIH

Winston

Jesper de Jong
Java Cowboy
Saloon Keeper
Posts: 15568
46
Craig Warnick wrote:I need some help understanding what a partial order interface with one method greaterThan(). I am working on an exercise to compare the volume of three boxes and this statement is the next step. Thank you,

It sounds like the exercise is to define an interface with one method, greaterThan(). You can do that in just 3 lines of code or so.

See What Is an Interface? in Oracle's Java Tutorials.

Craig Warnick
Greenhorn
Posts: 8
Thanks for all your responses and help. Got it figured out.

Campbell Ritchie
Sheriff
Posts: 50714
83
Well done, and welcome to the Ranch