Sort an array of object based on an attribute in Java

Question

I am trying to improve given algorithm of selecting a Storage box based on Summary info on the box. In order to retrieve the identifer (of summary object) which has the with the highest numItems attribute (of summary object), I would have to sort the Summary[], an array of objects but I just can't figure how to sort by an attribute.

I found numerous examples of creating ArrayList<Int> a = new ArrayList<Int>(); and then Using Collections to get maximum value but here, I am interested in other attribute and I just can't visualize how I would do that. Can you help?

public String selectNextDelivery(StorageBox.Summary[] summaries) throws NoBoxReadyException {
    if (summaries.length != 0) {
        for(StorageBox.Summary summary : summaries){
            if (summary.numItems > 0) {
                return summary.identifier;
            }
        }
    }
    // Otherwise no box is ready
    throw new NoBoxReadyException();
}

All you need to do is implement a Comparator interface. There already exists many ways to sort a collection of objects — OneCricketeer
– OneCricketeer, Commented Aug 22, 2016 at 5:03
Alternatively, you need to not immediately return an element from the list and instead look at all elements, then return the maximum — OneCricketeer
– OneCricketeer, Commented Aug 22, 2016 at 5:05
you could use Comparable interface by implementing it in your Summary class — mojtab23
– mojtab23, Commented Aug 22, 2016 at 5:05
@mojtab23 Oh, I accidentally left that part out - I did some search and I did come across implementing Comparable interface but by requirement of school assignment, I can't touch the Summary class. I have to deal with it here. — younghak
– younghak, Commented Aug 22, 2016 at 5:10

castletheperson · Accepted Answer · 2016-08-22 06:06:03Z

In Java 8, to get the maximum element in an array of objects using an attribute, use Stream#max() with Comparator.comparingInt()

return Stream.of(summaries)
    .max(Comparator.comparingInt(s -> s.numItems))
    .orElseThrow(() -> new NoBoxReadyException())
    .identifier;

Without Java 8, you can use Collections.max() with a custom Comparator:

try {
    return Collections.max(Arrays.asList(summaries), new Comparator<StorageBox.Summary>() {
        @Override
        public int compare(StorageBox.Summary s1, StorageBox.Summary s2) {
            return Integer.compare(s1.numItems, s2.numItems);
        }
    }).identifier;
} catch (NoSuchElementException nsee) {
    throw new NoBoxReadyException();
}

Or you could implement it yourself with a standard for loop:

if (summaries.length == 0)
    throw new NoBoxReadyException();
StorageBox.Summary max = summaries[0];
for (int i = 1; i < summaries.length; i++)
    if (summaries[i].numItems > max.numItems)
        max = summaries[i];
return max.identifier;

Daniil Dubrovsky · Accepted Answer · 2016-08-22 05:21:06Z

1

If you're familiar with Java 8 Streams you can do something like this:

Stream.of(summaries) //creating custom comparator which sorts based on identifier
    .sort(($1, $2) -> Integer.compare($1.identifier, $2.identifier))
    //Do something with your stream

If you want to get the first element from stream sorted by some attribute you can do:

 Stream.of(summaries)
     .max(Comparator.comparingInt(StorageBox.Summary::getIdentifier))
     .orElse(null);

Thanks 4castle for pointing out a Comparator.comparingInt() method

edited Aug 22, 2016 at 5:21

answered Aug 22, 2016 at 5:02

Daniil Dubrovsky

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1 Comment

castletheperson Over a year ago

Using sort and findFirst together is overkill. It makes an O(n) operation into O(nlogn)

Liping Huang · Accepted Answer · 2016-08-22 05:03:55Z

0

Maybe what you want is using the public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) api, where it has the comparator with which to determine the maximum element. A null value indicates that the elements' natural ordering should be used.

answered Aug 22, 2016 at 5:03

Liping Huang

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1 Comment

castletheperson Over a year ago

You didn't mention, but this method is located in the Collections class. It is very useful, though doesn't allow a custom exception.

A Joshi · Accepted Answer · 2016-08-22 05:08:35Z

0

You can use comparators to sort-

Arrays.sort(summaries, new Comparator<Summary>() {

    @Override
    public int compare(Summary o1, Summary o2) {

        return o1.numItems.compareTo(o2.numItems);
    }           
});

or Use the Collections api alongwith the Comparator to retrieve the max

Summary maxSummary = Collections.max(Arrays.asList(summaries), 
                                   new Comparator<Summary>() {

            @Override
            public int compare(Summary o1, Summary o2) {

                return o1.numItems.compareTo(o2.numItems);
            }           
        });

answered Aug 22, 2016 at 5:08

A Joshi

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