Java 8 Sort using 2 fields

Question

Based one some conditions, I have read data from MongoDB and creating a List<Document> with resultset.

List<Document> documentList = new ArrayList<Document>();

Sample Record Looks like:

documentList: [
    Document{
        { _id=5975ff00a213745b5e1a8ed9,
            u_id=,
            visblty = 1,
            c_id=5975ff00a213745b5e1a8ed8,                
            batchid=null,
            pdate=Tue Jul 11 17:52:25 IST 2017, 
            locale=en_US,
            subject = "Document2"
        }     },
    Document{
        { _id=597608aba213742554f537a6,
            u_id=,
            visblty = 1,
            c_id=597608aba213742554f537a3, 
            batchid=null,
            pdate=Fri Jul 28 01:26:22 IST 2017,
            locale=en_US,
            subject = "Document2"
        }    } 
]

Using this documentList, again i am filtering using some conditions and then I need to sort the filter record based on some conditions (which I will get in request).

List<Document> outList = documentList.stream()
                .filter(d -> d.getInteger("visblty") == 1
                && (!StringUtils.isEmpty(req.pdate())? (d.getDate(CommonConstants.PDATE).after(afterDate)): true) 
                && (!StringUtils.isEmpty(req.pdate())? (d.getDate(CommonConstants.PDATE).before(beforeDate)): true)
                .sorted().skip(4).limit()
                .collect(Collectors.toList());

Not sure how to sorted (Dynamically needs to change the sorting order based on input, it looks like "pdate by DESC" or "subject by ASC")

Like: "order by pdate DESC" or "order by pdate ASC"" or "order by subject DESC"

How to sort using Comparator object of Document class.

Note: I have tried couple of method which was suggested by the folks, but I didn't get any luck yet. Thank you in advance!

(!condition1? condition2: true) is an obfuscated variant of condition1 || condition2. You are maxing that out by repeating condition1 in two terms combined via &&. The entire filter predicate can be simplified to .filter( d -> d.getInteger("visblty")==1 && (StringUtils.isEmpty(req.pdate()) || d.getDate(CommonConstants.PDATE).after(afterDate)&&d.getDate(CommonConstants.PDATE) .before(beforeDate)) ). Though, StringUtils.isEmpty(req.pdate()) does not depend on the actual stream element, hence, could be even evaluated before the stream operation, rather than for each element. — Holger
– Holger, Commented Aug 2, 2017 at 10:50
I see your filter lambda gotten very big. I suggest you split it in parts and use function composition to add these parts together. — M. Prokhorov
– M. Prokhorov, Commented Aug 2, 2017 at 10:50
@MProkhorov: it’s big and redundant… I don’t know which date class it uses, but perhaps the combination of before and after could be replaced by a between call. Then, the code would be as simple as .filter(d -> d.getInteger("visblty")==1) .filter(d -> StringUtils.isEmpty(req.pdate()) || d.getDate(CommonConstants.PDATE).between(beforeDate, afterDate)) — Holger
– Holger, Commented Aug 2, 2017 at 10:58
@Holger, yes, even that is much better (after proper formatting ofc), without composing the function at all. — M. Prokhorov
– M. Prokhorov, Commented Aug 2, 2017 at 11:11

KayV · Accepted Answer · 2017-08-02 10:44:25Z

29

You can use group comparator and parallel stream as follows:

List<Document> outList = documentList.stream()
                               .filter(....)
                               .sorted(Comparator.comparing(Document::getPdate)
                                                 .thenComparing(Document::getSubject))   
                               .parallel();

answered Aug 2, 2017 at 10:44

KayV

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2 Comments

Pearl Over a year ago

@KayV The above example is easy to understand. But i have one clarification. What if we want to do a sorting dynamically.

KayV Over a year ago

@Pearl You can create custom comparator in that case. And based on some condition add the required comparator in your sorted -> comparing -> thenComparing chain

M. Prokhorov · Accepted Answer · 2017-08-02 10:49:05Z

In terms of Java code, equivalent sorting comparators look like the following:

order by pdate

Is represented by

Comparator.comparing(Document::getPDate)

order by subject

Is represented by

Comparator.comparing(Document::getSubject)

order by pdate, subject

Is represented by:

Comparator.comparing(Document::getPDate).thenComparing(Document::getSubject)

If at any point you need to have descending order, you can call reversed() on comparator, like this:

Comparator.comparing(Document::getPDate).reversed()
          .thenComparing(Comparator.comparing(Document::getSubject).reversed())

Note, that thenComparing method is overridden, providing ability to pass in:

a Comparator
a Function-extractor (works if whatever function extracts is-a Comparable)
a Function and a Comparator (for when extracted value is not itself a Comparable, so you cannot use its natural order and have to specify how it should be compared.

Szymon Stepniak · Accepted Answer · 2018-01-03 06:40:55Z

4

Instead of .sorted() try using .sorted(Comparator comparator) method.

You can create the Comparator used with .sorted(comparator) by using Comparator.comparing():

Comparator.comparing(Function keyExtractor)
    .reversed()
    .thenComparing(Function keyExtractor)

for example:

List<Document> outList = documentList.stream()
         // do your filtering here
        .sorted(Comparator.comparing(Document::getPdate).reversed()
                .thenComparing(Document::getSubject))
        .skip(4)
        .limit()
        .collect(Collectors.toList());

In this example you can use method reference Document::getPdate and Document::getSubject instead of lambda expression like d -> d.getPdate() and d -> d.getSubject()

edited Jan 3, 2018 at 6:40

answered Aug 2, 2017 at 10:37

Szymon Stepniak

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