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!

Answer 1

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();  

Answer 2

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

1. order by pdate

   Is represented by

   Comparator.comparing(Document::getPDate)
   

2. order by subject

   Is represented by

   Comparator.comparing(Document::getSubject)
   

3. 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.