Retrieve the first items of each group in Java

Question

In Java, how to get list down the latest/first item of each group after grouping by field(/s)? e.g From the given list of medical records, how to retrieve the latest record of each patient.


    Sample Data
    ---------------------------------------------------------------------------
    PatientName  |    ReportDate    | ReadingNo |    Measurement
    ---------------------------------------------------------------------------
    X            |    2020-01-02    |    1      |    255
    Y            |    2020-01-02    |    1      |    250
    X            |    2020-01-02    |    2      |    266
    Y            |    2020-01-02    |    2      |    270
    X            |    2020-01-02    |    3      |    298
    Y            |    2020-01-02    |    3      |    259
    X            |    2020-01-02    |    4      |    280
    Y            |    2020-01-02    |    4      |    285
    X            |    2020-01-03    |    1      |    260
    Y            |    2020-01-03    |    1      |    265
    X            |    2020-01-03    |    2      |    280
    Y            |    2020-01-03    |    2      |    260
    X            |    2020-01-03    |    3      |    285
    Y            |    2020-01-03    |    3      |    290
    X            |    2020-01-03    |    4      |    290
    Y            |    2020-01-03    |    4      |    280
    ---------------------------------------------------------------------------

    Expected result
    ---------------------------------------------------------------------------
    PatientName  |    ReportDate    | ReadingNo |    Measurement
    ---------------------------------------------------------------------------
    X            |    2020-01-03    |    4      |    290
    Y            |    2020-01-03    |    4      |    280
    ---------------------------------------------------------------------------

I can do it C# as below. Any one can help to translate it in Java?


    using System.Collections.Generic;
    using System.Linq;

    public class Program
    {
        public static void Main()
        {
            Console.WriteLine("---------------------------------------------------------------------------");
            Console.WriteLine(String.Format("{0}  |\t{1}\t| {2} |\t{3}", "PatientName", "ReportDate", "ReadingNo", "Measurement"));
            Console.WriteLine("---------------------------------------------------------------------------");
            List lst = new List
            {
                new Patient(){Name="X", ReportDate="2020-01-02", ReadingNo=1, Measurement=255 },
                new Patient(){Name="Y", ReportDate="2020-01-02", ReadingNo=1, Measurement=250 },
                new Patient(){Name="X", ReportDate="2020-01-02", ReadingNo=2, Measurement=266 },
                new Patient(){Name="Y", ReportDate="2020-01-02", ReadingNo=2, Measurement=270 },
                new Patient(){Name="X", ReportDate="2020-01-02", ReadingNo=3, Measurement=298 },
                new Patient(){Name="Y", ReportDate="2020-01-02", ReadingNo=3, Measurement=259 },
                new Patient(){Name="X", ReportDate="2020-01-02", ReadingNo=4, Measurement=280 },
                new Patient(){Name="Y", ReportDate="2020-01-02", ReadingNo=4, Measurement=285 },
                new Patient(){Name="X", ReportDate="2020-01-03", ReadingNo=1, Measurement=260 },
                new Patient(){Name="Y", ReportDate="2020-01-03", ReadingNo=1, Measurement=265 },
                new Patient(){Name="X", ReportDate="2020-01-03", ReadingNo=2, Measurement=280 },
                new Patient(){Name="Y", ReportDate="2020-01-03", ReadingNo=2, Measurement=260 },
                new Patient(){Name="X", ReportDate="2020-01-03", ReadingNo=3, Measurement=285 },
                new Patient(){Name="Y", ReportDate="2020-01-03", ReadingNo=3, Measurement=290 },
                new Patient(){Name="X", ReportDate="2020-01-03", ReadingNo=4, Measurement=290 },
                new Patient(){Name="Y", ReportDate="2020-01-03", ReadingNo=4, Measurement=280 }
            };
            lst.ForEach(p=>{
                Console.WriteLine(p.toString());
            });
            Console.WriteLine("---------------------------------------------------------------------------");

            var lstLatest = from p in lst
                  group p by p.Name
                      into g
                      select g.OrderByDescending(p => p.ReportDate).ThenByDescending(p=>p.ReadingNo).ToList();
            foreach(var p in lstLatest)
            {
                Console.WriteLine(p.First().toString());
            }


        }
    }

    public class Patient
    {
        public string Name { get; set;}
        public string ReportDate {get;set;}
        public int ReadingNo {get;set;}
        public int Measurement {get;set;}
        public string toString()
        {
            return String.Format("{0}\t\t\t|\t{1}\t|\t{2}  \t|\t{3}", this.Name, this.ReportDate, this.ReadingNo, this.Measurement);
        }
    }

What have you tried? Do you know Java at all and how to use streams, or are you just hoping we'll refactor the code for you? Did you try searching for how to group data in Java using streams? — Andreas, Jan 14 '20 at 02:51
You are correct. It is a *comment* to the question, which is why it is located in the *comment* section, not the *answer* section. — Andreas, Jan 14 '20 at 14:14

score 2 · Answer 1 · answered Jan 14 '20 at 07:58

Thanks @TemaTre. Your input helped. The working code looks like



    Map map = 
                lst.stream().collect(
                    Collectors.groupingBy(
                        Patient::getName,
                        Collectors.collectingAndThen(
                            Collectors.toList(), 
                            values -> values.stream()
                                .sorted(Comparator.comparing(Patient::getReportDate, Comparator.nullsFirst(Comparator.naturalOrder()))
                                        .thenComparing(Patient::getReadingNo, Comparator.nullsFirst(Comparator.naturalOrder())).reversed())
                                .collect(toList()).get(0)

                            )));

And the complete working code in Java is as below

import java.util.*;
    import java.util.stream.Collectors;
    import static java.util.stream.Collectors.*;
    public class Main{

         public static void main(String []args){
             List<Patient> lst = new ArrayList<>();
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-02");setReadingNo(1);setMeasurement(255);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-02");setReadingNo(1);setMeasurement(250);}});
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-02");setReadingNo(2);setMeasurement(266);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-02");setReadingNo(2);setMeasurement(270);}});
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-02");setReadingNo(3);setMeasurement(298);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-02");setReadingNo(3);setMeasurement(259);}});
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-02");setReadingNo(4);setMeasurement(280);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-02");setReadingNo(4);setMeasurement(285);}});
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-03");setReadingNo(1);setMeasurement(260);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-03");setReadingNo(1);setMeasurement(265);}});
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-03");setReadingNo(2);setMeasurement(280);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-03");setReadingNo(2);setMeasurement(260);}});
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-03");setReadingNo(3);setMeasurement(285);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-03");setReadingNo(3);setMeasurement(290);}});
             lst.add(new Patient(){{setName("X");setReportDate("2020-01-03");setReadingNo(4);setMeasurement(290);}});
             lst.add(new Patient(){{setName("Y");setReportDate("2020-01-03");setReadingNo(4);setMeasurement(280);}});

             for(Patient p: lst){
                 System.out.println(p.toString());
             }
             System.out.println("---------------------------");

             Map<String, Patient> map = 
                lst.stream().collect(
                    Collectors.groupingBy(
                        Patient::getName,
                        Collectors.collectingAndThen(
                            Collectors.toList(), 
                            values -> values.stream()
                                .sorted(Comparator.comparing(Patient::getReportDate, Comparator.nullsFirst(Comparator.naturalOrder()))
                                        .thenComparing(Patient::getReadingNo, Comparator.nullsFirst(Comparator.naturalOrder())).reversed())
                                .collect(toList()).get(0)

                            )));

            for (Map.Entry<String,Patient> entry : map.entrySet()) {
                System.out.println(entry.getValue().toString());
            } 


         }
    }

     class Patient{
        private String Name;
        public String getName(){
            return Name;
        }
        public void setName(String name){
            this.Name=name;
        }

        private String ReportDate;
        public String getReportDate(){
            return ReportDate;
        }
        public void setReportDate(String reportDate){
            this.ReportDate=reportDate;
        }

        private int ReadingNo;
        public int getReadingNo(){
            return ReadingNo;
        }
        public void setReadingNo(int readingNo){
            this.ReadingNo=readingNo;
        }

        private int Measurement;
        public int getMeasurement(){
            return Measurement;
        }
        public void setMeasurement(int measurement){
            this.Measurement=measurement;
        }

        public String toString(){
            return String.format("%s\t%s\t%d\t%d", Name, ReportDate, ReadingNo, Measurement);
        }
    }

Creating 16 classes, just to initialize 16 instances, is a very bad idea. Do not use the double-brace hack. [Efficiency of Java “Double Brace Initialization”?](https://stackoverflow.com/a/924326/5221149) --- Recommend adding a 4-arg constructor instead. If you don't want that, then add a static helper method to the `Main` class, so you can simply do `List lst = Arrays.asList(newPatient("X", "2020-01-02", 1, 255), newPatient("Y", "2020-01-02", 1, 250), ...);` — Andreas, Jan 14 '20 at 14:17

score 1 · Accepted Answer · answered Jan 14 '20 at 03:06

1

In java you should use streams api.

If you need group by you can use this example:

Map<String, Patient> map = 
getValute().stream().collect(
    Collectors.groupingBy(
        Patient::getName,
        Collectors.collectingAndThen(
            Collectors.toList(), 
            values -> values.get(0))));

As example if you want to use FirstOrDefault (only for education):

Animal cheetah = animals.stream()
    .filter((animal) -> animal.getNumber() == cheetahNumber)
    .findFirst()
    .orElse(Animal.DEFAULT);

answered Jan 14 '20 at 03:06

TemaTre

1,422
2
12
20

How to sort by the ReadingDate and ReadingNo before collecting the first item? – MohantaD Jan 14 '20 at 03:30
You can sort it firstly. And then after grouping, groups will sorts too – TemaTre Jan 14 '20 at 03:41

score 0 · Answer 3 · answered Jan 14 '20 at 22:49

import static java.util.Comparator.comparing;
import static java.util.Comparator.reverseOrder;
import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.groupingBy;



    Map<String, List<Patient>> map = lst.stream()
        .collect(groupingBy(Patient::getName, toList()));

    for (List<Patient> group : map.values()) {

        Patient first = group.stream()
            .sorted(comparing(Patient::getReportDate).thenComparing(Patient::getReadingNo, reverseOrder()))
            .findFirst()
            .orElseThrow();

        System.out.println(first);
    }

score 0 · Answer 4 · answered May 29 '22 at 06:52

Use Collectors.reducing to get one element each group

var dummyPatient = new Patient();
dummyPatient.setReportDate("2000-01-01");
dummyPatient.setReadingNo(-999);

Map<String, Patient> map = lst.stream().collect(
  Collectors.groupingBy(
      Patient::getName,
      Collectors.reducing(
          dummyPatient, 
          BinaryOperator.maxBy(
            Comparator.comparing(Patient::getReportDate).thenComparing(Patient::getReadingNo)
          )
      )
  )
);

Retrieve the first items of each group in Java

4 Answers4