在Java中，如何按字段分组后如何获得每个组的最新/第一项列表？ 例如，从给定的医疗记录列表中，如何检索每个患者的最新记录。
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
---------------------------------------------------------------------------
我可以按以下方式使用C＃进行操作。任何人都可以帮助翻译成Java吗？
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);
}
}
答案 0 :(得分：2)
感谢@TemaTre。您的输入有所帮助。工作代码看起来像
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)
)));
Java的完整工作代码如下
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);
}
}
答案 1 :(得分：1)
在Java中，您应该使用流api。
如果需要分组依据，可以使用以下示例：
Map<String, Patient> map =
getValute().stream().collect(
Collectors.groupingBy(
Patient::getName,
Collectors.collectingAndThen(
Collectors.toList(),
values -> values.get(0))));
例如，如果要使用FirstOrDefault（仅用于教育）：
Animal cheetah = animals.stream()
.filter((animal) -> animal.getNumber() == cheetahNumber)
.findFirst()
.orElse(Animal.DEFAULT);
答案 2 :(得分：0)
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);
}