如何按最新日期从 Arraylist 中删除重复项

Question

我需要根据最新日期删除重复项。

class Stat {

    private String country;
    private String state;
    private String fullname;
    Date indepdate;

    public Stat(String country,String state,String fullname,int indepdate){
        this.country=country;
        this.state=state;
        this.fullname=fullname;
        this.indepdate=indepdate;
    }

   //also getters and setters

    public String toString() {
         return "("+country+","+state+","+fullname+","+indepdate+")";
    } 
}

---

ArrayList<Stat> stats  =new  ArrayList();

Stats.add(new Stat("USA", "Florida", "John Jones", 5/1/2020));
Stats.add(new Stat("USA", "Florida", "Jeff Cane", 4/1/2016));
Stats.add(new Stat("USA", "California", "Lisa Smith", 3/1/2000));
Stats.add(new Stat("Germany", "Florida", "Tom Joseph", 5/1/2019));
Stats.add(new Stat("Germany", "Florida", "Chris Richard", 5/1/2018));
Stats.add(new Stat("Germany", "California", "Nancy Diaz", 4/3/2015));

我需要删除重复的国家，只保留最新日期的国家。

列表应如下所示：

USA, florida, John James, 5/1/2020
Germany,Florida,Tom Joseph, 5/1/2019

到目前为止，我有以下内容：

Map<String, Map<String, List<Stat>> 
           groupByCountryAndCity = stats.
             stream().
               collect(
                    Collectors.
                        groupingBy(
                            Stat::getCountry,
                            Collectors.
                                groupingBy(
                                     Stat::getIndepdate     
                                          )
                                   )
                     );

Answer 1

您可以将LocalDate 用于indepdate 字段。

import java.time.LocalDate;

public class Stat {

    private String country;
    private String state;
    private String fullname;
    private LocalDate indepdate;

    // ...
}

然后使用LocalDate.of()方法：LocalDate.of(2020, 5, 1)

试试这个：

List<Stat> output = stats.stream()
        .collect(Collectors.toMap(Stat::getCountry, Function.identity(),
                BinaryOperator.maxBy(Comparator.comparing(Stat::getIndepdate)),
                () -> new TreeMap<>(Comparator.comparing(String::toString).reversed())))
        .values().stream().collect(Collectors.toList());

输出：

[(USA,Florida,John Jones,2020-05-01), (Germany,Florida,Tom Joseph,2019-05-01)]

Answer 2

您可以使用以下Collectors 方法来做到这一点：

• groupingBy(Function<> classifier, Collector<> downstream)
• collectingAndThen(Collector<> downstream, Function<> finisher)
• maxBy(Comparator<> comparator)

List<Stat> stats = Arrays.asList(
        new Stat("USA", "Florida", "John Jones", "5/1/2020"),
        new Stat("USA", "Florida", "Jeff Cane", "4/1/2016"),
        new Stat("USA", "California", "Lisa Smith", "3/1/2000"),
        new Stat("Germany", "Florida", "Tom Joseph", "5/1/2019"),
        new Stat("Germany", "Florida", "Chris Richard", "5/1/2018"),
        new Stat("Germany", "California", "Nancy Diaz", "4/3/2015") );

Map<String, Stat> result = stats.stream()
        .collect(Collectors.groupingBy(Stat::getCountry,
                Collectors.collectingAndThen(
                        Collectors.maxBy(Comparator.comparing(Stat::getIndepdate)),
                        Optional::get)));

result.values().forEach(System.out::println);

class Stat {
    private final String country;
    private final String state;
    private final String fullname;
    private final LocalDate indepdate;

    public Stat(String country, String state, String fullname, String indepdate){
        this.country = country;
        this.state = state;
        this.fullname = fullname;
        this.indepdate = LocalDate.parse(indepdate, DateTimeFormatter.ofPattern("M/d/u"));
    }

    public String getCountry() {
        return this.country;
    }
    public String getState() {
        return this.state;
    }
    public String getFullname() {
        return this.fullname;
    }
    public LocalDate getIndepdate() {
        return this.indepdate;
    }

    @Override
    public String toString() {
        return "("+country+","+state+","+fullname+","+indepdate+")";
    }
}

输出

(USA,Florida,John Jones,2020-05-01)
(Germany,Florida,Tom Joseph,2019-05-01)

Answer 3

yu 可能会使用 Collectors 提供的减少流。某事。像这样：

stats.stream().collect(
        Collectors.groupingBy(Stat::getCountry,reducing(Comparator.comparing(Stat::getIndepDate))));