你可以用 LocalDate 为了 indepdate 现场。

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)]

您可以使用以下方法 Collectors 方法： groupingBy(Function<> classifier, Collector<> downstream) collectingAndThen(Collector<> downstream, Function<> finisher) maxBy(Comparator<> comparator) ```
List 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)

yu可以使用收集器提供的流的减少。像这样的东西：

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