兩個 LocalDateTimes 的 java 比較器

Question

名為MyCustomClass的自定義類有一個屬性，它是 joda LocalDateTime 。 我需要創建一個java.util.Comparator類，通過它們的TimeStamp屬性（ LocalDateTime類型）比較MyCustomClass的實例。 我已經閱讀了幾篇關於這個（包括這個）的帖子，我已經嘗試了所有的方法，但答案中顯示的方法似乎都不起作用。 例如，以下幾種方法會引發編譯錯誤：

import java.time.temporal.ChronoUnit;
import java.util.Comparator;

import org.joda.time.DateTime;
import org.joda.time.Duration;
import org.joda.time.Period;

import my.app.model.MyCustomClass;

public class MyCustomClassComparator implements Comparator<MyCustomClass>{
    public int compare(MyCustomClass mcc1, MyCustomClass mcc2) {
        //this first attempt throws a return type error for the method.
        return Period.fieldDifference(mcc2.getTimestamp(), mcc1.getTimestamp());
        //This next attempt says that LocalDateTimes are not valid arguments
        Duration.between(mcc2.getTimestamp(), mcc1.getTimestamp());
        //The next approach also says LocalDateTimes are not valid arguments.
        DateTime.parse(mcc2.getTimestamp()), mcc1.getTimestamp()).getSeconds();
        //This boilerplate approach says the minus sign is not valid for LocalDateTime
        return mcc1.getTimestamp() - mcc2.getTimestamp();
    }
}

我打算在代碼中的其他地方使用它，例如：

List<MyCustomClass> mccs = new ArrayList<MyCustomClass>();
// Sort by time stamp:
Collections.sort(mccs, new MyCustomClassComparator());

我如何編寫一個Comparator類來根據它們的 Joda LocalDateTime屬性比較MyCustomClass的實例？

Answer 1

如果您只需要一個比較器實例：

類似於Iłya Bursov的回答，因為 LocalDateTime 實現了可比性，您可以使用Comparator.comparing

Comparator.comparing(MyCustomClass::getTimestamp)

Answer 2

LocalDateTime實現了可比較的接口，因此您可以實現如下比較器：

public class MyCustomClassComparator implements Comparator<MyCustomClass> {
    @Override
    public int compare(final MyCustomClass mcc1, final MyCustomClass mcc2) {
        return mcc1.getTimestamp().compareTo(mcc2.getTimestamp());
    }
}

Answer 3

鑒於 Comparator 是一個函數式接口，我們可以使用單個 lambda 表達式來實現它。

Comparator<LocalDateTime> localDateTimeComparator = (o1, o2) -> o1.compareTo(o2);

//Then use the comparator as always....
LocalDateTime minDate = dates.stream()
                             .min(localDateTimeComparator)
                             .orElseThrow(Assertion::Error)
                   

對於重用，靜態實例可能會有所幫助

//interfaces are "more static" than classes with "static members" ;-)
public interface MyDateUtils { 

   Comparator<LocalDateTime> localDateTimeComparator = (o1, o2) -> o1.compareTo(o2);

}