java - BasicDBObject 的自定义排序未按预期排序

Question

在下面的代码中，我尝试按 BasicDBObject 列表的日期降序自定义排序。

打印的输出是:

{ "year" : "2015" , "month" : "12" , "day" : "1" , "time" : "07:30:59"}
{ "year" : "2015" , "month" : "12" , "day" : "1" , "time" : "07:30:20"}
{ "year" : "2015" , "month" : "12" , "day" : "1" , "time" : "07:30:00"}
{ "year" : "2015" , "month" : "01" , "day" : "23" , "time" : "07:30:59"}
{ "year" : "2015" , "month" : "09" , "day" : "26" , "time" : "07:30:59"}

什么时候应该是:

{ "year" : "2015" , "month" : "12" , "day" : "1" , "time" : "07:30:59"}
{ "year" : "2015" , "month" : "12" , "day" : "1" , "time" : "07:30:20"}
{ "year" : "2015" , "month" : "12" , "day" : "1" , "time" : "07:30:00"}
{ "year" : "2015" , "month" : "09" , "day" : "26" , "time" : "07:30:59"}
{ "year" : "2015" , "month" : "01" , "day" : "23" , "time" : "07:30:59"}

因为 9 月 (09) 发生在 1 月 (01) 之后。

排序比较方法有问题吗？

代码:

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormatter;

import com.mongodb.BasicDBObject;
import com.mongodb.DBObject;


public class SortByDate
{

    private void testSort()
    {

        List<DBObject> l = new ArrayList<DBObject>();

        DBObject dbo = new BasicDBObject();
        dbo.put("year", "2015");
        dbo.put("month", "12");
        dbo.put("day", "1");
        dbo.put("time", "07:30:20");
        l.add(dbo);

        dbo = new BasicDBObject();
        dbo.put("year", "2015");
        dbo.put("month", "09");
        dbo.put("day", "26");
        dbo.put("time", "07:30:59");
        l.add(dbo);

        dbo = new BasicDBObject();
        dbo.put("year", "2015");
        dbo.put("month", "12");
        dbo.put("day", "1");
        dbo.put("time", "07:30:59");
        l.add(dbo);

        dbo = new BasicDBObject();
        dbo.put("year", "2015");
        dbo.put("month", "01");
        dbo.put("day", "23");
        dbo.put("time", "07:30:59");
        l.add(dbo);

        dbo = new BasicDBObject();
        dbo.put("year", "2015");
        dbo.put("month", "12");
        dbo.put("day", "1");
        dbo.put("time", "07:30:00");

        l.add(dbo);

        DateTimeFormatter dtf = org.joda.time.format.DateTimeFormat.forPattern("MM/dd/yyyy HH:mm:ss");

        Collections.sort(l, new Comparator<DBObject>()
        {
            @Override
            public int compare(final DBObject o1, final DBObject o2)
            {
                long m1 = dtf.parseMillis((String) o1.get("month")+"/"+(String) o1.get("day")+"/"+(String) o1.get("year")+" "+(String) o1.get("time"));
                long m2 = dtf.parseMillis((String) o2.get("month")+"/"+(String) o2.get("day")+"/"+(String) o2.get("year")+" "+(String) o2.get("time"));

                return (int)m2 - (int)m1;
            }
        });

        for(DBObject d : l){ 
            System.out.println(d.toString()); 
        }

    }

    public static void main(String args[])
    {
        new SortByDate().testSort();
    }

}

Answer 1

问题出在这里:

return (int)m2 - (int)m1;

您将long millis转换为int，但值大于Integer.MAX_VALUE，因此发生溢出并且转换返回错误结果(它例如，可以为负数)。正确的转换可能是:

return (int)(m2 - m1);

但同样，如果日期相差很多，溢出仍然可能发生。正确且安全地执行您需要的操作的方法是使用 Long.compare :

return Long.compare(m2, m1);