Java - 如何按日期迭代单独对象的列表？

Question

I have two classes representing travel segments - a Segment class and a Camp class. A Camp is technically a type of Segment but I can't change the entity design at this point so assume they are unrelated. Both Segments and Camps contain checkIn and checkOut Date fields.

What I need to do is loop through a List of Segments and a List of Camps and call a certain function for each of the segments and camps in those lists. However, the function must be called in sequential date order for each of the segments and camps.

Obviously I can sort the Lists of Segments and Camps separately but the problem arises if there's a Camp for which the function should be called in between Segments.

So for example if I have the following:

Segment 1 - 10/16/14
Segment 2 - 10/24/14

Camp 1 - 10/17/14
Camp 2 - 10/18/14

I'd need to perform an operation on the segments/camps in the following order:
Segment 1
Camp 1
Camp 2
Segment 2

As I see it I have a few options to handle this but I'm wondering what is the most efficient / best way to do it. I could build some type of helper class that contains the fields of both Segments and Camps and create a list of objects of this helper class that contains either Segment or Camp detail.

This is what I've got currently. I've decided to go with a Map approach of mapping Dates to Camps, so I iterate through the keys of the map (Dates) and see if any of those dates are before the segment date, and if so, perform the operation on them. Is there a better way to do this? It seems ugly and confusing to me.

final Map<Date, List<Camp>> campMap = new HashMap();

for (Camp camp : pnr.getCamps()) {
    if (campMap.get(camp.getCheckInDate()) == null) {
        final List<Camp> camps = new ArrayList<Camp>();
        camps.add(camp);
        campMap.put(camp.getCheckInDate(), camps);
    } else {
        campMap.get(camp.getCheckInDate()).add(camp);
    }
}

final List<Date> datesToDelete = new ArrayList<Date>();

for (Segment segment : pnr.getSegments()) {
    // here is where I loop through the segments to perform an operation on each of them 

    for (Map.Entry<Date, List<Camp>> entry : campMap.entrySet()) {
        if (entry.getKey().before(segment.getSegmentDate())
                || entry.getKey().equals(segment.getSegmentDate())) {
            for (Camp camp : entry.getValue()) {
                // do something with the camp 
            }
            datesToDelete.add(entry.getKey());
        }
    }

    for (Date date : datesToDelete) {
        campMap.remove(date);
    }
    datesToDelete.clear();

    // do something with the segment

}

Basically I'm asking if there's a better/more intuitive/cleaner way to do this. Thanks so much for any help.

Answer 1

There's no "correct" way to tackle this; but for me I'd have your classes implement a common interface, something like:

public interface DateAware {

    long getDate();

}

After you've done that you can begin to work with them polymorphically. You could for example use a simple comparator to sort them with Collections.sort :

public class DateAwareComparator implements Comparator<DateAware> {

    int compare(DateAware o1, DateAware o2) {
        return Long.compare(o1.getDate(), o2.getDate());
    }

}

And you can now do things like:

List<DateAware> list = new ArrayList<>();
list.addAll(segments);
list.addAll(camps);

Using an interface also has the benefit that you're not placing any constraints on the inheritance hierarchy to achieve what your after.

Answer 2

有一个自定义Comparator （ http://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html ）可以比较两个段，两个阵营或两者的混合比较日期然后Collections.sort(segmentAndCampList, new SegmentAndCampComparator())