Sort objects in ArrayList by date?

问题:

Can someone help me with this? Every example I find is about doing this alphabetically, while I need my elements sorted by date.

My ArrayList contains objects on which one of the datamembers is a DateTime object. On DateTime I can call the functions:

lt() // less-than
lteq() // less-than-or-equal-to

So to compare I could do something like:

if(myList.get(i).lt(myList.get(j))){
    // ...
}

I don\'t really know what to do inside the if block. Any ideas?

回答1:

You can make your object comparable:

public static class MyObject implements Comparable<MyObject> {

  private Date dateTime;

  public Date getDateTime() {
    return dateTime;
  }

  public void setDateTime(Date datetime) {
    this.dateTime = datetime;
  }

  @Override
  public int compareTo(MyObject o) {
    return getDateTime().compareTo(o.getDateTime());
  }
}

And then you sort it by calling:

Collections.sort(myList);

However sometimes you don\'t want to change your model, like when you want to sort on several different properties. In that case, you can create comparator on the fly:

Collections.sort(myList, new Comparator<MyObject>() {
  public int compare(MyObject o1, MyObject o2) {
      return o1.getDateTime().compareTo(o2.getDateTime());
  }
});

However, the above works only if you\'re certain that dateTime is not null at the time of comparison. It\'s wise to handle null as well to avoid NullPointerExceptions:

public static class MyObject implements Comparable<MyObject> {

  private Date dateTime;

  public Date getDateTime() {
    return dateTime;
  }

  public void setDateTime(Date datetime) {
    this.dateTime = datetime;
  }

  @Override
  public int compareTo(MyObject o) {
    if (getDateTime() == null || o.getDateTime() == null)
      return 0;
    return getDateTime().compareTo(o.getDateTime());
  }
}

Or in the second example:

Collections.sort(myList, new Comparator<MyObject>() {
  public int compare(MyObject o1, MyObject o2) {
      if (o1.getDateTime() == null || o2.getDateTime() == null)
        return 0;
      return o1.getDateTime().compareTo(o2.getDateTime());
  }
});

回答2:

Since Java 8 the List interface provides the sort method. Combined with lambda expression the easiest solution would be

// sort DateTime typed list
list.sort((d1,d2) -> d1.compareTo(d2));
// or an object which has an DateTime attribute
list.sort((o1,o2) -> o1.getDateTime().compareTo(o2.getDateTime()));
// or like mentioned by Tunaki
list.sort(Comparator.comparing(o -> o.getDateTime()))

回答3:

You can use Collections.sort method. It\'s a static method. You pass it the list and a comparator. It uses a modified mergesort algorithm over the list. That\'s why you must pass it a comparator to do the pair comparisons.

Collections.sort(myList, new Comparator<MyObject> {
   public int compare(MyObject o1, MyObject o2) {
      DateTime a = o1.getDateTime();
      DateTime b = o2.getDateTime();
      if (a.lt(b)) 
        return -1;
      else if (a.lteq(b)) // it\'s equals
         return 0;
      else
         return 1;
   }
});

Note that if myList is of a comparable type (one that implements Comparable interface) (like Date, Integer or String) you can omit the comparator and the natural ordering will be used.

回答4:

Given MyObject that has a DateTime member with a getDateTime() method, you can sort an ArrayList that contains MyObject elements by the DateTime objects like this:

Collections.sort(myList, new Comparator<MyObject>() {
    public int compare(MyObject o1, MyObject o2) {
        return o1.getDateTime().lt(o2.getDateTime()) ? -1 : 1;
    }
});

回答5:

list.sort(Comparator.comparing(o -> o.getDateTime()));

The best answer IMHO from Tunaki using Java 8 lambda

回答6:

This is how I solved:

Collections.sort(MyList, (o1, o2) -> o1.getLastModified().compareTo(o2.getLastModified()));

Hope it help you.

回答7:

All the answers here I found to be un-neccesarily complex for a simple problem (at least to an experienced java developer, which I am not). I had a similar problem and chanced upon this (and other) solutions, and though they provided a pointer, for a beginner I found as stated above. My solution, depends on where in the the Object your Date is, in this case, the date is the first element of the Object[] where dataVector is the ArrayList containing your Objects.

Collections.sort(dataVector, new Comparator<Object[]>() {
    public int compare(Object[] o1, Object[] o2) {
        return ((Date)o1[0]).compareTo(((Date)o2[0]));
    }
});

回答8:

With introduction of Java 1.8, streams are very useful in solving this kind of problems:

Comparator <DateTime> myComparator = (arg1, arg2) 
                -> {
                    if(arg1.lt(arg2)) 
                       return -1;
                    else if (arg1.lteq(arg2))
                       return 0;
                    else
                       return 1;
                   };

ArrayList<DateTime> sortedList = myList
                   .stream()
                   .sorted(myComparator)
                   .collect(Collectors.toCollection(ArrayList::new));

回答9:

This may be an old response but I used some examples from this post to create a comparator that would sort an ArrayList of HashMap<String, String> by one object in the list, that being the timestamp.

I have these objects:

ArrayList<Map<String, String>> alList = new ArrayList<Map<String, String>>();

The map objects are as follows:

Map<String, Object> map = new HashMap<>();
        // of course this is the actual formatted date below in the timestamp
        map.put(\"timestamp\", \"MM/dd/yyyy HH:mm:ss\"); 
        map.put(\"item1\", \"my text goes here\");
        map.put(\"item2\", \"my text goes here\");

That mapping is what I use to load all my objects into the array list, using the alList.add(map) function, within a loop.

Now, I created my own comparator:

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

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;

 public class DateSorter implements Comparator {
     public int compare(Object firstObjToCompare, Object secondObjToCompare) {
    String firstDateString = ((HashMap<String, String>) firstObjToCompare).get(\"timestamp\");
    String secondDateString = ((HashMap<String, String>) secondObjToCompare).get(\"timestamp\");

    if (secondDateString == null || firstDateString == null) {
        return 0;
    }

    // Convert to Dates
    DateTimeFormatter dtf = DateTimeFormat.forPattern(\"MM/dd/yyyy HH:mm:ss\");
    DateTime firstDate = dtf.parseDateTime(firstDateString);
    DateTime secondDate = dtf.parseDateTime(secondDateString);

    if (firstDate.isAfter(secondDate)) return -1;
    else if (firstDate.isBefore(secondDate)) return 1;
    else return 0;
    }
}

I can now just call the Comparator at any time on the array and it will sort my array, giving me the Latest timestamp in position 0 (top of the list) and the earliest timestamp at the end of the list. New posts get put to the top basically.

Collections.sort(alList, new DateSorter());

This may help someone out, which is why I posted it. Take into consideration the return statements within the compare() function. There are 3 types of results. Returning 0 if they are equal, returning >0 if the first date is before the second date and returning <0 if the first date is after the second date. If you want your list to be reversed, then just switch those two return statements! Simple =]

回答10:

Pass the ArrayList In argument.

    private static void order(ArrayList<Object> list) {

    Collections.sort(list, new Comparator() {

        public int compare(Object o2, Object o1) {

            String x1 =  o1.Date;
            String x2 =  o2.Date;

                return  x1.compareTo(x2);

        }
    });
}

回答11:

DateTime.compare?