Sort objects in ArrayList by date

Question

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                    What should I do inside the if block

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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 lt MyObject gt       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

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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  - gt  d1 compareTo d2       or an object which has an DateTime attribute list sort  o1 o2  - gt  o1 getDateTime   compareTo o2 getDateTime         or like mentioned by Tunaki list sort Comparator comparing o - gt  o getDateTime        Reverse sorting  Java 8 comes also with some handy methods for reverse sorting     requested by lily list sort Comparator comparing o - gt  o getDateTime    reversed

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Pass the ArrayList In argument       private static void order ArrayList lt Object gt  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

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Future viewers  I think this is the simplest solution  if your model contains a string type date   2020-01-01 10 00 00  for example   then just write the following line to sort the data by date descending from newest to the oldest   Collections sort messages   o1  o2  - gt  o2 getMessageDate   compareTo o1 getMessageDate

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You can make your object comparable   public static class MyObject implements Comparable lt MyObject gt       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 lt MyObject gt        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 lt MyObject gt       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 lt MyObject gt        public int compare MyObject o1  MyObject o2          if  o1 getDateTime      null    o2 getDateTime      null          return 0        return o1 getDateTime   compareTo o2 getDateTime

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Use the below approach to identify dates are sort or not  SimpleDateFormat simpleDateFormat   new SimpleDateFormat  dd-MM-yyyy     boolean  decendingOrder   true      for int index 0 index lt date size   - 1  index              if simpleDateFormat parse date get index   getTime    lt  simpleDateFormat parse date get index 1   getTime                  decendingOrder   false              break                      if decendingOrder            System out println  Date are in Decending Order         else           System out println  Date not in Decending Order

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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 lt MyObject gt          public int compare MyObject o1  MyObject o2            return o1 getDateTime   lt o2 getDateTime      -1   1

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The Date class already implements Comparator interface  Assuming you have the  class below   public class A        private Date dateTime       public Date getDateTime             return dateTime                  other variables      And let s say you have a list of A objects as List lt A gt  aList  you can easily sort it with Java 8 s stream API  snippet below    import java util Comparator  import java util stream Collectors        aList   aList stream            sorted Comparator comparing A  getDateTime            collect Collectors toList

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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 lt String  String gt  by one object in the list  that being the timestamp     I have these objects   ArrayList lt Map lt String  String gt  gt  alList   new ArrayList lt Map lt String  String gt  gt       The map objects are as follows   Map lt String  Object gt  map   new HashMap lt  gt                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 lt String  String gt   firstObjToCompare  get  timestamp        String secondDateString     HashMap lt String  String gt   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  lt 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

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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 lt Object   gt          public int compare Object   o1  Object   o2            return   Date o1 0   compareTo   Date o2 0

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With introduction of Java 1 8  streams are very useful in solving this kind of problems   Comparator  lt DateTime gt  myComparator    arg1  arg2                   - gt                        if arg1 lt arg2                           return -1                      else if  arg1 lteq arg2                          return 0                      else                        return 1                         ArrayList lt DateTime gt  sortedList   myList                     stream                       sorted myComparator                      collect Collectors toCollection ArrayList  new

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This is how I solved   Collections sort MyList   o1  o2  - gt  o1 getLastModified   compareTo o2 getLastModified        Hope it help you

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list sort Comparator comparing o - gt  o getDateTime        The best answer IMHO from Tunaki using Java 8 lambda

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Here s the answer of how I achieve it  Mylist sort Comparator comparing myClass  getStarttime

[java] Sort objects in ArrayList by date?

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