How to sort a Collection T

Question

I have a generic Collection and am trying to work out how I can sort the items contained within it  I ve tried a few things but I can t get any of them working

User · Answer

You have two basic options provided by java util Collections     lt T extends Comparable lt   super T gt  gt  void sort List lt T gt  list    Use this if T implements Comparable and you re fine with that natural ordering   lt T gt  void sort List lt T gt  list  Comparator lt   super T gt  c    Use this if you want to provide your own Comparator     Depending on what the Collection is  you can also look at SortedSet or SortedMap

User · Answer

Collections by themselves do not have a predefined order  therefore you must convert them to a java util List   Then you can use one form of java util Collections sort  Collection lt  T  gt  collection         List lt  T  gt  list   new ArrayList lt  T  gt   collection     Collections sort  list        or Collections sort  list  new Comparator lt  T  gt                 list now is sorted

User · Answer

You can t if T is all you get  It must be injected by the provider   Collection lt T extends Comparable gt    or pass in the Comparator  Collections sort

User · Answer

Here is an example   I am using CompareToBuilder class from Apache for convenience  although this can be done without using it    import java util ArrayList  import java util Calendar  import java util Collections  import java util Comparator  import java util Date  import java util HashMap  import java util List  import org apache commons lang builder CompareToBuilder   public class Tester       boolean ascending   true       public static void main String args              Tester tester   new Tester            tester printValues               public void printValues             List lt HashMap lt String  Object gt  gt  list               new ArrayList lt HashMap lt String  Object gt  gt             HashMap lt String  Object gt  map               new HashMap lt String  Object gt              map put   actionId   new Integer 1234             map put   eventId    new Integer 21               map put   fromDate   getDate 1                    map put   toDate     getDate 7                    list add map            map   new HashMap lt String  Object gt             map put   actionId   new Integer 456             map put   eventId    new Integer 11              map put   fromDate   getDate 1                   map put   toDate     getDate 1                   list add map            map   new HashMap lt String  Object gt             map put   actionId   new Integer 1234             map put   eventId    new Integer 20               map put   fromDate   getDate 4                    map put   toDate     getDate 16                   list add map            map   new HashMap lt String  Object gt             map put   actionId   new Integer 1234             map put   eventId    new Integer 22               map put   fromDate   getDate 8                    map put   toDate     getDate 11                   list add map             map   new HashMap lt String  Object gt             map put   actionId   new Integer 1234             map put   eventId    new Integer 11               map put   fromDate   getDate 1                    map put   toDate     getDate 10                   list add map            map   new HashMap lt String  Object gt             map put   actionId   new Integer 1234             map put   eventId    new Integer 11               map put   fromDate   getDate 4                    map put   toDate     getDate 15                   list add map            map   new HashMap lt String  Object gt             map put   actionId   new Integer 567             map put   eventId    new Integer 12              map put   fromDate   getDate -1                  map put   toDate     getDate 1                   list add map            System out println   n Before Sorting  n             for  int j   0  j  lt  list size    j                 System out println list get j             Collections sort  list  new HashMapComparator2               System out println   n After Sorting  n             for  int j   0  j  lt  list size    j                 System out println list get j               public static Date getDate int days            Calendar cal   Calendar getInstance            cal setTime new Date             cal add Calendar DATE  days           return cal getTime               public class HashMapComparator2 implements Comparator           public int compare Object object1  Object object2                if  ascending                     return new CompareToBuilder                        append                            HashMap object1  get  actionId                              HashMap object2  get  actionId                                              append                            HashMap object2  get  eventId                              HashMap object1  get  eventId                                          toComparison                  else                   return new CompareToBuilder                        append                            HashMap object2  get  actionId                              HashMap object1  get  actionId                                              append                            HashMap object2  get  eventId                              HashMap object1  get  eventId                                          toComparison                                      If you have a specific code that you are working on and are having issues  you can post your pseudo code and we can try to help you out

User · Answer

A Collection does not have an ordering  so wanting to sort it does not make sense  You can sort List instances and arrays  and the methods to do that are Collections sort   and Arrays sort

User · Answer

Assuming you have a list of object of type Person  using Lambda expression  you can sort the last names of users for instance by doing the following    import java util Arrays  import java util Collections  import java util Comparator  import java util List   class Person           private String firstName          private String lastName           public Person String firstName  String lastName               this firstName   firstName              this lastName   lastName                     public String getLastName                return this lastName                     public String getFirstName                return this firstName                      Override         public String toString                return  Person     this getFirstName           this getLastName                         class TestSort           public static void main String   args               List lt Person gt  people   Arrays asList                                    new Person  John    Max                                       new Person  Coolio    Doe                                       new Person  Judith    Dan                                 Making use of lambda expression to sort the collection             people sort  p1  p2 - gt p1 getLastName   compareTo p2 getLastName                     Print sorted              printPeople people                      public static void printPeople List lt Person gt  people               for Person p   people                   System out println p

User · Answer

If your collections object is a list  I would use the sort method  as proposed in the other answers   However  if it is not a list  and you don t really care about what type of Collection object is returned  I think it is faster to create a TreeSet instead of a List   TreeSet sortedSet   new TreeSet myComparator   sortedSet addAll myCollectionToBeSorted

User · Answer

I came across a similar problem  Had to sort a list of 3rd party class  objects    List lt ThirdPartyClass gt  tpc   getTpcList         ThirdPartyClass does not implement the Java Comparable interface  I found an excellent illustration from mkyong on how to approach this problem  I had to use the Comparator approach to sorting     Sort ThirdPartyClass based on the value of some attribute function Collections sort tpc  Compare3rdPartyObjects tpcComp     where the Comparator is   public abstract class Compare3rdPartyObjects    public static Comparator lt ThirdPartyClass gt  tpcComp   new Comparator lt ThirdPartyClass gt           public int compare ThirdPartyClass tpc1  ThirdPartyClass tpc2             Integer tpc1Offset   compareUsing tpc1           Integer tpc2Offset   compareUsing tpc2              ascending order         return tpc1Offset compareTo tpc2Offset                Fetch the attribute value that you would like to use to compare the ThirdPartyClass instances  public static Integer compareUsing ThirdPartyClass tpc         Integer value   tpc getValueUsingSomeFunction        return value

[java] How to sort a Collection<T>?

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