Removing Duplicate Values from ArrayList

Question

I have one Arraylist of String and I have added Some Duplicate Value in that  and i just wanna remove that Duplicate value So how to remove it    Here Example I got one Idea   List lt String gt  list   new ArrayList lt String gt             list add  Krishna            list add  Krishna            list add  Kishan            list add  Krishn            list add  Aryan            list add  Harm             System out println  List  list            for  int i   1  i  lt  list size    i                  String a1   list get i               String a2   list get i-1               if  a1 equals a2                     list remove a1                                    System out println  List after short  list     But is there any Sufficient way remove that Duplicate form list  with out using For loop    And ya i can do it by using HashSet or some other way but using array list only  would like to have your suggestion for that  thank you for your answer in advance

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public static void main String   args         SuppressWarnings  serial       List lt Object gt  lst   new ArrayList lt Object gt               Override         public boolean add Object e                if  contains e               return super add e               else             return false                       lst add  ABC        lst add  ABC        lst add  ABCD        lst add  ABCD        lst add  ABCE        System out println lst        This is the better way

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You can create a LinkedHashSet from the list  The LinkedHashSet will contain each element only once  and in the same order as the List  Then create a new List from this LinkedHashSet  So effectively  it s a one-liner   list   new ArrayList lt String gt  new LinkedHashSet lt String gt  list     Any approach that involves List contains or List remove will probably decrease the asymptotic running time from O n   as in the above example  to O n 2       EDIT For the requirement mentioned in the comment  If you want to remove duplicate elements  but consider the Strings as equal ignoring the case  then you could do something like this   Set lt String gt  toRetain   new TreeSet lt String gt  String CASE INSENSITIVE ORDER   toRetain addAll list   Set lt String gt  set   new LinkedHashSet lt String gt  list   set retainAll new LinkedHashSet lt String gt  toRetain    list   new ArrayList lt String gt  set     It will have a running time of O n logn   which is still better than many other options  Note that this looks a little bit more complicated than it might have to be  I assumed that the order of the elements in the list may not be changed  If the order of the elements in the list does not matter  you can simply do  Set lt String gt  set   new TreeSet lt String gt  String CASE INSENSITIVE ORDER   set addAll list   list   new ArrayList lt String gt  set

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In case you just need to remove the duplicates using only ArrayList  no other Collection classes  then -     list is the original arraylist containing the duplicates as well List lt String gt  uniqueList   new ArrayList lt String gt         for int i 0 i lt list size   i              if  uniqueList contains list get i                uniqueList add list get i            Hope this helps

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private static void removeDuplicates List lt Integer gt  list        Collections sort list       int count   list size        for  int i   0  i  lt  count  i                   if i 1 lt count  amp  amp  list get i   list get i 1                list remove i               i--              count--

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Using java 8   public static  lt T gt  List lt T gt  removeDuplicates List lt T gt  list        return list stream   collect Collectors toSet    stream   collect Collectors toList

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You can make use of Google Guava utilities  as shown below   list   ImmutableSet copyOf list  asList       This is probably the most efficient way of eliminating the duplicates from the list and interestingly  it preserves the iteration order as well   UPDATE  But  in case  you don t want to involve Guava then duplicates can be removed as shown below   ArrayList lt String gt  list   new ArrayList lt String gt         list add  Krishna        list add  Krishna        list add  Kishan        list add  Krishn        list add  Aryan        list add  Harm     System out println  List  list   HashSet hs   new HashSet    hs addAll list   list clear    list addAll hs     But  of course  this will destroys the iteration order of the elements in the ArrayList   Shishir

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public static List lt String gt  removeDuplicateElements List lt String gt  array       List lt String gt  temp   new ArrayList lt String gt         List lt Integer gt  count   new ArrayList lt Integer gt         for  int i 0  i lt array size  -2  i             for  int j i 1 j lt array size  -1 j                                  if  array get i  compareTo array get j    0                        count add i                       int kk   i                                                    for  int i   count size   1 i gt 0 i--                array remove i                     return array

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List lt String gt  list   new ArrayList lt String gt             list add  Krishna            list add  Krishna            list add  Kishan            list add  Krishn            list add  Aryan            list add  Harm     HashSet lt String gt  hs new HashSet lt  gt  list    System out println           With Duplicate Element            System out println list   System out println           Removed Duplicate Element            System out println hs

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I don t think the list   new ArrayList lt String gt  new LinkedHashSet lt String gt  list   is not the best way   since we are using the LinkedHashset We could use directly LinkedHashset instead of ArrayList     Solution   import java util ArrayList  public class Arrays extends ArrayList    Override public boolean add Object e        if  contains e            return super add e        else          return false           public static void main String   args        Arrays element new Arrays        element add 1       element add 2       element add 2       element add 3        System out println element         Output   1  2  3   Here I am extending the ArrayList   as I am using the it with some changes by overriding the add method

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This will be the best way      List lt String gt  list   new ArrayList lt String gt         list add  Krishna        list add  Krishna        list add  Kishan        list add  Krishn        list add  Aryan        list add  Harm         Set lt String gt  set new HashSet lt  gt  list

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Java 8 stream function    You could use the distinct function like above to get the distinct elements of the list    stringList stream   distinct      From the documentation        Returns a stream consisting of the distinct elements  according to Object equals Object   of this stream      Another way  if you do not wish to use the equals method is by using the collect function like this     stringList stream          collect Collectors toCollection    - gt           new TreeSet lt String gt   p1  p2  - gt  p1 compareTo p2            From the documentation        Performs a mutable reduction operation on the elements of this stream using a Collector      Hope that helps

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list   list stream   distinct   collect Collectors toList     This could be one of the solutions using Java8 Stream API  Hope this helps

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if you want to use only arraylist then I am worried there is no better way which will create a huge performance benefit  But by only using arraylist i would check before adding into the list like following  void addToList String s     if  yourList contains s          yourList add s       In this cases using a Set is suitable

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It is better to use HastSet  1-a  A HashSet holds a set of objects  but in a way that it allows you to easily and quickly determine whether an object is already in the set or not  It does so by internally managing an array and storing the object using an index which is calculated from the hashcode of the object  Take a look here  1-b  HashSet is an unordered collection containing unique elements  It has the standard collection operations Add  Remove  Contains  but since it uses a hash-based implementation  these operation are O 1    As opposed to List for example  which is O n  for Contains and Remove   HashSet also provides standard set operations such as union  intersection  and symmetric difference Take a look here  2  There are different implementations of Sets  Some make insertion and lookup operations super fast by hashing elements  However that means that the order in which the elements were added is lost  Other implementations preserve the added order at the cost of slower running times   The HashSet class in C  goes for the first approach  thus not preserving the order of elements  It is much faster than a regular List  Some basic benchmarks showed that HashSet is decently faster when dealing with primary types  int  double  bool  etc    It is a lot faster when working with class objects  So that point is that HashSet is fast   The only catch of HashSet is that there is no access by indices  To access elements you can either use an enumerator or use the built-in function to convert the HashSet into a List and iterate through that Take a look here

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public void removeDuplicates         ArrayList lt Object gt  al   new ArrayList lt Object gt         al add  quot java quot        al add  a        al add  b        al add  a        al add  quot java quot        al add 10 3       al add  c        al add 14       al add  quot java quot        al add 12        System out println  quot Before Remove Duplicate elements  quot    al       for  int i   0  i  lt  al size    i              for  int j   i   1  j  lt  al size    j                  if  al get i  equals al get j                      al remove j                   j--                                    System out println  quot After Removing duplicate elements  quot    al      Before Remove Duplicate elements   java  a  b  a  java  10 3  c  14  java  12   After Removing duplicate elements   java  a  b  10 3  c  14  12

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public List lt Contact gt  removeDuplicates List lt Contact gt  list           Set set1   new LinkedHashSet list       Set set   new TreeSet new Comparator              Override         public int compare Object o1  Object o2                     if   Contact o1  getId   equalsIgnoreCase   Contact 2  getId                        return 0                            return 1                        set addAll list       final List newList   new ArrayList set       return newList

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Without a loop  No  Since ArrayList is indexed by order rather than by key  you can not found the target element without iterate the whole list   A good practice of programming is to choose proper data structure to suit your scenario  So if Set suits your scenario the most  the discussion of implementing it with List and trying to find the fastest way of using an improper data structure makes no sense

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Simple function for removing duplicates from list  private void removeDuplicates List lt   gt  list        int count   list size         for  int i   0  i  lt  count  i                   for  int j   i   1  j  lt  count  j                           if  list get i  equals list get j                                  list remove j--                   count--                                    Example  Input    1  2  2  3  1  3  3  2  3  1  2  3  3  4  4  4  1  Output   1  2  3  4

[java] Removing Duplicate Values from ArrayList

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