How do I remove repeated elements from ArrayList

Question

I have an ArrayList lt String gt   and I want to remove repeated strings from it  How can I do this

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In Java 8   List lt String gt  deduped   list stream   distinct   collect Collectors toList       Please note that the hashCode-equals contract for list members should be respected for the filtering to work properly

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public Set lt Object gt  findDuplicates List lt Object gt  list            Set lt Object gt  items   new HashSet lt Object gt             Set lt Object gt  duplicates   new HashSet lt Object gt             for  Object item   list                if  items contains item                     duplicates add item                     else                        items add item                                                 return duplicates

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As said before  you should use a class implementing the Set interface instead of List to be sure of the unicity of elements  If you have to keep the order of elements  the SortedSet interface can then be used  the TreeSet class implements that interface

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This is the right one  if you are concerned about the overhead of HashSet    public static ArrayList lt String gt  removeDuplicates  ArrayList lt String gt  arrayList       if  arrayList isEmpty    return null     return what makes sense for your app     Collections sort arrayList  String CASE INSENSITIVE ORDER         remove duplicates     ArrayList  lt String gt  arrayList mod   new ArrayList lt  gt         arrayList mod add arrayList get 0        for  int i 1  i lt arrayList size    i             if   arrayList get i  equals arrayList get i-1    arrayList mod add arrayList get i              return arrayList mod

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you can use nested loop in follow    ArrayList lt Class1 gt  l1   new ArrayList lt Class1 gt     ArrayList lt Class1 gt  l2   new ArrayList lt Class1 gt              Iterator iterator1   l1 iterator            boolean repeated   false           while  iterator1 hasNext                          Class1 c1    Class1  iterator1 next                for  Class1  c  l2                    if  c getId      c1 getId                        repeated   true                            if  repeated                  l2 add c1

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Time Complexity   O n    Without Set  private static void removeDup ArrayList lt String gt  listWithDuplicateElements        System out println   Original Duplicate List      listWithDuplicateElements       List lt String gt  listWithoutDuplicateElements   new ArrayList lt  gt  listWithDuplicateElements size          listWithDuplicateElements stream   forEach str - gt            if  listWithoutDuplicateElements indexOf str     -1                listWithoutDuplicateElements add str                               System out println   Without Duplicate List      listWithoutDuplicateElements

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If you want your list to automatically ignore duplicates and preserve its order  you could create a HashList a HashMap embedded List    public static class HashList lt T gt  extends ArrayList lt T gt           private HashMap  lt T T gt  hashMap          public HashList                hashMap new HashMap lt  gt                         Override         public boolean add T t               if hashMap get t   null                   hashMap put t t                   return super add t                else return false                      Override         public boolean addAll Collection lt   extends T gt  c               HashList lt T gt  addup  HashList lt T gt  c              for int i 0 i lt addup size   i                     add addup get i                 return true                     Usage Example   HashList lt String gt  hashlist new HashList lt  gt     hashList add  hello    hashList add  hello    System out println   HashList    hashlist

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This three lines of code can remove the duplicated element from ArrayList or any collection   List lt Entity gt  entities   repository findByUserId userId    Set lt Entity gt  s   new LinkedHashSet lt Entity gt  entities   entities clear    entities addAll s

User · Answer

In Java 8   List lt String gt  deduped   list stream   distinct   collect Collectors toList       Please note that the hashCode-equals contract for list members should be respected for the filtering to work properly

User · Answer

Here is my answer without using any other data structure like set or hashmap etc   public static  lt T gt  ArrayList lt T gt  uniquefy ArrayList lt T gt  myList         ArrayList  lt T gt  uniqueArrayList   new ArrayList lt T gt         for  int i   0  i  lt  myList size    i             if   uniqueArrayList contains myList get i                 uniqueArrayList add myList get i                         return uniqueArrayList

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There is also ImmutableSet from Guava as an option  here is the documentation    ImmutableSet copyOf list

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Probably a bit overkill  but I enjoy this kind of isolated problem      This code uses a temporary Set  for the uniqueness check  but removes elements directly inside the original list  Since element removal inside an ArrayList can induce a huge amount of array copying  the remove int -method is avoided   public static  lt T gt  void removeDuplicates ArrayList lt T gt  list        int size   list size        int out   0                final Set lt T gt  encountered   new HashSet lt T gt             for  int in   0  in  lt  size  in                  final T t   list get in               final boolean first   encountered add t               if  first                    list set out    t                                     while  out  lt  size            list remove --size             While we re at it  here s a version for LinkedList  a lot nicer     public static  lt T gt  void removeDuplicates LinkedList lt T gt  list        final Set lt T gt  encountered   new HashSet lt T gt         for  Iterator lt T gt  iter   list iterator    iter hasNext                final T t   iter next            final boolean first   encountered add t           if   first                iter remove                        Use the marker interface to present a unified solution for List   public static  lt T gt  void removeDuplicates List lt T gt  list        if  list instanceof RandomAccess               use first version here       else              use other version here           EDIT  I guess the generics-stuff doesn t really add any value here   Oh well

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for int a 0 a lt myArray size   a             for int b a 1 b lt myArray size   b                 if myArray get a  equalsIgnoreCase myArray get b                     myArray remove b                    dups                    b--

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Set lt String gt  strSet   strList stream   collect Collectors toSet       Is the easiest way to remove your duplicates

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This is used for your Custom Objects list      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  o2  getId       amp  amp                        Contact o1  getName   equalsIgnoreCase   Contact o2  getName                         return 0                            return 1                        set addAll list        final List newList   new ArrayList set       return newList

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It is possible to remove duplicates from arraylist without using HashSet or one more arraylist    Try this code        ArrayList lt String gt  lst   new ArrayList lt String gt         lst add  ABC        lst add  ABC        lst add  ABCD        lst add  ABCD        lst add  ABCE         System out println  Duplicates List   lst        Object   st   lst toArray          for  Object s   st            if  lst indexOf s     lst lastIndexOf s                 lst remove lst lastIndexOf s                            System out println  Distinct List   lst     Output is  Duplicates List  ABC  ABC  ABCD  ABCD  ABCE  Distinct List  ABC  ABCD  ABCE

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Code   List lt String gt  duplicatList   new ArrayList lt String gt     duplicatList   Arrays asList  AA   BB   CC   DD   DD   EE   AA   FF      above AA and DD are duplicate Set lt String gt  uniqueList   new HashSet lt String gt  duplicatList   duplicatList   new ArrayList lt String gt  uniqueList     let GC will doing free memory System out println  Removed Duplicate     duplicatList     Note  Definitely  there will be memory overhead

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ArrayList lt String gt  city new ArrayList lt String gt     city add  rajkot    city add  gondal    city add  rajkot    city add  gova    city add  baroda    city add  morbi    city add  gova     HashSet lt String gt  hashSet   new HashSet lt String gt     hashSet addAll city   city clear    city addAll hashSet   Toast makeText getActivity        city toString   Toast LENGTH SHORT  show

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If you want to preserve your Order then it is best to use LinkedHashSet  Because if you want to pass this List to an Insert Query by Iterating it  the order would be preserved   Try this  LinkedHashSet link new LinkedHashSet    List listOfValues new ArrayList    listOfValues add link     This conversion will be very helpful when you want to return a List but not a Set

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Would something like this work better   public static void removeDuplicates ArrayList lt String gt  list        Arraylist lt Object gt  ar       new Arraylist lt Object gt         Arraylist lt Object gt  tempAR   new Arraylist lt Object gt         while  list size   gt 0           ar add list 0            list removeall Collections singleton list 0               list addAll ar      That should maintain the order and also not be quadratic in run time

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LinkedHashSet will do the trick   String   arr2     5   1   2   3   3   4   1   2    Set lt String gt  set   new LinkedHashSet lt String gt  Arrays asList arr2    for String s1   set      System out println s1    System out println   ------------------------     String   arr3   set toArray new String 0    for int i   0  i  lt  arr3 length  i         System out println arr3 i  toString         output  5 1 2 3 4

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ArrayList lt String gt  list   new ArrayList lt String gt         HashSet lt String gt  unique   new LinkedHashSet lt String gt         HashSet lt String gt  dup   new LinkedHashSet lt String gt         boolean b   false      list add  Hello        list add  Hello        list add  how        list add  are        list add  u        list add  u         for Iterator iterator  list iterator   iterator hasNext                   String value    String iterator next            System out println value            if b  unique add value               dup add value           else             unique add value               System out println unique       System out println dup

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This is the right one  if you are concerned about the overhead of HashSet    public static ArrayList lt String gt  removeDuplicates  ArrayList lt String gt  arrayList       if  arrayList isEmpty    return null     return what makes sense for your app     Collections sort arrayList  String CASE INSENSITIVE ORDER         remove duplicates     ArrayList  lt String gt  arrayList mod   new ArrayList lt  gt         arrayList mod add arrayList get 0        for  int i 1  i lt arrayList size    i             if   arrayList get i  equals arrayList get i-1    arrayList mod add arrayList get i              return arrayList mod

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this can solve the problem   private List lt SomeClass gt  clearListFromDuplicateFirstName List lt SomeClass gt  list1          Map lt String  SomeClass gt  cleanMap   new LinkedHashMap lt String  SomeClass gt          for  int i   0  i  lt  list1 size    i               cleanMap put list1 get i  getFirstName    list1 get i                List lt SomeClass gt  list   new ArrayList lt SomeClass gt  cleanMap values          return list

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Although converting the ArrayList to a HashSet effectively removes duplicates  if you need to preserve insertion order  I d rather suggest you to use this variant     list is some List of Strings Set lt String gt  s   new LinkedHashSet lt  gt  list     Then  if you need to get back a List reference  you can use again the conversion constructor

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If you don t want duplicates  use a Set instead of a List  To convert a List to a Set you can use the following code      list is some List of Strings Set lt String gt  s   new HashSet lt String gt  list     If really necessary you can use the same construction to convert a Set back into a List

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If you don t want duplicates  use a Set instead of a List  To convert a List to a Set you can use the following code      list is some List of Strings Set lt String gt  s   new HashSet lt String gt  list     If really necessary you can use the same construction to convert a Set back into a List

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If you are using model type List lt  T  ArrayList lt  T    Hope it s help you   Here is my code without using any other data structure like set or hashmap  for  int i   0  i  lt  Models size    i     for  int j   i   1  j  lt  Models size    j              if  Models get i  getName   equals Models get j  getName            Models remove j      j--

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import java util    class RemoveDupFrmString       public static void main String   args                 String s  appsc            Set lt Character gt  unique   new LinkedHashSet lt Character gt               for char c   s toCharArray                   System out println unique add c                      for char dis unique               System out println dis

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public Set lt Object gt  findDuplicates List lt Object gt  list            Set lt Object gt  items   new HashSet lt Object gt             Set lt Object gt  duplicates   new HashSet lt Object gt             for  Object item   list                if  items contains item                     duplicates add item                     else                        items add item                                                 return duplicates

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This three lines of code can remove the duplicated element from ArrayList or any collection   List lt Entity gt  entities   repository findByUserId userId    Set lt Entity gt  s   new LinkedHashSet lt Entity gt  entities   entities clear    entities addAll s

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The  jonathan-stafford solution is OK  But this don t preserve the list order   If you want preserve the list order you have to use this   public static  lt T gt  void removeDuplicate List  lt T gt  list    Set  lt T gt  set   new HashSet  lt T gt     List  lt T gt  newList   new ArrayList  lt T gt     for  Iterator  lt T gt iter   list iterator       iter hasNext           Object element   iter next       if  set add  T  element         newList add  T  element           list clear       list addAll newList       It s only to complete the answer  Very good

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List lt String gt  result   new ArrayList lt String gt             Set lt String gt  set   new LinkedHashSet lt String gt             String s    ravi is a good boy  But ravi is very nasty fellow            StringTokenizer st   new StringTokenizer s                     while  st hasMoreTokens                  result add st nextToken                        System out println result            set addAll result           result clear            result addAll set           System out println result    output   ravi  is  a  good  boy  But  ravi  is  very  nasty  fellow   ravi  is  a  good  boy  But  very  nasty  fellow

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Code   List lt String gt  duplicatList   new ArrayList lt String gt     duplicatList   Arrays asList  AA   BB   CC   DD   DD   EE   AA   FF      above AA and DD are duplicate Set lt String gt  uniqueList   new HashSet lt String gt  duplicatList   duplicatList   new ArrayList lt String gt  uniqueList     let GC will doing free memory System out println  Removed Duplicate     duplicatList     Note  Definitely  there will be memory overhead

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If you are using model type List lt  T  ArrayList lt  T    Hope it s help you   Here is my code without using any other data structure like set or hashmap  for  int i   0  i  lt  Models size    i     for  int j   i   1  j  lt  Models size    j              if  Models get i  getName   equals Models get j  getName            Models remove j      j--

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If you don t want duplicates  use a Set instead of a List  To convert a List to a Set you can use the following code      list is some List of Strings Set lt String gt  s   new HashSet lt String gt  list     If really necessary you can use the same construction to convert a Set back into a List

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public static void main String   args       ArrayList lt Object gt  al   new ArrayList lt Object gt         al add  abc        al add  a        al add  b        al add  a        al add  abc        al add 10 3       al add  c        al add 10       al add  abc        al add 10       System out println  Before Duplicate Remove   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  After Removing duplicate   al

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this can solve the problem   private List lt SomeClass gt  clearListFromDuplicateFirstName List lt SomeClass gt  list1          Map lt String  SomeClass gt  cleanMap   new LinkedHashMap lt String  SomeClass gt          for  int i   0  i  lt  list1 size    i               cleanMap put list1 get i  getFirstName    list1 get i                List lt SomeClass gt  list   new ArrayList lt SomeClass gt  cleanMap values          return list

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List lt String gt  result   new ArrayList lt String gt             Set lt String gt  set   new LinkedHashSet lt String gt             String s    ravi is a good boy  But ravi is very nasty fellow            StringTokenizer st   new StringTokenizer s                     while  st hasMoreTokens                  result add st nextToken                        System out println result            set addAll result           result clear            result addAll set           System out println result    output   ravi  is  a  good  boy  But  ravi  is  very  nasty  fellow   ravi  is  a  good  boy  But  very  nasty  fellow

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This is used for your Custom Objects list      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  o2  getId       amp  amp                        Contact o1  getName   equalsIgnoreCase   Contact o2  getName                         return 0                            return 1                        set addAll list        final List newList   new ArrayList set       return newList

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As said before  you should use a class implementing the Set interface instead of List to be sure of the unicity of elements  If you have to keep the order of elements  the SortedSet interface can then be used  the TreeSet class implements that interface

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Would something like this work better   public static void removeDuplicates ArrayList lt String gt  list        Arraylist lt Object gt  ar       new Arraylist lt Object gt         Arraylist lt Object gt  tempAR   new Arraylist lt Object gt         while  list size   gt 0           ar add list 0            list removeall Collections singleton list 0               list addAll ar      That should maintain the order and also not be quadratic in run time

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If you re willing to use a third-party library  you can use the method distinct   in Eclipse Collections  formerly GS Collections    ListIterable lt Integer gt  integers   FastList newListWith 1  3  1  2  2  1   Assert assertEquals      FastList newListWith 1  3  2       integers distinct       The advantage of using distinct   instead of converting to a Set and then back to a List is that distinct   preserves the order of the original List  retaining the first occurrence of each element  It s implemented by using both a Set and a List   MutableSet lt T gt  seenSoFar   UnifiedSet newSet    int size   list size    for  int i   0  i  lt  size  i          T item   list get i       if  seenSoFar add item                 targetCollection add item           return targetCollection    If you cannot convert your original List into an Eclipse Collections type  you can use ListAdapter to get the same API   MutableList lt Integer gt  distinct   ListAdapter adapt integers  distinct      Note  I am a committer for Eclipse Collections

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When you are filling the ArrayList  use a condition for each element  For example       ArrayList lt  Integer  gt  al   new ArrayList lt  Integer  gt              fill 1      for   int i   0  i  lt   5  i              if    al contains  i                  al add  i             fill 2      for  int i   0  i  lt   10  i              if    al contains  i                  al add  i          for  Integer i  al                 System out print  i                      We will get an array  0  1  2  3  4  5  6  7  8  9  10

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Java 8 streams provide a very simple way to remove duplicate elements from a list  Using the distinct method  If we have a list of cities and we want to remove duplicates from that list it can be done in a single line -    List lt String gt  cityList   new ArrayList lt  gt      cityList add  Delhi     cityList add  Mumbai     cityList add  Bangalore     cityList add  Chennai     cityList add  Kolkata     cityList add  Mumbai      cityList   cityList stream   distinct   collect Collectors toList       How to remove duplicate elements from an arraylist

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Although converting the ArrayList to a HashSet effectively removes duplicates  if you need to preserve insertion order  I d rather suggest you to use this variant     list is some List of Strings Set lt String gt  s   new LinkedHashSet lt  gt  list     Then  if you need to get back a List reference  you can use again the conversion constructor

User · Answer

If you want your list to automatically ignore duplicates and preserve its order  you could create a HashList a HashMap embedded List    public static class HashList lt T gt  extends ArrayList lt T gt           private HashMap  lt T T gt  hashMap          public HashList                hashMap new HashMap lt  gt                         Override         public boolean add T t               if hashMap get t   null                   hashMap put t t                   return super add t                else return false                      Override         public boolean addAll Collection lt   extends T gt  c               HashList lt T gt  addup  HashList lt T gt  c              for int i 0 i lt addup size   i                     add addup get i                 return true                     Usage Example   HashList lt String gt  hashlist new HashList lt  gt     hashList add  hello    hashList add  hello    System out println   HashList    hashlist

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you can use nested loop in follow    ArrayList lt Class1 gt  l1   new ArrayList lt Class1 gt     ArrayList lt Class1 gt  l2   new ArrayList lt Class1 gt              Iterator iterator1   l1 iterator            boolean repeated   false           while  iterator1 hasNext                          Class1 c1    Class1  iterator1 next                for  Class1  c  l2                    if  c getId      c1 getId                        repeated   true                            if  repeated                  l2 add c1

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If you don t want duplicates in a Collection  you should consider why you re using a Collection that allows duplicates  The easiest way to remove repeated elements is to add the contents to a Set  which will not allow duplicates  and then add the Set back to the ArrayList   Set lt String gt  set   new HashSet lt  gt  yourList   yourList clear    yourList addAll set     Of course  this destroys the ordering of the elements in the ArrayList

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Here s a way that doesn t affect your list ordering   ArrayList l1   new ArrayList    ArrayList l2   new ArrayList     Iterator iterator   l1 iterator     while  iterator hasNext          YourClass o    YourClass  iterator next        if  l2 contains o   l2 add o       l1 is the original list  and l2 is the list without repeated items  Make sure YourClass has the equals method according to what you want to stand for equality

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If you want to preserve your Order then it is best to use LinkedHashSet  Because if you want to pass this List to an Insert Query by Iterating it  the order would be preserved   Try this  LinkedHashSet link new LinkedHashSet    List listOfValues new ArrayList    listOfValues add link     This conversion will be very helpful when you want to return a List but not a Set

User · Answer

When you are filling the ArrayList  use a condition for each element  For example       ArrayList lt  Integer  gt  al   new ArrayList lt  Integer  gt              fill 1      for   int i   0  i  lt   5  i              if    al contains  i                  al add  i             fill 2      for  int i   0  i  lt   10  i              if    al contains  i                  al add  i          for  Integer i  al                 System out print  i                      We will get an array  0  1  2  3  4  5  6  7  8  9  10

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import java util    class RemoveDupFrmString       public static void main String   args                 String s  appsc            Set lt Character gt  unique   new LinkedHashSet lt Character gt               for char c   s toCharArray                   System out println unique add c                      for char dis unique               System out println dis

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In Java  List permits ordered access of their elements  They can have duplicates because their lookup key is the position not some hash code  every element can be modified while they remain in the list where as Set represents a collection of unique elements and while elements are in set  they must not be modified While there is no restriction preventing you from modifying elements in a set  if an element is modified  then it could become forever lost in the set  public static void main String   args           List lt String gt  l   new ArrayList lt String gt            l add  quot A quot           l add  quot B quot           l add  quot C quot           l add  quot A quot           System out println  quot Before removing duplicates   quot           for  String s   l                System out println s                   Set lt String gt  set   new HashSet lt String gt  l          List lt String gt  newlist   new ArrayList lt String gt  set          System out println  quot after removing duplicates   quot           for  String s   newlist                System out println s                 for reference  refer this link How to remove duplicates from ArrayList

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There is also ImmutableSet from Guava as an option  here is the documentation    ImmutableSet copyOf list

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If you don t want duplicates  use a Set instead of a List  To convert a List to a Set you can use the following code      list is some List of Strings Set lt String gt  s   new HashSet lt String gt  list     If really necessary you can use the same construction to convert a Set back into a List

User · Answer

As said before  you should use a class implementing the Set interface instead of List to be sure of the unicity of elements  If you have to keep the order of elements  the SortedSet interface can then be used  the TreeSet class implements that interface

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Time Complexity   O n    Without Set  private static void removeDup ArrayList lt String gt  listWithDuplicateElements        System out println   Original Duplicate List      listWithDuplicateElements       List lt String gt  listWithoutDuplicateElements   new ArrayList lt  gt  listWithDuplicateElements size          listWithDuplicateElements stream   forEach str - gt            if  listWithoutDuplicateElements indexOf str     -1                listWithoutDuplicateElements add str                               System out println   Without Duplicate List      listWithoutDuplicateElements

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LinkedHashSet will do the trick   String   arr2     5   1   2   3   3   4   1   2    Set lt String gt  set   new LinkedHashSet lt String gt  Arrays asList arr2    for String s1   set      System out println s1    System out println   ------------------------     String   arr3   set toArray new String 0    for int i   0  i  lt  arr3 length  i         System out println arr3 i  toString         output  5 1 2 3 4

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ArrayList lt String gt  city new ArrayList lt String gt     city add  rajkot    city add  gondal    city add  rajkot    city add  gova    city add  baroda    city add  morbi    city add  gova     HashSet lt String gt  hashSet   new HashSet lt String gt     hashSet addAll city   city clear    city addAll hashSet   Toast makeText getActivity        city toString   Toast LENGTH SHORT  show

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Although converting the ArrayList to a HashSet effectively removes duplicates  if you need to preserve insertion order  I d rather suggest you to use this variant     list is some List of Strings Set lt String gt  s   new LinkedHashSet lt  gt  list     Then  if you need to get back a List reference  you can use again the conversion constructor

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for int a 0 a lt myArray size   a             for int b a 1 b lt myArray size   b                 if myArray get a  equalsIgnoreCase myArray get b                     myArray remove b                    dups                    b--

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Probably a bit overkill  but I enjoy this kind of isolated problem      This code uses a temporary Set  for the uniqueness check  but removes elements directly inside the original list  Since element removal inside an ArrayList can induce a huge amount of array copying  the remove int -method is avoided   public static  lt T gt  void removeDuplicates ArrayList lt T gt  list        int size   list size        int out   0                final Set lt T gt  encountered   new HashSet lt T gt             for  int in   0  in  lt  size  in                  final T t   list get in               final boolean first   encountered add t               if  first                    list set out    t                                     while  out  lt  size            list remove --size             While we re at it  here s a version for LinkedList  a lot nicer     public static  lt T gt  void removeDuplicates LinkedList lt T gt  list        final Set lt T gt  encountered   new HashSet lt T gt         for  Iterator lt T gt  iter   list iterator    iter hasNext                final T t   iter next            final boolean first   encountered add t           if   first                iter remove                        Use the marker interface to present a unified solution for List   public static  lt T gt  void removeDuplicates List lt T gt  list        if  list instanceof RandomAccess               use first version here       else              use other version here           EDIT  I guess the generics-stuff doesn t really add any value here   Oh well

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You can also do it this way  and preserve order      delete duplicates  if any  from  myArrayList  myArrayList   new ArrayList lt String gt  new LinkedHashSet lt String gt  myArrayList

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If you don t want duplicates in a Collection  you should consider why you re using a Collection that allows duplicates  The easiest way to remove repeated elements is to add the contents to a Set  which will not allow duplicates  and then add the Set back to the ArrayList   Set lt String gt  set   new HashSet lt  gt  yourList   yourList clear    yourList addAll set     Of course  this destroys the ordering of the elements in the ArrayList

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If you want to remove duplicates from ArrayList means find the below logic   public static Object   removeDuplicate Object   inputArray        long startTime   System nanoTime        int totalSize   inputArray length      Object   resultArray   new Object totalSize       int newSize   0      for int i 0  i lt totalSize  i                  Object value   inputArray i           if value    null                        continue                     for int j i 1  j lt totalSize  j                          if value equals inputArray j                                  inputArray j    null                                  resultArray newSize      value             long endTime   System nanoTime  -startTime      System out println  Total Time-B   endTime       return resultArray

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Here is my answer without using any other data structure like set or hashmap etc   public static  lt T gt  ArrayList lt T gt  uniquefy ArrayList lt T gt  myList         ArrayList  lt T gt  uniqueArrayList   new ArrayList lt T gt         for  int i   0  i  lt  myList size    i             if   uniqueArrayList contains myList get i                 uniqueArrayList add myList get i                         return uniqueArrayList

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Suppose we have a list of String like   List lt String gt  strList   new ArrayList lt  gt  5      insert up to five items to list            Then we can remove duplicate elements in multiple ways   Prior to Java 8  List lt String gt  deDupStringList   new ArrayList lt  gt  new HashSet lt  gt  strList      Note  If we want to maintain the insertion order then we need to use LinkedHashSet in place of HashSet  Using Guava  List lt String gt  deDupStringList2   Lists newArrayList Sets newHashSet strList      Using Java 8  List lt String gt  deDupStringList3   strList stream   distinct   collect Collectors toList       Note   In case we want to collect the result in a specific list implementation e g  LinkedList then we can modify the above example as   List lt String gt  deDupStringList3   strList stream   distinct                     collect Collectors toCollection LinkedList  new      We can use parallelStream also in the above code but it may not give expected performace benefits  Check this question for more

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If you don t want duplicates in a Collection  you should consider why you re using a Collection that allows duplicates  The easiest way to remove repeated elements is to add the contents to a Set  which will not allow duplicates  and then add the Set back to the ArrayList   Set lt String gt  set   new HashSet lt  gt  yourList   yourList clear    yourList addAll set     Of course  this destroys the ordering of the elements in the ArrayList

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Java 8 streams provide a very simple way to remove duplicate elements from a list  Using the distinct method  If we have a list of cities and we want to remove duplicates from that list it can be done in a single line -    List lt String gt  cityList   new ArrayList lt  gt      cityList add  Delhi     cityList add  Mumbai     cityList add  Bangalore     cityList add  Chennai     cityList add  Kolkata     cityList add  Mumbai      cityList   cityList stream   distinct   collect Collectors toList       How to remove duplicate elements from an arraylist

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You can also do it this way  and preserve order      delete duplicates  if any  from  myArrayList  myArrayList   new ArrayList lt String gt  new LinkedHashSet lt String gt  myArrayList

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Set lt String gt  strSet   strList stream   collect Collectors toSet       Is the easiest way to remove your duplicates

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In Java  List permits ordered access of their elements  They can have duplicates because their lookup key is the position not some hash code  every element can be modified while they remain in the list where as Set represents a collection of unique elements and while elements are in set  they must not be modified While there is no restriction preventing you from modifying elements in a set  if an element is modified  then it could become forever lost in the set  public static void main String   args           List lt String gt  l   new ArrayList lt String gt            l add  quot A quot           l add  quot B quot           l add  quot C quot           l add  quot A quot           System out println  quot Before removing duplicates   quot           for  String s   l                System out println s                   Set lt String gt  set   new HashSet lt String gt  l          List lt String gt  newlist   new ArrayList lt String gt  set          System out println  quot after removing duplicates   quot           for  String s   newlist                System out println s                 for reference  refer this link How to remove duplicates from ArrayList

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Here s a way that doesn t affect your list ordering   ArrayList l1   new ArrayList    ArrayList l2   new ArrayList     Iterator iterator   l1 iterator     while  iterator hasNext          YourClass o    YourClass  iterator next        if  l2 contains o   l2 add o       l1 is the original list  and l2 is the list without repeated items  Make sure YourClass has the equals method according to what you want to stand for equality

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Probably a bit overkill  but I enjoy this kind of isolated problem      This code uses a temporary Set  for the uniqueness check  but removes elements directly inside the original list  Since element removal inside an ArrayList can induce a huge amount of array copying  the remove int -method is avoided   public static  lt T gt  void removeDuplicates ArrayList lt T gt  list        int size   list size        int out   0                final Set lt T gt  encountered   new HashSet lt T gt             for  int in   0  in  lt  size  in                  final T t   list get in               final boolean first   encountered add t               if  first                    list set out    t                                     while  out  lt  size            list remove --size             While we re at it  here s a version for LinkedList  a lot nicer     public static  lt T gt  void removeDuplicates LinkedList lt T gt  list        final Set lt T gt  encountered   new HashSet lt T gt         for  Iterator lt T gt  iter   list iterator    iter hasNext                final T t   iter next            final boolean first   encountered add t           if   first                iter remove                        Use the marker interface to present a unified solution for List   public static  lt T gt  void removeDuplicates List lt T gt  list        if  list instanceof RandomAccess               use first version here       else              use other version here           EDIT  I guess the generics-stuff doesn t really add any value here   Oh well

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It is possible to remove duplicates from arraylist without using HashSet or one more arraylist    Try this code        ArrayList lt String gt  lst   new ArrayList lt String gt         lst add  ABC        lst add  ABC        lst add  ABCD        lst add  ABCD        lst add  ABCE         System out println  Duplicates List   lst        Object   st   lst toArray          for  Object s   st            if  lst indexOf s     lst lastIndexOf s                 lst remove lst lastIndexOf s                            System out println  Distinct List   lst     Output is  Duplicates List  ABC  ABC  ABCD  ABCD  ABCE  Distinct List  ABC  ABCD  ABCE

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Suppose we have a list of String like   List lt String gt  strList   new ArrayList lt  gt  5      insert up to five items to list            Then we can remove duplicate elements in multiple ways   Prior to Java 8  List lt String gt  deDupStringList   new ArrayList lt  gt  new HashSet lt  gt  strList      Note  If we want to maintain the insertion order then we need to use LinkedHashSet in place of HashSet  Using Guava  List lt String gt  deDupStringList2   Lists newArrayList Sets newHashSet strList      Using Java 8  List lt String gt  deDupStringList3   strList stream   distinct   collect Collectors toList       Note   In case we want to collect the result in a specific list implementation e g  LinkedList then we can modify the above example as   List lt String gt  deDupStringList3   strList stream   distinct                     collect Collectors toCollection LinkedList  new      We can use parallelStream also in the above code but it may not give expected performace benefits  Check this question for more

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Although converting the ArrayList to a HashSet effectively removes duplicates  if you need to preserve insertion order  I d rather suggest you to use this variant     list is some List of Strings Set lt String gt  s   new LinkedHashSet lt  gt  list     Then  if you need to get back a List reference  you can use again the conversion constructor

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If you want to remove duplicates from ArrayList means find the below logic   public static Object   removeDuplicate Object   inputArray        long startTime   System nanoTime        int totalSize   inputArray length      Object   resultArray   new Object totalSize       int newSize   0      for int i 0  i lt totalSize  i                  Object value   inputArray i           if value    null                        continue                     for int j i 1  j lt totalSize  j                          if value equals inputArray j                                  inputArray j    null                                  resultArray newSize      value             long endTime   System nanoTime  -startTime      System out println  Total Time-B   endTime       return resultArray

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Probably a bit overkill  but I enjoy this kind of isolated problem      This code uses a temporary Set  for the uniqueness check  but removes elements directly inside the original list  Since element removal inside an ArrayList can induce a huge amount of array copying  the remove int -method is avoided   public static  lt T gt  void removeDuplicates ArrayList lt T gt  list        int size   list size        int out   0                final Set lt T gt  encountered   new HashSet lt T gt             for  int in   0  in  lt  size  in                  final T t   list get in               final boolean first   encountered add t               if  first                    list set out    t                                     while  out  lt  size            list remove --size             While we re at it  here s a version for LinkedList  a lot nicer     public static  lt T gt  void removeDuplicates LinkedList lt T gt  list        final Set lt T gt  encountered   new HashSet lt T gt         for  Iterator lt T gt  iter   list iterator    iter hasNext                final T t   iter next            final boolean first   encountered add t           if   first                iter remove                        Use the marker interface to present a unified solution for List   public static  lt T gt  void removeDuplicates List lt T gt  list        if  list instanceof RandomAccess               use first version here       else              use other version here           EDIT  I guess the generics-stuff doesn t really add any value here   Oh well

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If you re willing to use a third-party library  you can use the method distinct   in Eclipse Collections  formerly GS Collections    ListIterable lt Integer gt  integers   FastList newListWith 1  3  1  2  2  1   Assert assertEquals      FastList newListWith 1  3  2       integers distinct       The advantage of using distinct   instead of converting to a Set and then back to a List is that distinct   preserves the order of the original List  retaining the first occurrence of each element  It s implemented by using both a Set and a List   MutableSet lt T gt  seenSoFar   UnifiedSet newSet    int size   list size    for  int i   0  i  lt  size  i          T item   list get i       if  seenSoFar add item                 targetCollection add item           return targetCollection    If you cannot convert your original List into an Eclipse Collections type  you can use ListAdapter to get the same API   MutableList lt Integer gt  distinct   ListAdapter adapt integers  distinct      Note  I am a committer for Eclipse Collections

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ArrayList lt String gt  list   new ArrayList lt String gt         HashSet lt String gt  unique   new LinkedHashSet lt String gt         HashSet lt String gt  dup   new LinkedHashSet lt String gt         boolean b   false      list add  Hello        list add  Hello        list add  how        list add  are        list add  u        list add  u         for Iterator iterator  list iterator   iterator hasNext                   String value    String iterator next            System out println value            if b  unique add value               dup add value           else             unique add value               System out println unique       System out println dup

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public static void main String   args       ArrayList lt Object gt  al   new ArrayList lt Object gt         al add  abc        al add  a        al add  b        al add  a        al add  abc        al add 10 3       al add  c        al add 10       al add  abc        al add 10       System out println  Before Duplicate Remove   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  After Removing duplicate   al

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As said before  you should use a class implementing the Set interface instead of List to be sure of the unicity of elements  If you have to keep the order of elements  the SortedSet interface can then be used  the TreeSet class implements that interface

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If you don t want duplicates in a Collection  you should consider why you re using a Collection that allows duplicates  The easiest way to remove repeated elements is to add the contents to a Set  which will not allow duplicates  and then add the Set back to the ArrayList   Set lt String gt  set   new HashSet lt  gt  yourList   yourList clear    yourList addAll set     Of course  this destroys the ordering of the elements in the ArrayList

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The  jonathan-stafford solution is OK  But this don t preserve the list order   If you want preserve the list order you have to use this   public static  lt T gt  void removeDuplicate List  lt T gt  list    Set  lt T gt  set   new HashSet  lt T gt     List  lt T gt  newList   new ArrayList  lt T gt     for  Iterator  lt T gt iter   list iterator       iter hasNext           Object element   iter next       if  set add  T  element         newList add  T  element           list clear       list addAll newList       It s only to complete the answer  Very good

[java] How do I remove repeated elements from ArrayList?

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