Create an ArrayList of unique values

Question

I have an ArrayList with values taken from a file  many lines  this is just an extract    20 03 2013 23 31 46 6870    6810    6800    6720    6860    6670    6700    6650    6750    6830    34864   34272 20 03 2013 23 31 46 6910    6780    6800    6720    6860    6680    6620    6690    6760    6790    35072   34496   Where the first two values per line are strings that contain data and are stored in a single element   What I want to do is compare the string data elements and delete  for example  the second one and all the elements referred to in that line   For now  I ve used a for loop that compares the string every 13 elements  in order to compare only data strings    My question  can I implement other better solutions   This is my code   import java util Scanner  import java util List  import java util ArrayList  import java io    import java text SimpleDateFormat  import java util Date   public class Main       public static void main String   args  throws Exception             The input file         Scanner s   new Scanner new File  prova txt                Saving each element of the input file in an arraylist          ArrayList lt String gt  list   new ArrayList lt String gt             while  s hasNext                 list add s next                       s close               Arraylist to save modified values         ArrayList lt String gt  ds   new ArrayList lt String gt                         int i          for i 0  i lt  list size  -13  i i 14                  combining the first to values to obtain data               String str   list get i      list get i 1               ds add str                 add all the other values to arraylist ds             int j              for j 2  j lt 14  j                     ds add list get i j                                 comparing data values             int k                for k 0  k lt  ds size  -12  k k 13                   ds get k     first data string element                     Comparing with other strings and delete                   TODO

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You can easily do this with a Hashmap  You obviously have a key  which is the String data  and some values   Loop on all your lines and add them to your Map   Map lt String  List lt Integer gt  gt  map   new HashMap lt  gt         while  s hasNext       String stringData         List lt Integer gt  values         map put stringData values       Note that in this case  you will keep the last occurence of duplicate lines  If you prefer keeping the first occurence and removing the others  you can add a check with Map containsKey String stringData   before putting in the map

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Pretty late to the party  but here s my two cents   Use a LinkedHashSet  I assume what you need is a collection which    disallows you to insert duplicates  retains insertion order    LinkedHashSet does this  The advantage over using an ArrayList is that LinkedHashSet has a complexity of O 1  for the contains operation  as opposed to ArrayList  which has O n      Of course  you need to implement your object s equals and hashCode methods properly

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Just Override the boolean equals   method of custom object  Say you have an ArrayList with custom field f1  f2      override   Override public boolean equals Object o        if  this    o  return true      if    o instanceof CustomObject   return false       CustomObject object    CustomObject  o       if   f1 equals object dob   return false      if   f2 equals object fullName   return false              return true      and check using ArrayList instance s contains   method  That s it

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You can read from file to map  where the key is the date and skip if the the whole row if the date is already in map          Map lt String  List lt String gt  gt  map   new HashMap lt String  List lt String gt  gt              int i   0          String lastData   null          while  s hasNext                  String str   s next                if  i   13    0                    if  map containsKey str                           skip the whole row                     lastData   null                    else                       lastData   str                      map put lastData  new ArrayList lt String gt                                      else if  lastData    null                    map get lastData  add str                               i

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Create an Arraylist of unique values   You could use Set toArray   method      A collection that contains no duplicate elements  More formally  sets   contain no pair of elements e1 and e2 such that e1 equals e2   and at   most one null element  As implied by its name  this interface models   the mathematical set abstraction    http   docs oracle com javase 6 docs api java util Set html

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Solution  1  HashSet  A good solution to the immediate problem of reading a file into an ArrayList with a uniqueness constraint is to simply keep a HashSet of seen items  Before processing a line  we check that its key is not already in the set  If it isn t  we add the key to the set to mark it as finished  then add the line data to the result ArrayList    import java util    import java io     public class Main       public static void main String   args           throws FileNotFoundException  IOException            String file    prova txt           ArrayList lt String   gt  data   new ArrayList lt  gt             HashSet lt String gt  seen   new HashSet lt  gt              try  BufferedReader br   new BufferedReader new FileReader file                  for  String line   line   br readLine       null                     String   split   line split    s                     String key   split 0          split 1                    if   seen contains key                         data add Arrays copyOfRange split  2  split length                        seen add key                                                      for  String   row   data                System out println Arrays toString row                          Solution  2  LinkedHashMap LinkedHashSet  Since we have key-value pairs in this particular dataset  we could roll everything into a LinkedHashMap lt String  ArrayList lt String gt  gt   see docs for LinkedHashMap  which preserves ordering but can t be indexed into  use-case driven decision  but amounts to the same strategy as above  ArrayList lt String gt  or String   is arbitrary here--it could be any data value   Note that this version makes it easy to preserve the most recently seen key rather than the oldest  remove the  data containsKey key  test    import java util    import java io     public class Main       public static void main String   args           throws FileNotFoundException  IOException            String file    prova txt           LinkedHashMap lt String  ArrayList lt String gt  gt  data   new LinkedHashMap lt  gt              try  BufferedReader br   new BufferedReader new FileReader file                  for  String line   line   br readLine       null                     String   split   line split    s                     String key   split 0          split 1                    if   data containsKey key                         ArrayList lt String gt  val   new ArrayList lt  gt                         String   sub   Arrays copyOfRange split  2  split length                        Collections addAll val  sub                       data put key  val                                                      for  Map Entry lt String  ArrayList lt String gt  gt  e   data entrySet                  System out println e getKey         gt      e getValue                           Solution  3  ArrayListSet  The above examples represent pretty narrow use cases  Here s a sketch for a general ArrayListSet class  which maintains the usual list behavior  add set remove etc  while preserving uniqueness   Basically  the class is an abstraction of solution  1 in this post  HashSet combined with ArrayList   but with a slightly different flavor  the data itself is used to determine uniqueness rather than a key  but it s a truer  ArrayList  structure    This class solves the problems of efficiency  ArrayList contains is linear  so we should reject that solution except in trivial cases   lack of ordering  storing everything directly in a HashSet doesn t help us   lack of ArrayList operations  LinkedHashSet is otherwise the best solution but we can t index into it  so it s not a true replacement for an ArrayList     Using a HashMap lt E  index gt  instead of a HashSet would speed up remove Object o  and indexOf Object o  functions  but slow down sort   A linear remove Object o  is the main drawback over a plain HashSet   import java util     public class ArrayListSet lt E gt  implements Iterable lt E gt   Set lt E gt        private ArrayList lt E gt  list      private HashSet lt E gt  set       public ArrayListSet             list   new ArrayList lt  gt             set   new HashSet lt  gt                public boolean add E e            return set add e   amp  amp  list add e              public boolean add int i  E e            if   set add e   return false          list add i  e           return true             public void clear             list clear            set clear               public boolean contains Object o            return set contains o              public E get int i            return list get i              public boolean isEmpty             return list isEmpty               public E remove int i                    E e   list remove i           set remove e           return e             public boolean remove Object o                    if  set remove o                 list remove o               return true                     return false             public boolean set int i  E e            if  set contains e   return false           set add e           set remove list set i  e            return true             public int size             return list size               public void sort Comparator lt   super E gt  c            Collections sort list  c              public Iterator lt E gt  iterator             return list iterator               public boolean addAll Collection lt   extends E gt  c            int before   size            for  E e   c  add e           return size      before             public boolean containsAll Collection lt   gt  c            return set containsAll c              public boolean removeAll Collection lt   gt  c            return set removeAll c   amp  amp  list removeAll c              public boolean retainAll Collection lt   gt  c             return set retainAll c   amp  amp  list retainAll c              public Object   toArray             return list toArray               public  lt T gt  T   toArray T   a            return list toArray a             Example usage   public class ArrayListSetDriver       public static void main String   args            ArrayListSet lt String gt  fruit   new ArrayListSet lt  gt             fruit add  apple            fruit add  banana            fruit add  kiwi            fruit add  strawberry            fruit add  apple            fruit add  strawberry             for  String item   fruit                System out print item              gt  apple banana kiwi strawberry                    fruit remove  kiwi            fruit remove 1           fruit add 0   banana            fruit set 2   cranberry            fruit set 0   cranberry            System out println             for  int i   0  i  lt  fruit size    i                  System out print fruit get i               gt  banana apple cranberry                    System out println                Solution  4  ArrayListMap  This class solves a drawback of ArrayListSet which is that the data we want to store and its associated key may not be the same  This class provides a put method that enforces uniqueness on a different object than the data stored in the underlying ArrayList  This is just what we need to solve the original problem posed in this thread  This gives us the ordering and iteration of an ArrayList but fast lookups and uniqueness properties of a HashMap  The HashMap contains the unique values mapped to their index locations in the ArrayList  which enforces ordering and provides iteration   This approach solves the scalability problems of using a HashSet in solution  1  That approach works fine for a quick file read  but without an abstraction  we d have to handle all consistency operations by hand and pass around multiple raw data structures if we needed to enforce that contract across multiple functions and over time   As with ArrayListSet  this can be considered a proof of concept rather than a full implementation   import java util     public class ArrayListMap lt K  V gt  implements Iterable lt V gt   Map lt K  V gt        private ArrayList lt V gt  list      private HashMap lt K  Integer gt  map       public ArrayListMap             list   new ArrayList lt  gt             map   new HashMap lt  gt                public void clear             list clear            map clear               public boolean containsKey Object key            return map containsKey key              public boolean containsValue Object value            return list contains value              public V get int i            return list get i              public boolean isEmpty             return map isEmpty               public V get Object key            return list get map get key               public V put K key  V value            if  map containsKey key                 int i   map get key               V v   list get i               list set i  value               return v                     list add value           map put key  list size   - 1           return null             public V putIfAbsent K key  V value            if  map containsKey key                 if  list get map get key      null                    list set map get key   value                   return null                             return list get map get key                       return put key  value              public V remove int i            V v   list remove i            for  Map Entry lt K  Integer gt  entry   map entrySet                  if  entry getValue      i                    map remove entry getKey                     break                                   decrementMapIndices i           return v             public V remove Object key            if  map containsKey key                 int i   map remove key               V v   list get i               list remove i               decrementMapIndices i               return v                     return null             private void decrementMapIndices int start            for  Map Entry lt K  Integer gt  entry   map entrySet                  int i   entry getValue                 if  i  gt  start                    map put entry getKey    i - 1                                      public int size             return list size               public void putAll Map lt   extends K    extends V gt  m            for  Map Entry lt   extends K    extends V gt  entry   m entrySet                  put entry getKey    entry getValue                          public Set lt Map Entry lt K  V gt  gt  entrySet             Set lt Map Entry lt K  V gt  gt  es   new HashSet lt  gt              for  Map Entry lt K  Integer gt  entry   map entrySet                  es add new AbstractMap SimpleEntry lt  gt                   entry getKey    list get entry getValue                                       return es             public Set lt K gt  keySet             return map keySet               public Collection lt V gt  values             return list             public Iterator lt V gt  iterator             return list iterator               public Object   toArray             return list toArray               public  lt T gt  T   toArray T   a            return list toArray a             Here s the class in action on the original problem   import java io     public class Main       public static void main String   args           throws FileNotFoundException  IOException            String file    prova txt           ArrayListMap lt String  String   gt  data   new ArrayListMap lt  gt              try  BufferedReader br   new BufferedReader new FileReader file                  for  String line   line   br readLine       null                     String   split   line split    s                     String key   split 0          split 1                   String   sub   Arrays copyOfRange split  2  split length                    data putIfAbsent key  sub                                     for  Map Entry lt String  String   gt  e   data entrySet                  System out println e getKey         gt                       java util Arrays toString e getValue                         for  String   a   data                System out println java util Arrays toString a

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I use helper class  Not sure it s good or bad  public class ListHelper lt T gt        private final T   t       public ListHelper T   t            this t   t             public List lt T gt  unique List lt T gt  list           Set lt T gt  set   new HashSet lt  gt  list           return Arrays asList set toArray t                Usage and test   import static org assertj core api Assertions assertThat    public class ListHelperTest         Test     public void unique             List lt String gt  s   Arrays asList  abc    cde    dfg    abc            List lt String gt  unique   new ListHelper lt  gt  new String 0   unique s           assertThat unique  hasSize 3             Or Java8 version   public class ListHelper lt T gt        public Function lt List lt T gt   List lt T gt  gt  unique             return l - gt  l stream   distinct   collect Collectors toList              public class ListHelperTest        Test     public void unique             List lt String gt  s   Arrays asList  abc    cde    dfg    abc            assertThat new ListHelper lt String gt    unique   apply s   hasSize 3

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Use Set                  Set lt String gt  list   new HashSet lt  gt           while  s hasNext              list add s next

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Saving each element of the input file in an arraylist      ArrayList lt String gt  list   new ArrayList lt String gt         while  s hasNext             list add s next              That s all you need list    ArrayList  list stream   distinct   collect Collectors toList

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If you want to make a list with unique values from an existing list you can use  List myUniqueList   myList stream   distinct   collect Collectors toList

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HashSet hs   new HashSet                    hs addAll arrayList                   arrayList clear                    arrayList addAll hs

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If you need unique values  you should use the implementation of the SET interface

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Try checking for duplicates with a  contains   method on the ArrayList  before adding a new element   It would look something like this     if  list contains data          list add data     That should prevent duplicates in the list  as well as not mess up the order of elements  like people seem to look for

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You could use a Set  It is a collection which doesn t accept duplicates

[java] Create an ArrayList of unique values

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