Java 8 Streams to find the duplicate elements

Question

I am trying to list out duplicate elements in the integer list say for eg   List lt Integer gt  numbers   Arrays asList new Integer   1 2 1 3 4 4          using Streams of jdk 8  Has anybody tried out  To remove the duplicates we can use the distinct   api  But what about finding the duplicated elements   Anybody can help me out

User · Answer

What about checking of indexes           numbers stream                filter integer - gt  numbers indexOf integer     numbers lastIndexOf integer                collect Collectors toSet                 forEach System out  println

User · Answer

You need a set  allItems below  to hold the entire array contents  but this is O n    Integer   numbers   new Integer     1  2  1  3  4  4    Set lt Integer gt  allItems   new HashSet lt  gt     Set lt Integer gt  duplicates   Arrays stream numbers           filter n - gt   allItems add n     Set add   returns false if the item was already in the set           collect Collectors toSet     System out println duplicates       1  4

User · Answer

You can use Collections frequency   numbers stream   filter i - gt  Collections frequency numbers  i   gt 1                   collect Collectors toSet    forEach System out  println

User · Answer

My StreamEx library which enhances the Java 8 streams provides a special operation distinct atLeast  which can retain only elements appearing at least the specified number of times  So your problem can be solved like this   List lt Integer gt  repeatingNumbers   StreamEx of numbers  distinct 2  toList      Internally it s similar to  Dave solution  it counts objects  to support other wanted quantities and it s parallel-friendly  it uses ConcurrentHashMap for parallelized stream  but HashMap for sequential   For big amounts of data you can get a speed-up using  parallel   distinct 2

User · Answer

You can get the duplicated like this    List lt Integer gt  numbers   Arrays asList 1  2  1  3  4  4   Set lt Integer gt  duplicated   numbers    stream      filter n - gt  numbers          stream            filter x - gt  x    n           count    gt  1      collect Collectors toSet

User · Answer

I think basic solutions to the question should be as below   Supplier supplier HashSet  new   HashSet has ls stream   collect Collectors toCollection supplier     List lst    List  ls stream   filter e- gt Collections frequency ls e  gt 1  distinct   collect Collectors toList       well  it is not recommended to perform a filter operation  but for better understanding  i have used it  moreover  there should be some custom filtration in future versions

User · Answer

Do you have to use the java 8 idioms  steams   Perphaps a simple solution would be to move the complexity to a map alike data structure that holds numbers as key  without repeating  and the times it ocurrs as a value  You could them iterate that map an only do something with those numbers that are ocurrs   1   import java lang Math  import java util Arrays  import java util List  import java util Map  import java util HashMap  import java util Iterator   public class RemoveDuplicates     public static void main String   args         List lt Integer gt  numbers   Arrays asList new Integer   1 2 1 3 4 4       Map lt Integer Integer gt  countByNumber   new HashMap lt Integer Integer gt        for Integer n numbers            Integer count   countByNumber get n        if  count    null           countByNumber put n count   1          else          countByNumber put n 1                  System out println countByNumber      Iterator it   countByNumber entrySet   iterator        while  it hasNext              Map Entry pair    Map Entry it next            System out println pair getKey             pair getValue

User · Answer

Basic example  First-half builds the frequency-map  second-half reduces it to a filtered list  Probably not as efficient as Dave s answer  but more versatile  like if you want to detect exactly two etc   List lt Integer gt  duplicates   IntStream of  1  2  3  2  1  2  3  4  2  2  2       boxed       collect  Collectors groupingBy  Function identity    Collectors counting           entrySet       stream       filter  p - gt  p getValue    gt  1       map  Map Entry  getKey       collect  Collectors toList

User · Answer

A multiset is a structure maintaining the number of occurrences for each element  Using Guava implementation   Set lt Integer gt  duplicated           ImmutableMultiset copyOf numbers  entrySet   stream                    filter entry - gt  entry getCount    gt  1                   map Multiset Entry  getElement                   collect Collectors toSet

User · Answer

If you only need to detect the presence of duplicates  instead of listing them  which is what the OP wanted   just convert them into both a List and Set  then compare the sizes       List lt Integer gt  list            Set lt Integer gt  set   new HashSet lt  gt  list       if  list size      set size               duplicates detected         I like this approach because it has less places for mistakes

User · Answer

the creating of an additional map or stream is time- and space-consuming     Set lt Integer gt  duplicates   numbers stream   collect  Collectors collectingAndThen    Collectors groupingBy  Function identity    Collectors counting        map - gt        map values   removeIf  cnt - gt  cnt  lt  2        return  map keySet                    1  4       and for the question of which is claimed to be a  duplicate   public static int   getDuplicatesStreamsToArray  int   input       return  IntStream of  input   boxed   collect  Collectors collectingAndThen        Collectors groupingBy  Function identity    Collectors counting            map - gt            map values   removeIf  cnt - gt  cnt  lt  2            return  map keySet                  stream   mapToInt  i - gt  i   toArray

User · Answer

I think I have good solution how to fix problem like this - List    List with grouping by Something a  amp  Something b  There is extended definition   public class Test        public static void test              class A               private int a              private int b              private float c              private float d               public A int a  int b  float c  float d                    this a   a                  this b   b                  this c   c                  this d   d                                    List lt A gt  list1   new ArrayList lt A gt              list1 addAll Arrays asList new A 1  2  3  4                   new A 2  3  4  5                   new A 1  2  3  4                   new A 2  3  4  5                   new A 1  2  3  4              Map lt Integer  A gt  map   list1 stream                    collect HashMap  new   m  v  - gt  m put                          Objects hash v a  v b  v c  v d   v                           HashMap  putAll            list1 clear            list1 addAll map values              System out println list1              class A  list1 it s just incoming data - magic is in the Objects hash

User · Answer

Try this solution   public class Anagramm    public static boolean isAnagramLetters String word  String anagramm        if  anagramm isEmpty              return false             Map lt Character  Integer gt  mapExistString   CharCountMap word       Map lt Character  Integer gt  mapCheckString   CharCountMap anagramm       return enoughLetters mapExistString  mapCheckString      private static Map lt Character  Integer gt  CharCountMap String chars        HashMap lt Character  Integer gt  charCountMap   new HashMap lt Character  Integer gt         for  char c   chars toCharArray              if  charCountMap containsKey c                 charCountMap put c  charCountMap get c    1             else               charCountMap put c  1                       return charCountMap     static boolean enoughLetters Map lt Character  Integer gt  mapExistString  Map lt Character Integer gt  mapCheckString        for  Entry lt Character  Integer gt  e   mapCheckString entrySet               Character letter   e getKey            Integer available   mapExistString get letter           if  available    null    e getValue    gt  available  return false            return true

User · Answer

An O n  way would be as below   List lt Integer gt  numbers   Arrays asList 1  2  1  3  4  4   Set lt Integer gt  duplicatedNumbersRemovedSet   new HashSet lt  gt     Set lt Integer gt  duplicatedNumbersSet   numbers stream   filter n - gt   duplicatedNumbersRemovedSet add n   collect Collectors toSet       The space complexity would go double in this approach  but that space is not a waste  in-fact  we now have the duplicated alone only as a Set as well as another Set with all the duplicates removed too

[java] Java 8, Streams to find the duplicate elements

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