Java Detect duplicates in ArrayList

Question

How could I go about detecting  returning true false  whether an ArrayList contains more than one of the same element in Java   Many thanks  Terry  Edit Forgot to mention that I am not looking to compare  Blocks  with each other but their integer values  Each  block  has an int and this is what makes them different  I find the int of a particular Block by calling a method named  getNum   e g  table1 0  2  getNum

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Improved code to return the duplicate elements    Can find duplicates in a Collection return the set of duplicates Unique Elements can be obtained from the Set     public static  lt T gt  List getDuplicate Collection lt T gt  list         final List lt T gt  duplicatedObjects   new ArrayList lt T gt         Set lt T gt  set   new HashSet lt T gt           Override     public boolean add T e            if  contains e                 duplicatedObjects add e                     return super add e                   for  T t   list            set add t             return duplicatedObjects      public static  lt T gt  boolean hasDuplicate Collection lt T gt  list        if  getDuplicate list  isEmpty            return false      return true

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To know the Duplicates in a List use the following code It will give you the set which contains duplicates    public Set lt   gt  findDuplicatesInList List lt   gt  beanList        System out println  findDuplicatesInList    beanList       Set lt Object gt  duplicateRowSet null      duplicateRowSet new LinkedHashSet lt Object gt                 for int i 0 i lt beanList size   i                     Object superString beanList get i                   System out println  findDuplicatesInList  superString    superString                   for int j 0 j lt beanList size   j                         if i  j                            Object subString beanList get j                            System out println  findDuplicatesInList  subString    subString                            if superString equals subString                                 duplicateRowSet add beanList get j                                                                                                 System out println  findDuplicatesInList  duplicationSet    duplicateRowSet           return duplicateRowSet

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With Java 8  you can use Stream API   boolean areAllDistinct List lt Block gt  blocksList        return blocksList stream   map Block  getNum  distinct   count      blockList size

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Simplest  dump the whole collection into a Set  using the Set Collection  constructor or Set addAll   then see if the Set has the same size as the ArrayList   List lt Integer gt  list        Set lt Integer gt  set   new HashSet lt Integer gt  list    if set size    lt  list size            There are duplicates        Update  If I m understanding your question correctly  you have a 2d array of Block  as in  Block table       and you want to detect if any row of them has duplicates   In that case  I could do the following  assuming that Block implements  equals  and  hashCode  correctly   for  Block   row   table       Set set   new HashSet lt Block gt         for  Block cell   row          set add cell           if  set size    lt  6      has duplicate          I m not 100  sure of that for syntax  so it might be safer to write it as  for  int i   0  i  lt  6  i         Set set   new HashSet lt Block gt         for  int j   0  j  lt  6  j        set add table i  j           Set add returns a boolean false if the item being added is already in the set  so you could even short circuit and bale out on any add that returns false if all you want to know is whether there are any duplicates

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best way to handle this issue is to use a HashSet    ArrayList lt String gt  listGroupCode   new ArrayList lt  gt     listGroupCode add  A    listGroupCode add  A    listGroupCode add  B    listGroupCode add  C    HashSet lt String gt  set   new HashSet lt  gt  listGroupCode   ArrayList lt String gt  result   new ArrayList lt  gt  set     Just print  result arraylist and see the result without duplicates

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Method to detect presence of duplicates in a generic list          Depends on the equals method of the concrete type  make sure to override it as required              public static  lt T gt  boolean hasDuplicates List lt T gt  list           int count   list size            T t1 t2           for int i 0 i lt count i                 t1   list get i               for int j i 1 j lt count j                     t2   list get j                   if t2 equals t1                        return true                                                    return false          An example of a concrete class that has overridden equals      public class Reminder      private long id      private int hour      private int minute       public Reminder long id  int hour  int minute           this id   id          this hour   hour          this minute   minute              Override     public boolean equals Object other           if other    null  return false          if this getClass      other getClass    return false          Reminder otherReminder    Reminder  other          if this hour    otherReminder hour  return false          if this minute    otherReminder minute  return false           return true

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private boolean isDuplicate         for  int i   0  i  lt  arrayList size    i              for  int j   i   1  j  lt  arrayList size    j                  if  arrayList get i  getName   trim   equalsIgnoreCase arrayList get j  getName   trim                       return true                                     return false

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If your elements are somehow Comparable  the fact that the order has any real meaning is indifferent -- it just needs to be consistent with your definition of equality   the fastest duplicate removal solution is going to sort the list   0 n log n     then to do a single pass and look for repeated elements  that is  equal elements that follow each other   this is O n     The overall complexity is going to be O n log n    which is roughly the same as what you would get with a Set  n times long n    but with a much smaller constant  This is because the constant in sort dedup results from the cost of comparing elements  whereas the cost from the set is most likely to result from a hash computation  plus one  possibly several  hash comparisons  If you are using a hash-based Set implementation  that is  because a Tree based is going to give you a O  n log   n     which is even worse   As I understand it  however  you do not need to remove duplicates  but merely test for their existence  So you should hand-code a merge or heap sort algorithm on your array  that simply exits returning true  i e   there is a dup   if your comparator returns 0  and otherwise completes the sort  and traverse the sorted array testing for repeats  In a merge or heap sort  indeed  when the sort is completed  you will have compared every duplicate pair unless both elements were already in their final positions  which is unlikely   Thus  a tweaked sort algorithm should yield a huge performance improvement  I would have to prove that  but I guess the tweaked algorithm should be in the O log n   on uniformly random data

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Improved code  using return value of Set add instead of comparing the size of list and set   public static  lt T gt  boolean hasDuplicate Iterable lt T gt  all        Set lt T gt  set   new HashSet lt T gt            Set add returns false if the set does not change  which        indicates that a duplicate element has been added      for  T each  all  if   set add each   return true      return false

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If you want the set of duplicate values   import java util ArrayList  import java util HashSet  import java util List  import java util Set   public class FindDuplicateInArrayList        public static void main String   args             Set lt String gt  uniqueSet   new HashSet lt String gt             List lt String gt  dupesList   new ArrayList lt String gt             for  String a   args                if  uniqueSet contains a                   dupesList add a               else                 uniqueSet add a                     System out println uniqueSet size       distinct words      uniqueSet           System out println dupesList size       dupesList words      dupesList             And probably also think about trimming values or using lowercase     depending on your case

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ArrayList lt String gt  withDuplicates   new ArrayList lt  gt         withDuplicates add  1        withDuplicates add  2        withDuplicates add  1        withDuplicates add  3        HashSet lt String gt  set   new HashSet lt  gt  withDuplicates       ArrayList lt String gt  withoutDupicates   new ArrayList lt  gt  set        ArrayList lt String gt  duplicates   new ArrayList lt String gt          Iterator lt String gt  dupIter   withDuplicates iterator        while dupIter hasNext              String dupWord   dupIter next        if withDuplicates contains dupWord                 duplicates add dupWord        else          withoutDupicates add dupWord                 System out println duplicates     System out println withoutDupicates

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If you are looking to avoid having duplicates at all  then you should just cut out the middle process of detecting duplicates and use a Set

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String tempVal   null      for  int i   0  i  lt  l size    i              tempVal   l get i     take the ith object out of list         while  l contains tempVal                 l remove tempVal     remove all matching entries                   l add tempVal     at last add one entry         Note  this will have major performance hit though as items are removed from start of the list  To address this  we have two options  1  iterate in reverse order and remove elements  2  Use LinkedList instead of ArrayList  Due to biased questions asked in interviews to remove duplicates from List without using any other collection  above example is the answer  In real world though  if I have to achieve this  I will put elements from List to Set  simple

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This answer is wrriten in Kotlin  but can easily be translated to Java   If your arraylist s size is within a fixed small range  then this is a great solution   var duplicateDetected   false     if arrList size  gt  1           for i in 0 until arrList size               for j in 0 until arrList size                   if i    j  amp  amp  arrList get i     arrList get j                        duplicateDetected   true

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I needed to do a similar operation for a Stream  but couldn t find a good example  Here s what I came up with   public static  lt T gt  boolean areUnique final Stream lt T gt  stream        final Set lt T gt  seen   new HashSet lt  gt         return stream allMatch seen  add       This has the advantage of short-circuiting when duplicates are found early rather than having to process the whole stream and isn t much more complicated than just putting everything in a Set and checking the size  So this case would roughly be   List lt T gt  list       boolean allDistinct   areUnique list stream

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Simply put  1  make sure all items are comparable 2  sort the array 2  iterate over the array and find duplicates

[java] Java: Detect duplicates in ArrayList?

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