How to count the number of occurrences of an element in a List

Question

I have an ArrayList  a Collection class of Java  as follows   ArrayList lt String gt  animals   new ArrayList lt String gt     animals add  bat    animals add  owl    animals add  bat    animals add  bat      As you can see  the animals ArrayList consists of 3 bat elements and one owl element  I was wondering if there is any API in the Collection framework that returns the number of bat occurrences or if there is another way to determine the number of occurrences   I found that Google s Collection Multiset does have an API that returns the total number of occurrences of an element  But that is compatible only with JDK 1 5  Our product is currently in JDK 1 6  so I cannot use it

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A slightly more efficient approach might be  Map lt String  AtomicInteger gt  instances   new HashMap lt String  AtomicInteger gt      void add String name         AtomicInteger value   instances get name        if  value    null           instances put name  new AtomicInteger 1         else         value incrementAndGet

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You can use groupingBy feature of Java 8 for your use case  import java util ArrayList  import java util List  import java util Map  import java util function Function  import java util stream Collectors   public class Test       public static void main String   args            List lt String gt  animals   new ArrayList lt  gt              animals add  quot bat quot            animals add  quot owl quot            animals add  quot bat quot            animals add  quot bat quot             Map lt String Long gt  occurrenceMap                   animals stream   collect Collectors groupingBy Function identity   Collectors counting              System out println  quot occurrenceMap    quot    occurrenceMap            Output occurrenceMap    bat 3  owl 1

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So do it the old fashioned way and roll your own   Map lt String  Integer gt  instances   new HashMap lt String  Integer gt      void add String name         Integer value   instances get name        if  value    null            value   new Integer 0           instances put name  value               instances put name  value

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I m pretty sure the static frequency-method in Collections would come in handy here   int occurrences   Collections frequency animals   bat      That s how I d do it anyway  I m pretty sure this is jdk 1 6 straight up

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If you use Eclipse Collections  you can use a Bag   A MutableBag can be returned from any implementation of RichIterable by calling toBag     MutableList lt String gt  animals   Lists mutable with  bat    owl    bat    bat    MutableBag lt String gt  bag   animals toBag    Assert assertEquals 3  bag occurrencesOf  bat     Assert assertEquals 1  bag occurrencesOf  owl       The HashBag implementation in Eclipse Collections is backed by a MutableObjectIntMap   Note  I am a committer for Eclipse Collections

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So do it the old fashioned way and roll your own   Map lt String  Integer gt  instances   new HashMap lt String  Integer gt      void add String name         Integer value   instances get name        if  value    null            value   new Integer 0           instances put name  value               instances put name  value

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Sorry there s no simple method call that can do it  All you d need to do though is create a map and count frequency with it   HashMap lt String int gt  frequencymap   new HashMap lt String int gt     foreach String a in animals      if frequencymap containsKey a         frequencymap put a  frequencymap get a  1         else  frequencymap put a  1

User · Answer

There is no native method in Java to do that for you   However  you can use IterableUtils countMatches   from Apache Commons-Collections to do it for you

User · Answer

Put the elements of the arraylist in the hashMap to count the frequency

User · Answer

A slightly more efficient approach might be  Map lt String  AtomicInteger gt  instances   new HashMap lt String  AtomicInteger gt      void add String name         AtomicInteger value   instances get name        if  value    null           instances put name  new AtomicInteger 1         else         value incrementAndGet

User · Answer

Put the elements of the arraylist in the hashMap to count the frequency

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Actually  Collections class has a static method called   frequency Collection c  Object o  which returns the number of occurrences of the element you are searching for  by the way  this will work perfectly for you    ArrayList lt String gt  animals   new ArrayList lt String gt     animals add  bat    animals add  owl    animals add  bat    animals add  bat    System out println  Freq of bat    Collections frequency animals   bat

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There is no native method in Java to do that for you   However  you can use IterableUtils countMatches   from Apache Commons-Collections to do it for you

User · Answer

Alternative Java 8 solution using Streams   long count   animals stream   filter animal - gt   bat  equals animal   count

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package traversal   import java util ArrayList  import java util List   public class Occurrance       static int count       public static void main String   args            List lt String gt  ls   new ArrayList lt String gt             ls add  aa            ls add  aa            ls add  bb            ls add  cc            ls add  dd            ls add  ee            ls add  ee            ls add  aa            ls add  aa             for  int i   0  i  lt  ls size    i                  if  ls get i      aa                     count   count   1                                  System out println count                Output  4

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Actually  Collections class has a static method called   frequency Collection c  Object o  which returns the number of occurrences of the element you are searching for  by the way  this will work perfectly for you    ArrayList lt String gt  animals   new ArrayList lt String gt     animals add  bat    animals add  owl    animals add  bat    animals add  bat    System out println  Freq of bat    Collections frequency animals   bat

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In Java 8   Map lt String  Long gt  counts       list stream   collect Collectors groupingBy e - gt  e  Collectors counting

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I didn t want to make this case more difficult and made it with two iterators I have a HashMap with LastName -  FirstName  And my method should delete items with dulicate FirstName   public static void removeTheFirstNameDuplicates HashMap lt String  String gt  map         Iterator lt Map Entry lt String  String gt  gt  iter   map entrySet   iterator        Iterator lt Map Entry lt String  String gt  gt  iter2   map entrySet   iterator        while iter hasNext                  Map Entry lt String  String gt  pair   iter next            String name   pair getValue            int i   0           while iter2 hasNext                           Map Entry lt String  String gt  nextPair   iter2 next                if  nextPair getValue   equals name                   i                       if  i  gt  1              iter remove

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If you are a user of my ForEach DSL  it can be done with a Count query   Count lt String gt  query   Count from list   for  Count lt Foo gt  each  query  each yield    bat  equals each element   int number   query result

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Simple Way to find the occurrence of string value in an array using Java 8 features   public void checkDuplicateOccurance             List lt String gt  duplicateList   new ArrayList lt String gt             duplicateList add  Cat            duplicateList add  Dog            duplicateList add  Cat            duplicateList add  cow            duplicateList add  Cow            duplicateList add  Goat                      Map lt String  Long gt  couterMap   duplicateList stream   collect Collectors groupingBy e - gt  e toString   Collectors counting              System out println couterMap           Output    Cat 2  Goat 1  Cow 1  cow 1  Dog 1   You can notice  Cow  and cow are not considered as same string  in case you required it under same count  use  toLowerCase    Please find the snippet below for the same   Map lt String  Long gt  couterMap   duplicateList stream   collect Collectors groupingBy e - gt  e toString   toLowerCase   Collectors counting        Output    cat 2  cow 2  goat 1  dog 1

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Java 8 - another method  String searched    bat   long n   IntStream range 0  animals size                 filter i - gt  searched equals animals get i                 count

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To get the occurrences of the object from the list directly   int noOfOccurs   Collections frequency animals   bat      To get the occurrence of the Object collection inside list  override the equals method in the Object class as    Override public boolean equals Object o       Animals e      if   o instanceof Animals            return false       else          e  Animals o          if this type  e type                 return true                      return false     Animals int type       this type   type      Call the Collections frequency as   int noOfOccurs   Collections frequency animals  new Animals 1

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A slightly more efficient approach might be  Map lt String  AtomicInteger gt  instances   new HashMap lt String  AtomicInteger gt      void add String name         AtomicInteger value   instances get name        if  value    null           instances put name  new AtomicInteger 1         else         value incrementAndGet

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What you want is a Bag - which is like a set but also counts the number of occurances  Unfortunately the java Collections framework - great as they are dont have a Bag impl  For that one must use the Apache Common Collection link text

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List lt String gt  lst   new ArrayList lt String gt      lst add  Ram    lst add  Ram    lst add  Shiv    lst add  Boss     Map lt String  Integer gt  mp   new HashMap lt String  Integer gt      for  String string   lst         if mp keySet   contains string                 mp put string  mp get string  1         else               mp put string  1            System out println   mp   mp     Output    mp   Ram 2  Boss 1  Shiv 1

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Sorry there s no simple method call that can do it  All you d need to do though is create a map and count frequency with it   HashMap lt String int gt  frequencymap   new HashMap lt String int gt     foreach String a in animals      if frequencymap containsKey a         frequencymap put a  frequencymap get a  1         else  frequencymap put a  1

User · Answer

There is no native method in Java to do that for you   However  you can use IterableUtils countMatches   from Apache Commons-Collections to do it for you

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This shows  why it is important to  Refer to objects by their interfaces  as described in Effective Java book   If you code to the implementation and use ArrayList in let s say  50 places in your code  when you find a good  List  implementation that count the items  you will have to change all those 50 places  and probably you ll have to break your code   if it is only used by you there is not a big deal  but if it is used by someone else uses  you ll break their code too    By programming to the interface you can let those 50 places unchanged and replace the implementation from ArrayList to  CountItemsList   for instance   or some other class   Below is a very basic sample on how this could be written  This is only a sample  a production ready List would be much more complicated   import java util     public class CountItemsList lt E gt  extends ArrayList lt E gt             This is private  It is not visible from outside      private Map lt E Integer gt  count   new HashMap lt E Integer gt             There are several entry points to this class        this is just to show one of them      public boolean add  E element               if   count containsKey  element                  count put  element  1              else                count put  element  count get  element     1                      return super add  element                  This method belongs to CountItemList interface   or class           to used you have to cast      public int getCount  E element              if    count containsKey  element                   return 0                    return count get  element               public static void main  String    args              List lt String gt  animals   new CountItemsList lt String gt             animals add  bat            animals add  owl            animals add  bat            animals add  bat             System out println     CountItemsList lt String gt   animals  getCount   bat                OO principles applied here  inheritance  polymorphism  abstraction  encapsulation

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To achieve that one can do it in several ways  namely  Methods that return the number of occurrence of a single element  Collection Frequency Collections frequency animals   quot bat quot     Java Stream  Filter animals stream   filter  quot bat quot   equals  count     Just iteration thought the list public static long manually Collection lt   gt  c  Object o       int count   0      for Object e   c          if e equals o               count        return count     Methods that create a map of frequencies  Collectors groupingBy Map lt String  Long gt  counts           animals stream                  collect Collectors groupingBy Function identity    Collectors counting       merge Map lt String  Long gt  map   new HashMap lt  gt     c forEach e - gt  map merge e  1L  Long  sum     Manually Map lt String  Integer gt  mp   new HashMap lt  gt             animals forEach animal - gt  mp compute animal   k  v  - gt   v    null    1   v   1     A running example with all the methods  public class Frequency        public static int frequency Collection lt   gt  c  Object o           return Collections frequency c  o              public static long filter Collection lt   gt  c  Object o           return c stream   filter o  equals  count               public static long manually Collection lt   gt  c  Object o           int count   0          for Object e   c              if e equals o                   count            return count             public static Map lt    Long gt  mapGroupBy Collection lt   gt  c           return c stream                    collect Collectors groupingBy Function identity     Collectors counting                 public static Map lt Object  Long gt  mapMerge Collection lt   gt  c            Map lt Object  Long gt  map   new HashMap lt  gt              c forEach e - gt  map merge e  1L  Long  sum             return map             public static Map lt Object  Long gt  manualMap Collection lt   gt  c           Map lt Object  Long gt  map   new HashMap lt  gt             c forEach e - gt  map compute e   k  v  - gt   v    null    1   v   1            return map              public static void main String   args           List lt String gt  animals   new ArrayList lt  gt             animals add  quot bat quot            animals add  quot owl quot            animals add  quot bat quot            animals add  quot bat quot             System out println frequency animals   quot bat quot             System out println filter animals  quot bat quot             System out println manually animals  quot bat quot             mapGroupBy animals  forEach  k  v  - gt  System out println k    quot  - gt   quot  v            mapMerge animals  forEach  k  v  - gt  System out println k    quot  - gt   quot  v            manualMap animals  forEach  k  v  - gt  System out println k    quot  - gt   quot  v             The methods name should have reflected what those methods are doing  however  I used the name to reflect the approach being used instead  given that in the current context it is okey

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This shows  why it is important to  Refer to objects by their interfaces  as described in Effective Java book   If you code to the implementation and use ArrayList in let s say  50 places in your code  when you find a good  List  implementation that count the items  you will have to change all those 50 places  and probably you ll have to break your code   if it is only used by you there is not a big deal  but if it is used by someone else uses  you ll break their code too    By programming to the interface you can let those 50 places unchanged and replace the implementation from ArrayList to  CountItemsList   for instance   or some other class   Below is a very basic sample on how this could be written  This is only a sample  a production ready List would be much more complicated   import java util     public class CountItemsList lt E gt  extends ArrayList lt E gt             This is private  It is not visible from outside      private Map lt E Integer gt  count   new HashMap lt E Integer gt             There are several entry points to this class        this is just to show one of them      public boolean add  E element               if   count containsKey  element                  count put  element  1              else                count put  element  count get  element     1                      return super add  element                  This method belongs to CountItemList interface   or class           to used you have to cast      public int getCount  E element              if    count containsKey  element                   return 0                    return count get  element               public static void main  String    args              List lt String gt  animals   new CountItemsList lt String gt             animals add  bat            animals add  owl            animals add  bat            animals add  bat             System out println     CountItemsList lt String gt   animals  getCount   bat                OO principles applied here  inheritance  polymorphism  abstraction  encapsulation

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So do it the old fashioned way and roll your own   Map lt String  Integer gt  instances   new HashMap lt String  Integer gt      void add String name         Integer value   instances get name        if  value    null            value   new Integer 0           instances put name  value               instances put name  value

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This shows  why it is important to  Refer to objects by their interfaces  as described in Effective Java book   If you code to the implementation and use ArrayList in let s say  50 places in your code  when you find a good  List  implementation that count the items  you will have to change all those 50 places  and probably you ll have to break your code   if it is only used by you there is not a big deal  but if it is used by someone else uses  you ll break their code too    By programming to the interface you can let those 50 places unchanged and replace the implementation from ArrayList to  CountItemsList   for instance   or some other class   Below is a very basic sample on how this could be written  This is only a sample  a production ready List would be much more complicated   import java util     public class CountItemsList lt E gt  extends ArrayList lt E gt             This is private  It is not visible from outside      private Map lt E Integer gt  count   new HashMap lt E Integer gt             There are several entry points to this class        this is just to show one of them      public boolean add  E element               if   count containsKey  element                  count put  element  1              else                count put  element  count get  element     1                      return super add  element                  This method belongs to CountItemList interface   or class           to used you have to cast      public int getCount  E element              if    count containsKey  element                   return 0                    return count get  element               public static void main  String    args              List lt String gt  animals   new CountItemsList lt String gt             animals add  bat            animals add  owl            animals add  bat            animals add  bat             System out println     CountItemsList lt String gt   animals  getCount   bat                OO principles applied here  inheritance  polymorphism  abstraction  encapsulation

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Put the elements of the arraylist in the hashMap to count the frequency

User · Answer

What you want is a Bag - which is like a set but also counts the number of occurances  Unfortunately the java Collections framework - great as they are dont have a Bag impl  For that one must use the Apache Common Collection link text

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List lt String gt  list   Arrays asList  as    asda    asd    urff    dfkjds    hfad    asd    qadasd    as    asda            asd    urff    dfkjds    hfad    asd    qadasd     as    asda    asd    urff    dfkjds    hfad    asd            qadasd    as    asda    asd    urff    dfkjds    hfad    asd    qadasd      Method 1   Set lt String gt  set   new LinkedHashSet lt  gt     set addAll list    for  String s   set         System out println s           Collections frequency list  s        Method 2   int count   1  Map lt String  Integer gt  map   new HashMap lt  gt     Set lt String gt  set1   new LinkedHashSet lt  gt     for  String s   list        if   set1 add s             count   map get s    1            map put s  count       count   1     System out println map

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So do it the old fashioned way and roll your own   Map lt String  Integer gt  instances   new HashMap lt String  Integer gt      void add String name         Integer value   instances get name        if  value    null            value   new Integer 0           instances put name  value               instances put name  value

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I wonder  why you can t use that Google s Collection API with JDK 1 6  Does it say so  I think you can  there should not be any compatibility issues  as it is built for a lower version  The case would have been different if that were built for 1 6 and you are running 1 5   Am I wrong somewhere

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Map lt String Integer gt  hm   new HashMap lt String  Integer gt     for String i   animals        Integer j   hm get i       hm put i  j  null   1   j 1      for Map Entry lt String  Integer gt  val   hm entrySet          System out println val getKey     occurs     val getValue     times

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Put the elements of the arraylist in the hashMap to count the frequency

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You can use groupingBy feature of Java 8 for your use case  import java util ArrayList  import java util List  import java util Map  import java util function Function  import java util stream Collectors   public class Test       public static void main String   args            List lt String gt  animals   new ArrayList lt  gt              animals add  quot bat quot            animals add  quot owl quot            animals add  quot bat quot            animals add  quot bat quot             Map lt String Long gt  occurrenceMap                   animals stream   collect Collectors groupingBy Function identity   Collectors counting              System out println  quot occurrenceMap    quot    occurrenceMap            Output occurrenceMap    bat 3  owl 1

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In Java 8   Map lt String  Long gt  counts       list stream   collect Collectors groupingBy e - gt  e  Collectors counting

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To get the occurrences of the object from the list directly   int noOfOccurs   Collections frequency animals   bat      To get the occurrence of the Object collection inside list  override the equals method in the Object class as    Override public boolean equals Object o       Animals e      if   o instanceof Animals            return false       else          e  Animals o          if this type  e type                 return true                      return false     Animals int type       this type   type      Call the Collections frequency as   int noOfOccurs   Collections frequency animals  new Animals 1

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What you want is a Bag - which is like a set but also counts the number of occurances  Unfortunately the java Collections framework - great as they are dont have a Bag impl  For that one must use the Apache Common Collection link text

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List lt String gt  lst   new ArrayList lt String gt      lst add  Ram    lst add  Ram    lst add  Shiv    lst add  Boss     Map lt String  Integer gt  mp   new HashMap lt String  Integer gt      for  String string   lst         if mp keySet   contains string                 mp put string  mp get string  1         else               mp put string  1            System out println   mp   mp     Output    mp   Ram 2  Boss 1  Shiv 1

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Simple Way to find the occurrence of string value in an array using Java 8 features   public void checkDuplicateOccurance             List lt String gt  duplicateList   new ArrayList lt String gt             duplicateList add  Cat            duplicateList add  Dog            duplicateList add  Cat            duplicateList add  cow            duplicateList add  Cow            duplicateList add  Goat                      Map lt String  Long gt  couterMap   duplicateList stream   collect Collectors groupingBy e - gt  e toString   Collectors counting              System out println couterMap           Output    Cat 2  Goat 1  Cow 1  cow 1  Dog 1   You can notice  Cow  and cow are not considered as same string  in case you required it under same count  use  toLowerCase    Please find the snippet below for the same   Map lt String  Long gt  couterMap   duplicateList stream   collect Collectors groupingBy e - gt  e toString   toLowerCase   Collectors counting        Output    cat 2  cow 2  goat 1  dog 1

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Alternative Java 8 solution using Streams   long count   animals stream   filter animal - gt   bat  equals animal   count

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Sorry there s no simple method call that can do it  All you d need to do though is create a map and count frequency with it   HashMap lt String int gt  frequencymap   new HashMap lt String int gt     foreach String a in animals      if frequencymap containsKey a         frequencymap put a  frequencymap get a  1         else  frequencymap put a  1

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I wonder  why you can t use that Google s Collection API with JDK 1 6  Does it say so  I think you can  there should not be any compatibility issues  as it is built for a lower version  The case would have been different if that were built for 1 6 and you are running 1 5   Am I wrong somewhere

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If you are a user of my ForEach DSL  it can be done with a Count query   Count lt String gt  query   Count from list   for  Count lt Foo gt  each  query  each yield    bat  equals each element   int number   query result

User · Answer

There is no native method in Java to do that for you   However  you can use IterableUtils countMatches   from Apache Commons-Collections to do it for you

User · Answer

I wonder  why you can t use that Google s Collection API with JDK 1 6  Does it say so  I think you can  there should not be any compatibility issues  as it is built for a lower version  The case would have been different if that were built for 1 6 and you are running 1 5   Am I wrong somewhere

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If you use Eclipse Collections  you can use a Bag   A MutableBag can be returned from any implementation of RichIterable by calling toBag     MutableList lt String gt  animals   Lists mutable with  bat    owl    bat    bat    MutableBag lt String gt  bag   animals toBag    Assert assertEquals 3  bag occurrencesOf  bat     Assert assertEquals 1  bag occurrencesOf  owl       The HashBag implementation in Eclipse Collections is backed by a MutableObjectIntMap   Note  I am a committer for Eclipse Collections

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Map lt String Integer gt  hm   new HashMap lt String  Integer gt     for String i   animals        Integer j   hm get i       hm put i  j  null   1   j 1      for Map Entry lt String  Integer gt  val   hm entrySet          System out println val getKey     occurs     val getValue     times

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Sorry there s no simple method call that can do it  All you d need to do though is create a map and count frequency with it   HashMap lt String int gt  frequencymap   new HashMap lt String int gt     foreach String a in animals      if frequencymap containsKey a         frequencymap put a  frequencymap get a  1         else  frequencymap put a  1

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List lt String gt  list   Arrays asList  as    asda    asd    urff    dfkjds    hfad    asd    qadasd    as    asda            asd    urff    dfkjds    hfad    asd    qadasd     as    asda    asd    urff    dfkjds    hfad    asd            qadasd    as    asda    asd    urff    dfkjds    hfad    asd    qadasd      Method 1   Set lt String gt  set   new LinkedHashSet lt  gt     set addAll list    for  String s   set         System out println s           Collections frequency list  s        Method 2   int count   1  Map lt String  Integer gt  map   new HashMap lt  gt     Set lt String gt  set1   new LinkedHashSet lt  gt     for  String s   list        if   set1 add s             count   map get s    1            map put s  count       count   1     System out println map

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package traversal   import java util ArrayList  import java util List   public class Occurrance       static int count       public static void main String   args            List lt String gt  ls   new ArrayList lt String gt             ls add  aa            ls add  aa            ls add  bb            ls add  cc            ls add  dd            ls add  ee            ls add  ee            ls add  aa            ls add  aa             for  int i   0  i  lt  ls size    i                  if  ls get i      aa                     count   count   1                                  System out println count                Output  4

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This shows  why it is important to  Refer to objects by their interfaces  as described in Effective Java book   If you code to the implementation and use ArrayList in let s say  50 places in your code  when you find a good  List  implementation that count the items  you will have to change all those 50 places  and probably you ll have to break your code   if it is only used by you there is not a big deal  but if it is used by someone else uses  you ll break their code too    By programming to the interface you can let those 50 places unchanged and replace the implementation from ArrayList to  CountItemsList   for instance   or some other class   Below is a very basic sample on how this could be written  This is only a sample  a production ready List would be much more complicated   import java util     public class CountItemsList lt E gt  extends ArrayList lt E gt             This is private  It is not visible from outside      private Map lt E Integer gt  count   new HashMap lt E Integer gt             There are several entry points to this class        this is just to show one of them      public boolean add  E element               if   count containsKey  element                  count put  element  1              else                count put  element  count get  element     1                      return super add  element                  This method belongs to CountItemList interface   or class           to used you have to cast      public int getCount  E element              if    count containsKey  element                   return 0                    return count get  element               public static void main  String    args              List lt String gt  animals   new CountItemsList lt String gt             animals add  bat            animals add  owl            animals add  bat            animals add  bat             System out println     CountItemsList lt String gt   animals  getCount   bat                OO principles applied here  inheritance  polymorphism  abstraction  encapsulation

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I wonder  why you can t use that Google s Collection API with JDK 1 6  Does it say so  I think you can  there should not be any compatibility issues  as it is built for a lower version  The case would have been different if that were built for 1 6 and you are running 1 5   Am I wrong somewhere

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What you want is a Bag - which is like a set but also counts the number of occurances  Unfortunately the java Collections framework - great as they are dont have a Bag impl  For that one must use the Apache Common Collection link text

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I didn t want to make this case more difficult and made it with two iterators I have a HashMap with LastName -  FirstName  And my method should delete items with dulicate FirstName   public static void removeTheFirstNameDuplicates HashMap lt String  String gt  map         Iterator lt Map Entry lt String  String gt  gt  iter   map entrySet   iterator        Iterator lt Map Entry lt String  String gt  gt  iter2   map entrySet   iterator        while iter hasNext                  Map Entry lt String  String gt  pair   iter next            String name   pair getValue            int i   0           while iter2 hasNext                           Map Entry lt String  String gt  nextPair   iter2 next                if  nextPair getValue   equals name                   i                       if  i  gt  1              iter remove

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I m pretty sure the static frequency-method in Collections would come in handy here   int occurrences   Collections frequency animals   bat      That s how I d do it anyway  I m pretty sure this is jdk 1 6 straight up

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To achieve that one can do it in several ways  namely  Methods that return the number of occurrence of a single element  Collection Frequency Collections frequency animals   quot bat quot     Java Stream  Filter animals stream   filter  quot bat quot   equals  count     Just iteration thought the list public static long manually Collection lt   gt  c  Object o       int count   0      for Object e   c          if e equals o               count        return count     Methods that create a map of frequencies  Collectors groupingBy Map lt String  Long gt  counts           animals stream                  collect Collectors groupingBy Function identity    Collectors counting       merge Map lt String  Long gt  map   new HashMap lt  gt     c forEach e - gt  map merge e  1L  Long  sum     Manually Map lt String  Integer gt  mp   new HashMap lt  gt             animals forEach animal - gt  mp compute animal   k  v  - gt   v    null    1   v   1     A running example with all the methods  public class Frequency        public static int frequency Collection lt   gt  c  Object o           return Collections frequency c  o              public static long filter Collection lt   gt  c  Object o           return c stream   filter o  equals  count               public static long manually Collection lt   gt  c  Object o           int count   0          for Object e   c              if e equals o                   count            return count             public static Map lt    Long gt  mapGroupBy Collection lt   gt  c           return c stream                    collect Collectors groupingBy Function identity     Collectors counting                 public static Map lt Object  Long gt  mapMerge Collection lt   gt  c            Map lt Object  Long gt  map   new HashMap lt  gt              c forEach e - gt  map merge e  1L  Long  sum             return map             public static Map lt Object  Long gt  manualMap Collection lt   gt  c           Map lt Object  Long gt  map   new HashMap lt  gt             c forEach e - gt  map compute e   k  v  - gt   v    null    1   v   1            return map              public static void main String   args           List lt String gt  animals   new ArrayList lt  gt             animals add  quot bat quot            animals add  quot owl quot            animals add  quot bat quot            animals add  quot bat quot             System out println frequency animals   quot bat quot             System out println filter animals  quot bat quot             System out println manually animals  quot bat quot             mapGroupBy animals  forEach  k  v  - gt  System out println k    quot  - gt   quot  v            mapMerge animals  forEach  k  v  - gt  System out println k    quot  - gt   quot  v            manualMap animals  forEach  k  v  - gt  System out println k    quot  - gt   quot  v             The methods name should have reflected what those methods are doing  however  I used the name to reflect the approach being used instead  given that in the current context it is okey

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A slightly more efficient approach might be  Map lt String  AtomicInteger gt  instances   new HashMap lt String  AtomicInteger gt      void add String name         AtomicInteger value   instances get name        if  value    null           instances put name  new AtomicInteger 1         else         value incrementAndGet

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Java 8 - another method  String searched    bat   long n   IntStream range 0  animals size                 filter i - gt  searched equals animals get i                 count

[java] How to count the number of occurrences of an element in a List

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