What is the best way to filter a Java Collection

Question

I want to filter a java util Collection based on a predicate

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Consider Google Collections for an updated Collections framework that supports generics.

UPDATE: The google collections library is now deprecated. You should use the latest release of Guava instead. It still has all the same extensions to the collections framework including a mechanism for filtering based on a predicate.

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I wrote an extended Iterable class that support applying functional algorithms without copying the collection content   Usage   List lt Integer gt  myList   new ArrayList lt Integer gt     1  2  3  4  5    Iterable lt Integer gt  filtered   Iterable wrap myList  select new Predicate1 lt Integer gt          public Boolean call Integer n  throws FunctionalException               return n   2    0            for  int n   filtered         System out println n       The code above will actually execute  for  int n   myList         if  n   2    0                  System out println n

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The setup   public interface Predicate lt T gt      public boolean filter T t      void filterCollection Collection lt T gt  col  Predicate lt T gt  predicate      for  Iterator i   col iterator    i hasNext           T obj   i next        if  predicate filter obj           i remove                  The usage   List lt MyObject gt  myList        filterCollection myList  new Predicate lt MyObject gt        public boolean filter MyObject obj        return obj shouldFilter

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The Collections2 filter Collection Predicate  method in Google s Guava library does just what you re looking for

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Let   s look at how to filter a built-in JDK List and a MutableList using Eclipse Collections   List lt Integer gt  jdkList   Arrays asList 1  2  3  4  5   MutableList lt Integer gt  ecList   Lists mutable with 1  2  3  4  5     If you wanted to filter the numbers less than 3  you would expect the following outputs   List lt Integer gt  selected   Lists mutable with 1  2   List lt Integer gt  rejected   Lists mutable with 3  4  5     Here   s how you can filter using a Java 8 lambda as the Predicate   Assert assertEquals selected  Iterate select jdkList  each - gt  each  lt  3    Assert assertEquals rejected  Iterate reject jdkList  each - gt  each  lt  3     Assert assertEquals selected  ecList select each - gt  each  lt  3    Assert assertEquals rejected  ecList reject each - gt  each  lt  3      Here   s how you can filter using an anonymous inner class as the Predicate   Predicate lt Integer gt  lessThan3   new Predicate lt Integer gt          public boolean accept Integer each                return each  lt  3            Assert assertEquals selected  Iterate select jdkList  lessThan3    Assert assertEquals selected  ecList select lessThan3      Here are some alternatives to filtering JDK lists and Eclipse Collections MutableLists using the Predicates factory   Assert assertEquals selected  Iterate select jdkList  Predicates lessThan 3     Assert assertEquals selected  ecList select Predicates lessThan 3       Here is a version that doesn t allocate an object for the predicate  by using the Predicates2 factory instead with the selectWith method that takes a Predicate2   Assert assertEquals      selected  ecList selectWith Predicates2  lt Integer gt lessThan    3      Sometimes you want to filter on a negative condition   There is a special method in Eclipse Collections for that called reject   Assert assertEquals rejected  Iterate reject jdkList  lessThan3    Assert assertEquals rejected  ecList reject lessThan3      The method partition will return two collections  containing the elements selected by and rejected by the Predicate   PartitionIterable lt Integer gt  jdkPartitioned   Iterate partition jdkList  lessThan3   Assert assertEquals selected  jdkPartitioned getSelected     Assert assertEquals rejected  jdkPartitioned getRejected      PartitionList lt Integer gt  ecPartitioned   gscList partition lessThan3   Assert assertEquals selected  ecPartitioned getSelected     Assert assertEquals rejected  ecPartitioned getRejected       Note  I am a committer for Eclipse Collections

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Are you sure you want to filter the Collection itself  rather than an iterator   see org apache commons collections iterators FilterIterator  or using version 4 of apache commons org apache commons collections4 iterators FilterIterator

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Use Collection Query Engine  CQEngine   It is by far the fastest way to do this   See also  How do you query object collections in Java  Criteria SQL-like

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Java 8  2014  solves this problem using streams and lambdas in one line of code   List lt Person gt  beerDrinkers   persons stream        filter p - gt  p getAge    gt  16  collect Collectors toList       Here s a tutorial   Use Collection removeIf to modify the collection in place   Notice  In this case  the predicate will remove objects who satisfy the predicate    persons removeIf p - gt  p getAge    lt   16       lambdaj allows filtering collections without writing loops or inner classes   List lt Person gt  beerDrinkers   select persons  having on Person class  getAge        greaterThan 16       Can you imagine something more readable   Disclaimer  I am a contributor on lambdaj

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My answer builds on that from Kevin Wong  here as a one-liner using CollectionUtils from spring and a Java 8 lambda expression   CollectionUtils filter list  p - gt    Person  p  getAge    gt  16     This is as concise and readable as any alternative I have seen  without using aspect-based libraries   Spring CollectionUtils is available from spring version 4 0 2 RELEASE  and remember you need JDK 1 8 and language level 8

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In Java 8  You can directly use this filter method and then do that    List lt String gt  lines   Arrays asList  java    pramod    example      List lt String gt  result   lines stream                           filter line - gt    pramod  equals line                  collect Collectors toList                     result forEach System out  println

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I needed to filter a list depending on the values already present in the list  For example  remove all values following that is less than the current value   2 5 3 4 7 5  -   2 5 7   Or for example to remove all duplicates  3 5 4 2 3 5 6  -   3 5 4 2 6    public class Filter       public static  lt T gt  void List List lt T gt  list  Chooser lt T gt  chooser            List lt Integer gt  toBeRemoved   new ArrayList lt  gt             leftloop          for  int right   1  right  lt  list size      right                for  int left   0  left  lt  right    left                    if  toBeRemoved contains left                         continue                                    Keep keep   chooser choose list get left   list get right                    switch  keep                        case LEFT                          toBeRemoved add right                           continue leftloop                      case RIGHT                          toBeRemoved add left                           break                      case NONE                          toBeRemoved add left                           toBeRemoved add right                           continue leftloop                                                     Collections sort toBeRemoved  new Comparator lt Integer gt                   Override             public int compare Integer o1  Integer o2                    return o2 - o1                                     for  int i   toBeRemoved                if  i  gt   0  amp  amp  i  lt  list size                      list remove i                                      public static  lt T gt  void List List lt T gt  list  Keeper lt T gt  keeper            Iterator lt T gt  iterator   list iterator            while  iterator hasNext                  if   keeper keep iterator next                       iterator remove                                       public interface Keeper lt E gt            boolean keep E obj              public interface Chooser lt E gt            Keep choose E left  E right              public enum Keep           LEFT  RIGHT  BOTH  NONE            This will bee used like this   List lt String gt  names   new ArrayList lt  gt     names add  Anders    names add  Stefan    names add  Anders    Filter List names  new Filter Chooser lt String gt           Override     public Filter Keep choose String left  String right            return left equals right    Filter Keep LEFT   Filter Keep BOTH

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The simple pre-Java8 solution   ArrayList lt Item gt  filtered   new ArrayList lt Item gt      for  Item item   items  if  condition item   filtered add item     Unfortunately this solution isn t fully generic  outputting a list rather than the type of the given collection  Also  bringing in libraries or writing functions that wrap this code seems like overkill to me unless the condition is complex  but then you can write a function for the condition

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Best  way is too wide a request  Is it  shortest    Fastest    Readable   Filter in place or into another collection   Simplest  but not most readable  way is to iterate it and use Iterator remove   method   Iterator lt Foo gt  it   col iterator    while  it hasNext         Foo foo   it next      if   condition foo    it remove        Now  to make it more readable  you can wrap it into a utility method  Then invent a IPredicate interface  create an anonymous implementation of that interface and do something like   CollectionUtils filterInPlace col    new IPredicate lt Foo gt         public boolean keepIt Foo foo          return foo isBar                  where filterInPlace   iterate the collection and calls Predicate keepIt   to learn if the instance to be kept in the collection   I don t really see a justification for bringing in a third-party library just for this task

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How about some plain and straighforward Java   List lt Customer gt  list       List lt Customer gt  newList   new ArrayList lt  gt      for  Customer c   list       if  c getName   equals  dd    newList add c        Simple  readable and easy  and works in Android   But if you re using Java 8 you can do it in a sweet one line   List lt Customer gt  newList   list stream   filter c - gt  c getName   equals  dd    collect toList       Note that toList   is statically imported

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Since the early release of Java 8  you could try something like   Collection lt T gt  collection        Stream lt T gt  stream   collection stream   filter         For example  if you had a list of integers and you wanted to filter the numbers that are   10 and then print out those numbers to the console  you could do something like   List lt Integer gt  numbers   Arrays asList 12  74  5  8  16   numbers stream   filter n - gt  n  gt  10  forEach System out  println

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With the ForEach DSL you may write  import static ch akuhn util query Query select  import static ch akuhn util query Query  result  import ch akuhn util query Select   Collection lt String gt  collection        for  Select lt String gt  each   select collection         each yield   each value length    gt  3     Collection lt String gt  result    result      Given a collection of  The  quick  brown  fox  jumps  over  the  lazy  dog  this results in  quick  brown  jumps  over  lazy   ie all strings longer than three characters   All iteration styles supported by the ForEach DSL are   AllSatisfy AnySatisfy Collect Counnt CutPieces Detect GroupedBy IndexOf InjectInto Reject Select   For more details  please refer to https   www iam unibe ch scg svn repos Sources ForEach

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Since java 9 Collectors filtering is enabled   public static  lt T  A  R gt      Collector lt T     R gt  filtering Predicate lt   super T gt  predicate                                   Collector lt   super T  A  R gt  downstream    Thus filtering should be   collection stream   collect Collectors filtering predicate  collector     Example   List lt Integer gt  oddNumbers   List of 1  19  15  10  -10  stream                collect Collectors filtering i - gt  i   2    1  Collectors toList

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I ll throw RxJava in the ring  which is also available on Android  RxJava might not always be the best option  but it will give you more flexibility if you wish add more transformations on your collection or handle errors while filtering   Observable from Arrays asList 1  2  3  4  5        filter new Func1 lt Integer  Boolean gt              public Boolean call Integer i                return i   2    0                        subscribe new Action1 lt Integer gt              public void call Integer i                System out println i                       Output   1 3 5   More details on RxJava s filter can be found here

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JFilter http   code google com p jfilter  is best suited for your requirement   JFilter is a simple and high performance open source library to query collection of Java beans   Key features   Support of collection  java util Collection  java util Map and Array  properties  Support of collection inside collection of any depth  Support of inner queries  Support of parameterized queries  Can filter 1 million records in few 100 ms  Filter   query  is given in simple json format  it is like Mangodb queries  Following are some examples     id     le   10    where object id property is less than equals to 10      id      in    0    100      where object id property is 0 or 100     lineItems    lineAmount   1     where lineItems collection property of parameterized type  has lineAmount equals to 1       and     id    0      billingAddress    city   DEL       where id property is 0 and billingAddress city property is DEL     lineItems    taxes     key    code   GST     value     gt    1 01       where lineItems collection property of parameterized type  which has taxes map type property of parameteriszed type  has code equals to GST value greater than 1 01      or     code   10     skus      and     price     in    20    40        code   RedApple         Select all products where product code is 10 or sku price in 20 and 40 and sku code is  RedApple

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Using java 8  specifically lambda expression  you can do it simply like the below example   myProducts stream   filter prod - gt  prod price gt 10  collect Collectors toList      where for each product inside myProducts collection  if prod price gt 10  then add this product to the new filtered list

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Wait for Java 8   List lt Person gt  olderThan30        Create a Stream from the personList   personList stream        filter the element to select only those with age  gt   30   filter p - gt  p age  gt   30       put those filtered elements into a new List    collect Collectors toList

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https   code google com p joquery   Supports different possibilities   Given collection   Collection lt Dto gt  testList   new ArrayList lt  gt       of type   class Dto       private int id      private String text       public int getId                 return id             public int getText                 return text            Filter  Java 7  Filter lt Dto gt  query   CQ  lt Dto gt filter testList       where        property  id   eq   value 1   Collection lt Dto gt  filtered   query list      Java 8  Filter lt Dto gt  query   CQ  lt Dto gt filter testList       where        property Dto  getId       eq   value 1   Collection lt Dto gt  filtered   query list      Also   Filter lt Dto gt  query   CQ  lt Dto gt filter            from testList           where            property Dto  getId  between   value 1  value 2           and            property Dto  grtText  in   value new string    a   b       Sorting  also available for the Java 7   Filter lt Dto gt  query   CQ  lt Dto gt filter testList           orderBy            property Dto  getId           property Dto  getName      Collection lt Dto gt  sorted   query list      Grouping  also available for the Java 7   GroupQuery lt Integer Dto gt  query   CQ  lt Dto Dto gt query testList           group            groupBy Dto  getId      Collection lt Grouping lt Integer Dto gt  gt  grouped   query list      Joins  also available for the Java 7   Given   class LeftDto       private int id      private String text       public int getId                 return id             public int getText                 return text           class RightDto       private int id      private int leftId      private String text       public int getId                 return id             public int getLeftId                         return leftId                 public int getText                 return text           class JoinedDto       private int leftId      private int rightId      private String text       public JoinedDto int leftId int rightId String text                this leftId   leftId          this rightId   rightId          this text   text             public int getLeftId                 return leftId             public int getRightId                         return rightId                 public int getText                 return text           Collection lt LeftDto gt  leftList   new ArrayList lt  gt      Collection lt RightDto gt  rightList   new ArrayList lt  gt       Can be Joined like   Collection lt JoinedDto gt  results   CQ  lt LeftDto  LeftDto gt query   from leftList                    lt RightDto  JoinedDto gt innerJoin CQ  lt RightDto  RightDto gt query   from rightList                    on LeftFyo  getId  RightDto  getLeftId                   transformDirect selection - gt   new JoinedDto selection getLeft   getText                                                          selection getLeft   getId                                                          selection getRight   getId                                                        list      Expressions  Filter lt Dto gt  query   CQ  lt Dto gt filter        from testList       where        exec s - gt  s getId     1  eq   value 2

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Some really great great answers here  Me  I d like to keep thins as simple and readable as possible   public abstract class AbstractFilter lt T gt                    Method that returns whether an item is to be included or not          param item an item from the given collection          return true if this item is to be included in the collection  false in case it has to be removed              protected abstract boolean excludeItem T item        public void filter Collection lt T gt  collection            if  CollectionUtils isNotEmpty collection                 Iterator lt T gt  iterator   collection iterator                while  iterator hasNext                      if  excludeItem iterator next                           iterator remove

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Assuming that you are using Java 1 5  and that you cannot add Google Collections  I would do something very similar to what the Google guys did  This is a slight variation on Jon s comments   First add this interface to your codebase   public interface IPredicate lt T gt    boolean apply T type       Its implementers can answer when a certain predicate is true of a certain type  E g  If T were User and AuthorizedUserPredicate lt User gt  implements IPredicate lt T gt   then AuthorizedUserPredicate apply returns whether the passed in User is authorized   Then in some utility class  you could say  public static  lt T gt  Collection lt T gt  filter Collection lt T gt  target  IPredicate lt T gt  predicate        Collection lt T gt  result   new ArrayList lt T gt         for  T element  target            if  predicate apply element                 result add element                       return result      So  assuming that you have the use of the above might be  Predicate lt User gt  isAuthorized   new Predicate lt User gt          public boolean apply User user               binds a boolean method in User to a reference         return user isAuthorized                allUsers is a Collection lt User gt  Collection lt User gt  authorizedUsers   filter allUsers  isAuthorized     If performance on the linear check is of concern  then I might want to have a domain object that has the target collection  The domain object that has the target collection would have filtering logic for the methods that initialize  add and set the target collection   UPDATE    In the utility class  let s say Predicate   I have added a select method with an option for default value when the predicate doesn t return the expected value  and also a static property for params to be used inside the new IPredicate   public class Predicate       public static Object predicateParams       public static  lt T gt  Collection lt T gt  filter Collection lt T gt  target  IPredicate lt T gt  predicate            Collection lt T gt  result   new ArrayList lt T gt             for  T element   target                if  predicate apply element                     result add element                                   return result             public static  lt T gt  T select Collection lt T gt  target  IPredicate lt T gt  predicate            T result   null          for  T element   target                if   predicate apply element                   continue              result   element              break                    return result             public static  lt T gt  T select Collection lt T gt  target  IPredicate lt T gt  predicate  T defaultValue            T result   defaultValue          for  T element   target                if   predicate apply element                   continue              result   element              break                    return result            The following example looks for missing objects between collections   List lt MyTypeA gt  missingObjects    List lt MyTypeA gt   Predicate filter myCollectionOfA      new IPredicate lt MyTypeA gt              public boolean apply MyTypeA objectOfA                Predicate predicateParams   objectOfA getName                return Predicate select myCollectionB  new IPredicate lt MyTypeB gt                      public boolean apply MyTypeB objectOfB                        return objectOfB getName   equals Predicate predicateParams toString                                         null                      The following example  looks for an instance in a collection  and returns the first element of the collection as default value when the instance is not found   MyType myObject   Predicate select collectionOfMyType  new IPredicate lt MyType gt      public boolean apply MyType objectOfMyType        return objectOfMyType isDefault        collectionOfMyType get 0      UPDATE  after Java 8 release    It s been several years since I  Alan  first posted this answer  and I still cannot believe I am collecting SO points for this answer  At any rate  now that Java 8 has introduced closures to the language  my answer would now be considerably different  and simpler  With Java 8  there is no need for a distinct static utility class  So if you want to find the 1st element that matches your predicate   final UserService userService          perhaps injected IoC final Optional lt UserModel gt  userOption   userCollection stream   filter u - gt        boolean isAuthorized   userService isAuthorized u       return isAuthorized     findFirst      The JDK 8 API for optionals has the ability to get    isPresent    orElse defaultUser   orElseGet userSupplier  and orElseThrow exceptionSupplier   as well as other  monadic  functions such as map  flatMap and filter   If you want to simply collect all the users which match the predicate  then use the Collectors to terminate the stream in the desired collection   final UserService userService          perhaps injected IoC final List lt UserModel gt  userOption   userCollection stream   filter u - gt        boolean isAuthorized   userService isAuthorized u       return isAuthorized     collect Collectors toList       See here for more examples on how Java 8 streams work

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With the ForEach DSL you may write  import static ch akuhn util query Query select  import static ch akuhn util query Query  result  import ch akuhn util query Select   Collection lt String gt  collection        for  Select lt String gt  each   select collection         each yield   each value length    gt  3     Collection lt String gt  result    result      Given a collection of  The  quick  brown  fox  jumps  over  the  lazy  dog  this results in  quick  brown  jumps  over  lazy   ie all strings longer than three characters   All iteration styles supported by the ForEach DSL are   AllSatisfy AnySatisfy Collect Counnt CutPieces Detect GroupedBy IndexOf InjectInto Reject Select   For more details  please refer to https   www iam unibe ch scg svn repos Sources ForEach

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This  combined with the lack of real closures  is my biggest gripe for Java  Honestly  most of the methods mentioned above are pretty easy to read and REALLY efficient  however  after spending time with  Net  Erlang  etc    list comprehension integrated at the language level makes everything so much cleaner  Without additions at the language level  Java just cant be as clean as many other languages in this area   If performance is a huge concern  Google collections is the way to go  or write your own simple predicate utility   Lambdaj syntax is more readable for some people  but it is not quite as efficient   And then there is a library I wrote  I will ignore any questions in regard to its efficiency  yea  its that bad        Yes  i know its clearly reflection based  and no I don t actually use it  but it does work   LinkedList lt Person gt  list          LinkedList lt Person gt  filtered               Query from list  where Condition ensure  age   Op GTE  21      OR  LinkedList lt Person gt  list        LinkedList lt Person gt  filtered   Query from list  where  x   gt  x age  gt   21

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With Guava   Collection lt Integer gt  collection   Lists newArrayList 1  2  3  4  5    Iterators removeIf collection iterator    new Predicate lt Integer gt           Override     public boolean apply Integer i            return i   2    0             System out println collection      Prints 1  3  5

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Use CollectionUtils filter Collection Predicate   from Apache Commons

[java] What is the best way to filter a Java Collection?

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