Java 8 lambda get and remove element from list

Question

Given a list of elements  I want to get the element with a given property and remove it from the list  The best solution I found is   ProducerDTO p   producersProcedureActive                  stream                    filter producer - gt  producer getPod   equals pod                    findFirst                    get    producersProcedureActive remove p     Is it possible to combine get and remove in a lambda expression

User · Answer

Although the thread is quite old  still thought to provide solution - using Java8   Make the use of removeIf function  Time complexity is O n   producersProcedureActive removeIf producer - gt  producer getPod   equals pod      API reference  removeIf docs  Assumption  producersProcedureActive is a List  NOTE  With this approach you won t be able to get the hold of the deleted item

User · Answer

Consider using vanilla java iterators to perform the task   public static  lt T gt  T findAndRemoveFirst Iterable lt   extends T gt  collection  Predicate lt   super T gt  test        T value   null      for  Iterator lt   extends T gt  it   collection iterator    it hasNext             if  test test value   it next                   it remove                return value                return null      Advantages    It is plain and obvious  It traverses only once and only up to the matching element  You can do it on any Iterable even without stream   support  at least those implementing remove   on their iterator     Disadvantages    You cannot do it in place as a single expression  auxiliary method or variable required      As for the     Is it possible to combine get and remove in a lambda expression    other answers clearly show that it is possible  but you should be aware of   Search and removal may traverse the list twice ConcurrentModificationException may be thrown when removing element from the list being iterated

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With Eclipse Collections you can use detectIndex along with remove int  on any java util List   List lt Integer gt  integers   Lists mutable with 1  2  3  4  5   int index   Iterate detectIndex integers  i - gt  i  gt  2   if  index  gt  -1        integers remove index      Assert assertEquals Lists mutable with 1  2  4  5   integers     If you use the MutableList type from Eclipse Collections  you can call the detectIndex method directly on the list   MutableList lt Integer gt  integers   Lists mutable with 1  2  3  4  5   int index   integers detectIndex i - gt  i  gt  2   if  index  gt  -1        integers remove index      Assert assertEquals Lists mutable with 1  2  4  5   integers     Note  I am a committer for Eclipse Collections

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To Remove element from the list   objectA removeIf x - gt  conditions     eg   objectA removeIf x - gt  blockedWorkerIds contains x     List lt String gt  str1   new ArrayList lt String gt     str1 add  A    str1 add  B    str1 add  C    str1 add  D     List lt String gt  str2   new ArrayList lt String gt     str2 add  D    str2 add  E     str1 removeIf x - gt  str2 contains x      str1 forEach System out  println     OUTPUT  A B C

User · Answer

When we want to get multiple elements from a List into a new list  filter using a predicate  and remove them from the existing list  I could not find a proper answer anywhere   Here is how we can do it using Java Streaming API partitioning   Map lt Boolean  List lt ProducerDTO gt  gt  classifiedElements   producersProcedureActive      stream        collect Collectors partitioningBy producer - gt  producer getPod   equals pod         get two new lists  List lt ProducerDTO gt  matching   classifiedElements get true   List lt ProducerDTO gt  nonMatching   classifiedElements get false       OR get non-matching elements to the existing list producersProcedureActive   classifiedElements get false     This way you effectively remove the filtered elements from the original list and add them to a new list   Refer the 5 2  Collectors partitioningBy section of this article

User · Answer

I m sure this will be an unpopular answer  but it works     ProducerDTO   p   new ProducerDTO 1   producersProcedureActive              stream                filter producer - gt  producer getPod   equals pod                findFirst                ifPresent producer - gt   producersProcedureActive remove producer   p 0    producer     p 0  will either hold the found element or be null   The  trick  here is circumventing the  effectively final  problem by using an array reference that is effectively final  but setting its first element

User · Answer

the task is  get  and  remove element from list  p stream   collect  Collectors collectingAndThen  Collector of      ArrayDeque  new       a  producer  - gt          if  producer getPod   equals  pod             a addLast  producer                a1  a2  - gt          return  a1               rslt - gt  rslt pollFirst           e  - gt        if  e    null         p remove  e        remove     return  e          get

User · Answer

The below logic is the solution without modifying the original list  List lt String gt  str1   new ArrayList lt String gt     str1 add  A    str1 add  B    str1 add  C    str1 add  D     List lt String gt  str2   new ArrayList lt String gt     str2 add  D    str2 add  E     List lt String gt  str3   str1 stream                            filter item - gt   str2 contains item                            collect Collectors toList      str1      A    B    C    D   str2      D    E   str3      A    B    C

User · Answer

As others have suggested  this might be a use case for loops and iterables  In my opinion  this is the simplest approach  If you want to modify the list in-place  it cannot be considered  real  functional programming anyway  But you could use Collectors partitioningBy   in order to get a new list with elements which satisfy your condition  and a new list of those which don t  Of course with this approach  if you have multiple elements satisfying the condition  all of those will be in that list and not only the first

User · Answer

Combining my initial idea and your answers I reached what seems to be the solution to my own question   public ProducerDTO findAndRemove String pod        ProducerDTO p   null      try           p   IntStream range 0  producersProcedureActive size                  filter i - gt  producersProcedureActive get i  getPod   equals pod                 boxed                 findFirst                 map i - gt  producersProcedureActive remove  int i                 get            logger debug p         catch  NoSuchElementException e            logger error  No producer found with POD      pod                   return p      It lets remove the object using remove int  that do not traverse again the  list  as suggested by  Tunaki  and it lets return the removed object to the function caller   I read your answers that suggest me to choose safe methods like ifPresent instead of get but I do not find a way to use them in this scenario   Are there any important drawback in this kind of solution    Edit following  Holger advice  This should be the function I needed  public ProducerDTO findAndRemove String pod        return IntStream range 0  producersProcedureActive size                 filter i - gt  producersProcedureActive get i  getPod   equals pod                      boxed                                                                                findFirst                map i - gt  producersProcedureActive remove  int i                orElseGet    - gt                    logger error  No producer found with POD      pod                          return null

User · Answer

Use can use filter of Java 8  and create another list if you don t want to change the old list   List lt ProducerDTO gt  result   producersProcedureActive                              stream                                filter producer - gt  producer getPod   equals pod                                collect Collectors toList

User · Answer

The direct solution would be to invoke ifPresent consumer  on the Optional returned by findFirst    This consumer will be invoked when the optional is not empty  The benefit also is that it won t throw an exception if the find operation returned an empty optional  like your current code would do  instead  nothing will happen   If you want to return the removed value  you can map the Optional to the result of calling remove   producersProcedureActive stream                            filter producer - gt  producer getPod   equals pod                            findFirst                            map p - gt                                producersProcedureActive remove p                               return p                                But note that the remove Object  operation will again traverse the list to find the element to remove  If you have a list with random access  like an ArrayList  it would be better to make a Stream over the indexes of the list and find the first index matching the predicate   IntStream range 0  producersProcedureActive size              filter i - gt  producersProcedureActive get i  getPod   equals pod             boxed             findFirst             map i - gt  producersProcedureActive remove  int  i      With this solution  the remove int  operation operates directly on the index

[java] Java 8 lambda get and remove element from list

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