Modifying Objects within stream in Java8 while iterating

Question

In Java8 streams  am I allowed to modify update objects within  For eg  List lt User gt  users   users stream   forEach u - gt  u setProperty  value

User · Answer

Instead of creating strange things  you can just filter   and then map   your result   This is much more readable and sure  Streams will make it in only one loop

User · Answer

Yes  you can modify or update the values of objects in the list in your case likewise  users stream   forEach u - gt  u setProperty  quot some value quot     However  the above statement will make updates on the source objects  Which may not be acceptable in most cases  Luckily  we do have another way like  List lt Users gt  updatedUsers   users stream   map u - gt  u setProperty  quot some value quot    collect Collectors toList      Which returns an updated list back  without hampering the old one

User · Answer

The functional way would imho be   import static java util stream Collectors toList  import java util Arrays  import java util List  import java util function Predicate   public class PredicateTestRun        public static void main String   args             List lt String gt  lines   Arrays asList  a    b    c            System out println lines       a  b  c          Predicate lt   super String gt  predicate   value - gt   b  equals value           lines   lines stream   filter predicate negate    collect toList              System out println lines       a  c            In this solution the original list is not modified  but should contain your expected result in a new list that is accessible under the same variable as the old one

User · Answer

This might be a little late  But here is one of the usage  This to get the count of the number of files    Create a pointer to memory  a new obj in this case  and have the property of the object modified  Java 8 stream doesn t allow to modify the pointer itself and hence if you declare just count as a variable and try to increment within the stream it will never work and throw a compiler exception in the first place  Path path   Paths get   Users XXXX static test txt       Count c   new Count                c setCount 0               Files lines path  forEach item - gt                    c setCount c getCount   1                   System out println item                  System out println  line count   c    public static class Count          private int count           public int getCount                 return count                     public void setCount int count                this count   count                      Override         public String toString                 return  Count  count     count

User · Answer

As it was mentioned before - you can t modify original list  but you can stream  modify and collect items into new list  Here is simple example how to modify string element   public class StreamTest         Test     public void replaceInsideStream              List lt String gt  list   Arrays asList  test1    test2 attr    test3            List lt String gt  output   list stream   map value - gt  value replace   attr        collect Collectors toList             System out println  Output      output      Output   test1  test2  test3

User · Answer

To do structural modification on the source of the stream  as Pshemo mentioned in his answer  one solution is to create a new instance of a Collection like ArrayList with the items inside your primary list  iterate over the new list  and do the operations on the primary list   new ArrayList lt  gt  users  stream   forEach u - gt  users remove u

User · Answer

Yes  you can modify state of objects inside your stream  but most often you should avoid modifying state of source of stream  From non-interference section of stream package documentation we can read that   For most data sources  preventing interference means ensuring that the data source is not modified at all during the execution of the stream pipeline  The notable exception to this are streams whose sources are concurrent collections  which are specifically designed to handle concurrent modification  Concurrent stream sources are those whose Spliterator reports the CONCURRENT characteristic   So this is OK   List lt User gt  users   getUsers      users stream   forEach u - gt  u setProperty value                                                 but this in most cases is not   users stream   forEach u - gt  users remove u                                                and may throw ConcurrentModificationException or even other unexpected exceptions like NPE  List lt Integer gt  list   IntStream range 0  10  boxed   collect Collectors toList      list stream        filter i - gt  i  gt  5       forEach i - gt  list remove i       throws NullPointerException

User · Answer

To get rid from ConcurrentModificationException Use CopyOnWriteArrayList

User · Answer

You can make use of the removeIf to remove data from a list conditionally   Eg - If you want to remove all even numbers from a list  you can do it as follows       final List lt Integer gt  list   IntStream range 1 100  boxed   collect Collectors toList          list removeIf number - gt  number   2    0

[java] Modifying Objects within stream in Java8 while iterating

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