How to use if-else logic in Java 8 stream forEach

Question

What I want to do is shown below in 2 stream calls  I want to split a collection into 2 new collections based on some condition  Ideally I want to do it in 1  I ve seen conditions used for the  map function of streams  but couldn t find anything for the forEach  What is the best way to achieve what I want       animalMap entrySet   stream                filter pair- gt  pair getValue      null               forEach pair- gt  myMap put pair getKey    pair getValue           animalMap entrySet   stream                filter pair- gt  pair getValue      null               forEach pair- gt  myList add pair getKey

User · Answer

The problem by using stream   forEach     with a call to add or put inside the forEach  so you mutate the external myMap or myList instance  is that you can run easily into concurrency issues if someone turns the stream in parallel and the collection you are modifying is not thread safe   One approach you can take is to first partition the entries in the original map  Once you have that  grab the corresponding list of entries and collect them in the appropriate map and list   Map lt Boolean  List lt Map Entry lt K  V gt  gt  gt  partitions       animalMap entrySet                 stream                 collect partitioningBy e - gt  e getValue      null     Map lt K  V gt  myMap        partitions get false                 stream                  collect toMap Map Entry  getKey  Map Entry  getValue     List lt K gt  myList       partitions get true                 stream                  map Map Entry  getKey                  collect toList           or if you want to do it in one pass  implement a custom collector  assuming a Tuple2 lt E1  E2 gt  class exists  you can create your own   e g   public static  lt K V gt  Collector lt Map Entry lt K  V gt      Tuple2 lt Map lt K  V gt   List lt K gt  gt  gt  customCollector         return Collector of                 - gt  new Tuple2 lt  gt  new HashMap lt  gt     new ArrayList lt  gt                   pair  entry  - gt                    if entry getValue      null                        pair  2 add entry getKey                       else                       pair  1 put entry getKey    entry getValue                                                   p1  p2  - gt                    p1  1 putAll p2  1                   p1  2 addAll p2  2                   return p1                      with its usage   Tuple2 lt Map lt K  V gt   List lt K gt  gt  pair        animalMap entrySet   parallelStream   collect customCollector       You can tune it more if you want  for example by providing a predicate as parameter

User · Answer

I think it s possible in Java 9   animalMap entrySet   stream                    forEach                          pair - gt  Optional ofNullable pair getValue                                     ifPresentOrElse v - gt  myMap put pair getKey    v   v - gt  myList add pair getKey                           Need the ifPresentOrElse for it to work though    I think a for loop looks better

User · Answer

In most cases  when you find yourself using forEach on a Stream  you should rethink whether you are using the right tool for your job or whether you are using it the right way   Generally  you should look for an appropriate terminal operation doing what you want to achieve or for an appropriate Collector  Now  there are Collectors for producing Maps and Lists  but no out of-the-box collector for combining two different collectors  based on a predicate   Now  this answer contains a collector for combining two collectors  Using this collector  you can achieve the task as  Pair lt Map lt KeyType  Animal gt   List lt KeyType gt  gt  pair   animalMap entrySet   stream        collect conditional entry - gt  entry getValue      null              Collectors toMap Map Entry  getKey  Map Entry  getValue               Collectors mapping Map Entry  getKey  Collectors toList        Map lt KeyType Animal gt  myMap   pair a  List lt KeyType gt  myList   pair b      But maybe  you can solve this specific task in a simpler way  One of you results matches the input type  it   s the same map just stripped off the entries which map to null  If your original map is mutable and you don   t need it afterwards  you can just collect the list and remove these keys from the original map as they are mutually exclusive   List lt KeyType gt  myList animalMap entrySet   stream        filter pair - gt  pair getValue      null       map Map Entry  getKey       collect Collectors toList      animalMap keySet   removeAll myList     Note that you can remove mappings to null even without having the list of the other keys   animalMap values   removeIf Objects  isNull     or  animalMap values   removeAll Collections singleton null        If you can   t  or don   t want to  modify the original map  there is still a solution without a custom collector  As hinted in Alexis C    s answer  partitioningBy is going into the right direction  but you may simplify it   Map lt Boolean Map lt KeyType Animal gt  gt  tmp   animalMap entrySet   stream        collect Collectors partitioningBy pair - gt  pair getValue      null                   Collectors toMap Map Entry  getKey  Map Entry  getValue     Map lt KeyType Animal gt  myMap   tmp get true   List lt KeyType gt  myList   new ArrayList lt  gt  tmp get false  keySet       The bottom line is  don   t forget about ordinary Collection operations  you don   t have to do everything with the new Stream API

User · Answer

Just put the condition into the lambda itself  e g   animalMap entrySet   stream            forEach                  pair - gt                        if  pair getValue      null                            myMap put pair getKey    pair getValue                           else                           myList add pair getKey                                                          Of course  this assumes that both collections  myMap and myList  are declared and initialized prior to the above piece of code      Update  using Map forEach makes the code shorter  plus more efficient and readable  as Jorn Vernee kindly suggested        animalMap forEach               key  value  - gt                    if  value    null                        myMap put key  value                     else                       myList add key

[java] How to use if-else logic in Java 8 stream forEach

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