Is it possible to cast a Stream in Java 8

Question

Is it possible to cast a stream in Java 8  Say I have a list of objects  I can do something like this to filter out all the additional objects   Stream of objects  filter c - gt  c instanceof Client    After this though  if I want to do something with the clients I would need to cast each of them   Stream of objects  filter c - gt  c instanceof Client       map c - gt    Client  c  getID    forEach System out  println     This looks a little ugly  Is it possible to cast an entire stream to a different type  Like cast Stream lt Object gt  to a Stream lt Client gt    Please ignore the fact that doing things like this would probably mean bad design  We do stuff like this in my computer science class  so I was looking into the new features of java 8 and was curious if this was possible

User · Answer

This looks a little ugly  Is it possible to cast an entire stream to a different type  Like cast Stream lt Object gt  to a Stream lt Client gt     No that wouldn t be possible  This is not new in Java 8  This is specific to generics  A List lt Object gt  is not a super type of List lt String gt   so you can t just cast a List lt Object gt  to a List lt String gt     Similar is the issue here  You can t cast Stream lt Object gt  to Stream lt Client gt   Of course you can cast it indirectly like this   Stream lt Client gt  intStream    Stream lt Client gt    Stream lt   gt  stream    but that is not safe  and might fail at runtime  The underlying reason for this is  generics in Java are implemented using erasure  So  there is no type information available about which type of Stream it is at runtime  Everything is just Stream   BTW  what s wrong with your approach  Looks fine to me

User · Answer

Late to the party  but I think it is a useful answer   flatMap would be the shortest way to do it   Stream of objects  flatMap o- gt  o instanceof Client  Stream of  Client o  Stream empty      If o is a Client then create a Stream with a single element  otherwise use the empty stream  These streams will then be flattened into a Stream lt Client gt

User · Answer

I don t think there is a way to do that out-of-the-box  A possibly cleaner solution would be   Stream of objects       filter c - gt  c instanceof Client       map c - gt   Client  c       map Client  getID       forEach System out  println     or  as suggested in the comments  you could use the cast method - the former may be easier to read though   Stream of objects       filter Client class  isInstance       map Client class  cast       map Client  getID       forEach System out  println

User · Answer

Along the lines of ggovan s answer  I do this as follows          Provides various high-order functions      public final class F                  When the returned   code Function  is passed as an argument to          link Stream flatMap   the result is a stream of instances of          code cls               public static  lt E gt  Function lt Object  Stream lt E gt  gt  instancesOf Class lt E gt  cls            return o - gt  cls isInstance o                    Stream of cls cast o                     Stream empty              Using this helper function   Stream of objects  flatMap F instancesOf Client class            map Client  getId           forEach System out  println

[java] Is it possible to cast a Stream in Java 8?

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