How should we manage jdk8 stream for null values

Question

Hello fellow Java developers  I know the subject may be a bit in advance as the JDK8 is not yet released  and not for now anyway    but I was reading some articles about the Lambda expressions and particularly the part related to the new collection API known as Stream  Here is the example as given in the Java Magazine article  it is an otter population algorithm     Set lt Otter gt  otters   getOtters    System out println otters stream        filter o - gt   o isWild         map o - gt  o getKeeper         filter k - gt  k isFemale         into new ArrayList lt  gt          size      My question is what happen if in the middle of the Set internal iteration  one of the otter is null  I would expect a NullPointerException to be thrown but maybe am I still stuck in the previous development paradigm  non-functional   can someone enlighten me as how this should be handled  If this really throw a NullPointerException  I find the feature quite dangerous and will have to be used only as below   Developer to ensure there is no null value  maybe using a previous  filter o - gt  o    null   Developer to ensure the application is never generating null otter or a special NullOtter object to deal with   What is the best option  or any other option  Thanks

User · Answer

Stuart s answer provides a great explanation  but I d like to provide another example   I ran into this issue when attempting to perform a reduce on a Stream containing null values  actually it was LongStream average    which is a type of reduction   Since average   returns OptionalDouble  I assumed the Stream could contain nulls but instead a NullPointerException was thrown  This is due to Stuart s explanation of null v  empty   So  as the OP suggests  I added a filter like so   list stream        filter o - gt  o    null       reduce        Or as tangens pointed out below  use the predicate provided by the Java API   list stream        filter Objects  nonNull       reduce        From the mailing list discussion Stuart linked  Brian Goetz on nulls in Streams

User · Answer

If you just want to filter null values out of a stream  you can simply use a method reference to java util Objects nonNull Object   From its documentation      This method exists to be used as a Predicate  filter Objects  nonNull    For example   List lt String gt  list   Arrays asList  null   Foo   null   Bar   null  null    list stream        filter  Objects  nonNull        lt -- Filter out null values      forEach  System out  println      This will print   Foo Bar

User · Answer

Although the answers are 100  correct  a small suggestion to improve null case handling of the list itself with Optional    List lt String gt  listOfStuffFiltered   Optional ofNullable listOfStuff                   orElseGet Collections  emptyList                   stream                    filter Objects  nonNull                   collect Collectors toList       The part Optional ofNullable listOfStuff  orElseGet Collections  emptyList  will allow you to handle nicely the case when listOfStuff is null and return an emptyList instead of failing with NullPointerException

User · Answer

An example how to avoid null e g  use filter before groupingBy  Filter out the null instances before groupingBy  Here is an example  MyObjectlist stream                filter p - gt  p getSomeInstance      null               collect Collectors groupingBy MyObject  getSomeInstance

User · Answer

Current thinking seems to be to  tolerate  nulls  that is  to allow them in general  although some operations are less tolerant and may end up throwing NPE  See the discussion of nulls on the Lambda Libraries expert group mailing list  specifically this message  Consensus around option  3 subsequently emerged  with a notable objection from Doug Lea   So yes  the OP s concern about pipelines blowing up with NPE is valid   It s not for nothing that Tony Hoare referred to nulls as the  Billion Dollar Mistake   Dealing with nulls is a real pain  Even with classic collections  without considering lambdas or streams  nulls are problematic  As fge mentioned in a comment  some collections allow nulls and others do not  With collections that allow nulls  this introduces ambiguities into the API  For example  with Map get    a null return indicates either that the key is present and its value is null  or that the key is absent  One has to do extra work to disambiguate these cases   The usual use for null is to denote the absence of a value  The approach for dealing with this proposed for Java SE 8 is to introduce a new java util Optional type  which encapsulates the presence absence of a value  along with behaviors of supplying a default value  or throwing an exception  or calling a function  etc  if the value is absent  Optional is used only by new APIs  though  everything else in the system still has to put up with the possibility of nulls   My advice is to avoid actual null references to the greatest extent possible  It s hard to see from the example given how there could be a  null  Otter  But if one were necessary  the OP s suggestions of filtering out null values  or mapping them to a sentinel object  the Null Object Pattern  are fine approaches

[java] How should we manage jdk8 stream for null values

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