Java 8 NullPointerException in Collectors toMap

Question

The Java 8 Collectors toMap throws a NullPointerException if one of the values is  null   I don t understand this behaviour  maps can contain null pointers as value without any problems  Is there a good reason why values cannot be null for Collectors toMap   Also  is there a nice Java 8 way of fixing this  or should I revert to plain old for loop   An example of my problem   import java util ArrayList  import java util List  import java util Map  import java util stream Collectors    class Answer       private int id       private Boolean answer       Answer                Answer int id  Boolean answer            this id   id          this answer   answer             public int getId             return id             public void setId int id            this id   id             public Boolean getAnswer             return answer             public void setAnswer Boolean answer            this answer   answer           public class Main       public static void main String   args            List lt Answer gt  answerList   new ArrayList lt  gt              answerList add new Answer 1  true            answerList add new Answer 2  true            answerList add new Answer 3  null             Map lt Integer  Boolean gt  answerMap           answerList                  stream                    collect Collectors toMap Answer  getId  Answer  getAnswer              Stacktrace   Exception in thread  main  java lang NullPointerException     at java util HashMap merge HashMap java 1216      at java util stream Collectors lambda toMap 168 Collectors java 1320      at java util stream Collectors  Lambda 5 1528902577 accept Unknown Source      at java util stream ReduceOps 3ReducingSink accept ReduceOps java 169      at java util ArrayList ArrayListSpliterator forEachRemaining ArrayList java 1359      at java util stream AbstractPipeline copyInto AbstractPipeline java 512      at java util stream AbstractPipeline wrapAndCopyInto AbstractPipeline java 502      at java util stream ReduceOps ReduceOp evaluateSequential ReduceOps java 708      at java util stream AbstractPipeline evaluate AbstractPipeline java 234      at java util stream ReferencePipeline collect ReferencePipeline java 499      at Main main Main java 48      at sun reflect NativeMethodAccessorImpl invoke0 Native Method      at sun reflect NativeMethodAccessorImpl invoke NativeMethodAccessorImpl java 62      at sun reflect DelegatingMethodAccessorImpl invoke DelegatingMethodAccessorImpl java 43      at java lang reflect Method invoke Method java 483      at com intellij rt execution application AppMain main AppMain java 134    This problem still exists in Java 11

User · Answer

Retaining all questions ids with small tweak  Map lt Integer  Boolean gt  answerMap      answerList stream                collect Collectors toMap Answer  getId  a - gt                          Boolean TRUE equals a getAnswer

User · Answer

NullPointerException is by far the most frequently encountered exception  at least in my case   To avoid this I go defensive and add bunch of null checks and I end up having bloated and ugly code  Java 8 introduces Optional to handle null references so you can define nullable and non-nullable values    That said  I would wrap all the nullable references in Optional container  We should also not break backward compatibility as well  Here is the code   class Answer       private int id      private Optional lt Boolean gt  answer       Answer                Answer int id  Boolean answer            this id   id          this answer   Optional ofNullable answer              public int getId             return id             public void setId int id            this id   id                        Gets the answer which can be a null value  Use   link  getAnswerAsOptional    instead                 return the answer which can be a null value             public Boolean getAnswer                What should be the default value  If we return null the callers will be at higher risk of having NPE         return answer orElse null                         Gets the optional answer                 return the answer which is contained in   code Optional               public Optional lt Boolean gt  getAnswerAsOptional             return answer                        Gets the answer or the supplied default value                 return the answer or the supplied default value              public boolean getAnswerOrDefault boolean defaultValue            return answer orElse defaultValue              public void setAnswer Boolean answer            this answer   Optional ofNullable answer            public class Main       public static void main String   args            List lt Answer gt  answerList   new ArrayList lt  gt              answerList add new Answer 1  true            answerList add new Answer 2  true            answerList add new Answer 3  null                map with optional answers  i e  with null          Map lt Integer  Optional lt Boolean gt  gt  answerMapWithOptionals   answerList stream                    collect Collectors toMap Answer  getId  Answer  getAnswerAsOptional                map in which null values are removed         Map lt Integer  Boolean gt  answerMapWithoutNulls   answerList stream                    filter a - gt  a getAnswerAsOptional   isPresent                     collect Collectors toMap Answer  getId  Answer  getAnswer                map in which null values are treated as false by default         Map lt Integer  Boolean gt  answerMapWithDefaults   answerList stream                    collect Collectors toMap a - gt  a getId    a - gt  a getAnswerOrDefault false              System out println  With Optional      answerMapWithOptionals           System out println  Without Nulls      answerMapWithoutNulls           System out println  Wit Defaults      answerMapWithDefaults

User · Answer

Sorry to reopen an old question  but since it was edited recently saying that the  issue  still remains in Java 11  I felt like I wanted to point out this   answerList          stream            collect Collectors toMap Answer  getId  Answer  getAnswer      gives you the null pointer exception because the map does not allow null as a value  This makes sense because if you look in a map for the key k and it is not present  then the returned value is already null  see javadoc   So if you were able to put in k the value null  the map would look like it s behaving oddly   As someone said in the comments  it s pretty easy to solve this by using filtering   answerList          stream            filter a - gt  a getAnswer      null           collect Collectors toMap Answer  getId  Answer  getAnswer      in this way no null values will be inserted in the map  and STILL you will get null as the  value  when looking for an id that does not have an answer in the map   I hope this makes sense to everyone

User · Answer

Yep  a late answer from me  but I think it may help to understand what s happening under the hood in case anyone wants to code some other Collector-logic   I tried to solve the problem by coding a more native and straight forward approach  I think it s as direct as possible   public class LambdaUtilities               In contrast to   link Collectors toMap Function  Function   the result map      may have null values          public static  lt T  K  U  M extends Map lt K  U gt  gt  Collector lt T  M  M gt  toMapWithNullValues Function lt   super T    extends K gt  keyMapper  Function lt   super T    extends U gt  valueMapper        return toMapWithNullValues keyMapper  valueMapper  HashMap  new                   In contrast to   link Collectors toMap Function  Function  BinaryOperator  Supplier        the result map may have null values          public static  lt T  K  U  M extends Map lt K  U gt  gt  Collector lt T  M  M gt  toMapWithNullValues Function lt   super T    extends K gt  keyMapper  Function lt   super T    extends U gt  valueMapper  Supplier lt Map lt K  U gt  gt  supplier        return new Collector lt T  M  M gt              Override       public Supplier lt M gt  supplier             return    - gt               SuppressWarnings  unchecked             M map    M  supplier get              return map                             Override       public BiConsumer lt M  T gt  accumulator             return  map  element  - gt              K key   keyMapper apply element             if  map containsKey key                 throw new IllegalStateException  Duplicate key     key                         map put key  valueMapper apply element                               Override       public BinaryOperator lt M gt  combiner             return  left  right  - gt              int total   left size     right size              left putAll right             if  left size    lt  total                throw new IllegalStateException  Duplicate key s                           return left                             Override       public Function lt M  M gt  finisher             return Function identity                    Override       public Set lt Collector Characteristics gt  characteristics             return Collections unmodifiableSet EnumSet of Collector Characteristics IDENTITY FINISH                             And the tests using JUnit and assertj      Test   public void testToMapWithNullValues   throws Exception       Map lt Integer  Integer gt  result   Stream of 1  2  3           collect LambdaUtilities toMapWithNullValues Function identity    x - gt  x   2    1   x   null         assertThat result           isExactlyInstanceOf HashMap class           hasSize 3           containsEntry 1  1           containsEntry 2  null           containsEntry 3  3           Test   public void testToMapWithNullValuesWithSupplier   throws Exception       Map lt Integer  Integer gt  result   Stream of 1  2  3           collect LambdaUtilities toMapWithNullValues Function identity    x - gt  x   2    1   x   null  LinkedHashMap  new         assertThat result           isExactlyInstanceOf LinkedHashMap class           hasSize 3           containsEntry 1  1           containsEntry 2  null           containsEntry 3  3           Test   public void testToMapWithNullValuesDuplicate   throws Exception       assertThatThrownBy    - gt  Stream of 1  2  3  1           collect LambdaUtilities toMapWithNullValues Function identity    x - gt  x   2    1   x   null                 isExactlyInstanceOf IllegalStateException class               hasMessage  Duplicate key 1            Test   public void testToMapWithNullValuesParallel   throws Exception       Map lt Integer  Integer gt  result   Stream of 1  2  3           parallel      this causes  combiner   to be called          collect LambdaUtilities toMapWithNullValues Function identity    x - gt  x   2    1   x   null         assertThat result           isExactlyInstanceOf HashMap class           hasSize 3           containsEntry 1  1           containsEntry 2  null           containsEntry 3  3           Test   public void testToMapWithNullValuesParallelWithDuplicates   throws Exception       assertThatThrownBy    - gt  Stream of 1  2  3  1  2  3           parallel      this causes  combiner   to be called          collect LambdaUtilities toMapWithNullValues Function identity    x - gt  x   2    1   x   null                 isExactlyInstanceOf IllegalStateException class               hasCauseExactlyInstanceOf IllegalStateException class               hasStackTraceContaining  Duplicate key          And how do you use it  Well  just use it instead of toMap   like the tests show  This makes the calling code look as clean as possible   EDIT  implemented Holger s idea below  added a test method

User · Answer

I wrote a Collector which  unlike the default java one  does not crash when you have null values   public static  lt T  K  U gt          Collector lt T     Map lt K  U gt  gt  toMap Function lt   super T    extends K gt  keyMapper                  Function lt   super T    extends U gt  valueMapper        return Collectors collectingAndThen              Collectors toList                list - gt                    Map lt K  U gt  result   new HashMap lt  gt                     for  T item   list                        K key   keyMapper apply item                       if  result putIfAbsent key  valueMapper apply item      null                            throw new IllegalStateException String format  Duplicate key  s   key                                                            return result                      Just replace your Collectors toMap   call to a call to this function and it ll fix the problem

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Here s somewhat simpler collector than proposed by  EmmanuelTouzery  Use it if you like   public static  lt T  K  U gt  Collector lt T     Map lt K  U gt  gt  toMapNullFriendly          Function lt   super T    extends K gt  keyMapper          Function lt   super T    extends U gt  valueMapper         SuppressWarnings  unchecked       U none    U  new Object        return Collectors collectingAndThen              Collectors  lt T  K  U gt  toMap keyMapper                      valueMapper andThen v - gt  v    null   none   v    map - gt                    map replaceAll  k  v  - gt  v    none   null   v                   return map                      We just replace null with some custom object none and do the reverse operation in the finisher

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public static  lt T  K  V gt  Collector lt T  HashMap lt K  V gt   HashMap lt K  V gt  gt  toHashMap          Function lt   super T    extends K gt  keyMapper          Function lt   super T    extends V gt  valueMapper         return Collector of              HashMap  new               map  t  - gt  map put keyMapper apply t   valueMapper apply t                 map1  map2  - gt                    map1 putAll map2                   return map1                          public static  lt T  K gt  Collector lt T  HashMap lt K  T gt   HashMap lt K  T gt  gt  toHashMap          Function lt   super T    extends K gt  keyMapper         return toHashMap keyMapper  Function identity

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For completeness  I m posting a version of the toMapOfNullables with a mergeFunction param  public static  lt T  K  U gt  Collector lt T     Map lt K  U gt  gt  toMapOfNullables Function lt   super T    extends K gt  keyMapper  Function lt   super T    extends U gt  valueMapper  BinaryOperator lt U gt  mergeFunction        return Collectors collectingAndThen Collectors toList    list - gt            Map lt K  U gt  result   new HashMap lt  gt             for T item   list                K key   keyMapper apply item               U newValue   valueMapper apply item               U value   result containsKey key    mergeFunction apply result get key   newValue    newValue              result put key  value                     return result

User · Answer

It is not possible with the static methods of Collectors  The javadoc of toMap explains that toMap is based on Map merge       param mergeFunction a merge function  used to resolve collisions between values associated with the same key  as supplied to Map merge Object  Object  BiFunction     and the javadoc of Map merge says        throws NullPointerException if the specified key is null and this map   does not support null keys or the value or remappingFunction is   null   You can avoid the for loop by using the forEach method of your list   Map lt Integer   Boolean gt  answerMap   new HashMap lt  gt     answerList forEach  answer  - gt  answerMap put answer getId    answer getAnswer        but it is not really simple than the old way   Map lt Integer  Boolean gt  answerMap   new HashMap lt  gt     for  Answer answer   answerList        answerMap put answer getId    answer getAnswer

User · Answer

I have slightly modified Emmanuel Touzery s implementation   This version    Allows null keys Allows null values Detects duplicate keys  even if they are null  and throws IllegalStateException as in the original JDK implementation  Detects duplicate keys also when the key already mapped to the null value  In other words  separates a mapping with null-value from no-mapping    public static  lt T  K  U gt  Collector lt T     Map lt K  U gt  gt  toMapOfNullables Function lt   super T    extends K gt  keyMapper  Function lt   super T    extends U gt  valueMapper        return Collectors collectingAndThen          Collectors toList            list - gt                Map lt K  U gt  map   new LinkedHashMap lt  gt                 list forEach item - gt                    K key   keyMapper apply item                   if  map containsKey key                         throw new IllegalStateException String format  Duplicate key  s   key                                      map put key  valueMapper apply item                                return map                       Unit tests    Test public void toMapOfNullables WhenHasNullKey         assertEquals singletonMap null   value            Stream of  ignored   collect Utils toMapOfNullables i - gt  null  i - gt   value               Test public void toMapOfNullables WhenHasNullValue         assertEquals singletonMap  key   null           Stream of  ignored   collect Utils toMapOfNullables i - gt   key   i - gt  null              Test public void toMapOfNullables WhenHasDuplicateNullKeys         assertThrows new IllegalStateException  Duplicate key null               - gt  Stream of 1  2  3  collect Utils toMapOfNullables i - gt  null  i - gt  i              Test public void toMapOfNullables WhenHasDuplicateKeys NoneHasNullValue         assertThrows new IllegalStateException  Duplicate key duplicated-key               - gt  Stream of 1  2  3  collect Utils toMapOfNullables i - gt   duplicated-key   i - gt  i              Test public void toMapOfNullables WhenHasDuplicateKeys OneHasNullValue         assertThrows new IllegalStateException  Duplicate key duplicated-key               - gt  Stream of 1  null  3  collect Utils toMapOfNullables i - gt   duplicated-key   i - gt  i              Test public void toMapOfNullables WhenHasDuplicateKeys AllHasNullValue         assertThrows new IllegalStateException  Duplicate key duplicated-key               - gt  Stream of null  null  null  collect Utils toMapOfNullables i - gt   duplicated-key   i - gt  i

User · Answer

You can work around this known bug in OpenJDK with this  Map lt Integer  Boolean gt  collect   list stream            collect HashMap  new   m v - gt m put v getId    v getAnswer     HashMap  putAll    It is not that much pretty  but it works  Result  1  true 2  true 3  null   this tutorial helped me the most   EDIT  Unlike Collectors toMap  this will silently replace values if you have the same key multiple times  as  mmdemirbas pointed out in the comments  If you don t want this  look at the link in the comment

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If the value is a String  then this might work   map entrySet   stream   collect Collectors toMap e - gt  e getKey    e - gt   Optional ofNullable e getValue    orElse

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According to the Stacktrace  Exception in thread  main  java lang NullPointerException at java util HashMap merge HashMap java 1216  at java util stream Collectors lambda toMap 148 Collectors java 1320  at java util stream Collectors  Lambda 5 391359742 accept Unknown Source  at java util stream ReduceOps 3ReducingSink accept ReduceOps java 169  at java util ArrayList ArrayListSpliterator forEachRemaining ArrayList java 1359  at java util stream AbstractPipeline copyInto AbstractPipeline java 512  at java util stream AbstractPipeline wrapAndCopyInto AbstractPipeline java 502  at java util stream ReduceOps ReduceOp evaluateSequential ReduceOps java 708  at java util stream AbstractPipeline evaluate AbstractPipeline java 234  at java util stream ReferencePipeline collect ReferencePipeline java 499  at com guice Main main Main java 28  at sun reflect NativeMethodAccessorImpl invoke0 Native Method  at sun reflect NativeMethodAccessorImpl invoke NativeMethodAccessorImpl java 62  at sun reflect DelegatingMethodAccessorImpl invoke DelegatingMethodAccessorImpl java 43  at java lang reflect Method invoke Method java 483  at com intellij rt execution application AppMain main AppMain java 134    When is called the map merge          BiConsumer lt M  T gt  accumulator                map  element  - gt  map merge keyMapper apply element                                             valueMapper apply element   mergeFunction     It will do a null check as first thing  if  value    null      throw new NullPointerException      I don t use Java 8 so often so i don t know if there are a better way to fix it  but fix it is a bit hard   You could do   Use filter to filter all NULL values  and in the Javascript code check if the server didn t send any answer for this id means that he didn t reply to it   Something like this   Map lt Integer  Boolean gt  answerMap           answerList                  stream                    filter  a  - gt  a getAnswer      null                   collect Collectors toMap Answer  getId  Answer  getAnswer      Or use peek  which is used to alter the stream element for element  Using peek you could change the answer to something more acceptable for map but it means edit your logic a bit   Sounds like if you want to keep the current design you should avoid Collectors toMap

[java] Java 8 NullPointerException in Collectors.toMap

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