Modifying local variable from inside lambda

Question

Modifying a local variable in forEach gives a compile error   Normal      int ordinal   0      for  Example s   list            s setOrdinal ordinal           ordinal            With Lambda      int ordinal   0      list forEach s - gt            s setOrdinal ordinal           ordinal              Any idea how to resolve this

User · Answer

If you are on Java 10  you can use var for that   var ordinal   new Object     int value     list forEach s - gt        s setOrdinal ordinal value       ordinal value

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Yes  you can modify local variables from inside lambdas  in the way shown by the other answers   but you should not do it  Lambdas have been made for functional style of programming and this means  No side effects  What you want to do is considered bad style  It is also dangerous in case of parallel streams  You should either find a solution without side effects or use a traditional for loop

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Use a wrapper Any kind of wrapper is good  With Java 8   use either an AtomicInteger  AtomicInteger ordinal   new AtomicInteger 0   list forEach s - gt      s setOrdinal ordinal getAndIncrement              or an array  int   ordinal     0    list forEach s - gt      s setOrdinal ordinal 0           With Java 10   var wrapper   new Object    int ordinal   0     list forEach s - gt      s setOrdinal wrapper ordinal          Note  be very careful if you use a parallel stream  You might not end up with the expected result  Other solutions like Stuart s might be more adapted for those cases  For types other than int Of course  this is still valid for types other than int  You only need to change the wrapping type to an AtomicReference or an array of that type  For instance  if you use a String  just do the following  AtomicReference lt String gt  value   new AtomicReference lt  gt     list forEach s - gt      value set  quot blah quot         Use an array  String   value     null    list forEach s- gt      value 0     quot blah quot        Or with Java 10   var wrapper   new Object    String value     list forEach s- gt     wrapper value    quot blah quot

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An alternative to AtomicInteger is to use an array  or any other object able to store a value   final int ordinal     new int     0    list forEach   s - gt  s setOrdinal   ordinal  0           But see the Stuart s answer  there might be a better way to deal with your case

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To have a more general solution  you can write a generic Wrapper class   public static class Wrapper lt T gt        public T obj      public Wrapper T obj    this obj   obj          Wrapper lt Integer gt  w   new Wrapper lt  gt  0   this forEach s - gt        s setOrdinal w obj       w obj           this is a variant of the solution given by Almir Campos    In the specific case this is not a good solution  as Integer is worse than int for your purpose  anyway this solution is more general I think

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This is fairly close to an XY problem  That is  the question being asked is essentially how to mutate a captured local variable from a lambda  But the actual task at hand is how to number the elements of a list   In my experience  upward of 80  of the time there is a question of how to mutate a captured local from within a lambda  there s a better way to proceed  Usually this involves reduction  but in this case the technique of running a stream over the list indexes applies well   IntStream range 0  list size              forEach i - gt  list get i  setOrdinal i

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You can wrap it up to workaround the compiler but please remember that side effects in lambdas are discouraged    To quote the javadoc      Side-effects in behavioral parameters to stream operations are  in general  discouraged  as they can often lead to unwitting violations of the statelessness requirement   A small number of stream operations  such as forEach   and peek    can operate only via side-effects  these should be used with care

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I had a slightly different problem   Instead of incrementing a local variable in the forEach  I needed to assign an object to the local variable   I solved this by defining a private inner domain class that wraps both the list I want to iterate over  countryList  and the output I hope to get from that list  foundCountry    Then using Java 8  forEach   I iterate over the list field  and when the object I want is found  I assign that object to the output field   So this assigns a value to a field of the local variable  not changing the local variable itself   I believe that since the local variable itself is not changed  the compiler doesn t complain   I can then use the value that I captured in the output field  outside of the list    Domain Object   public class Country        private int id      private String countryName       public Country int id  String countryName           this id   id          this countryName   countryName             public int getId             return id             public void setId int id            this id   id             public String getCountryName             return countryName             public void setCountryName String countryName            this countryName   countryName            Wrapper object    private class CountryFound      private final List lt Country gt  countryList      private Country foundCountry      public CountryFound List lt Country gt  countryList  Country foundCountry           this countryList   countryList          this foundCountry   foundCountry            public List lt Country gt  getCountryList             return countryList            public void setCountryList List lt Country gt  countryList            this countryList   countryList            public Country getFoundCountry             return foundCountry            public void setFoundCountry Country foundCountry            this foundCountry   foundCountry            Iterate operation   int id   5  CountryFound countryFound   new CountryFound countryList  null   countryFound getCountryList   forEach c - gt        if c getId      id           countryFound setFoundCountry c             System out println  Country found      countryFound getFoundCountry   getCountryName       You could remove the wrapper class method  setCountryList    and make the field  countryList  final  but I did not get compilation errors leaving these details as-is

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As the used variables from outside the lamda have to be  implicitly  final  you have to use something like AtomicInteger or write your own data structure   See https   docs oracle com javase tutorial java javaOO lambdaexpressions html accessing-local-variables

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I know that s an old question  but if you are comfortable with a workaround  considering the fact that the external variable should be final  you can simply do this   final int   ordinal   new int 1   list forEach s - gt        s setOrdinal ordinal 0        ordinal 0           Maybe not the most elegant  or even the most correct  but it will do

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If you only need to pass the value from the outside into the lambda  and not get it out  you can do it with a regular anonymous class instead of a lambda   list forEach new Consumer lt Example gt          int ordinal   0      public void accept Example s            s setOrdinal ordinal           ordinal

[java] Modifying local variable from inside lambda

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