How to negate a method reference predicate

Question

In Java 8  you can use a method reference to filter a stream  for example   Stream lt String gt  s        long emptyStrings   s filter String  isEmpty  count      Is there a way to create a method reference that is the negation of an existing one  i e  something like   long nonEmptyStrings   s filter not String  isEmpty   count      I could create the not method like below but I was wondering if the JDK offered something similar   static  lt T gt  Predicate lt T gt  not Predicate lt T gt  p    return o - gt   p test o

User · Answer

I m planning to static import the following to allow for the method reference to be used inline    public static  lt T gt  Predicate lt T gt  not Predicate lt T gt  t        return t negate        e g   Stream lt String gt  s        long nonEmptyStrings   s filter not String  isEmpty   count      Update  Starting from Java-11  the JDK offers a similar solution built-in as well

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Predicate not         java-11 offers a new method Predicate not  So you can negate the method reference   Stream lt String gt  s        long nonEmptyStrings   s filter Predicate not String  isEmpty   count

User · Answer

Predicate has methods and  or and negate   However  String  isEmpty is not a Predicate  it s just a String - gt  Boolean lambda and it could still become anything  e g  Function lt String  Boolean gt   Type inference is what needs to happen first  The filter method infers type implicitly  But if you negate it before passing it as an argument  it no longer happens  As  axtavt mentioned  explicit inference can be used as an ugly way   s filter   Predicate lt String gt   String  isEmpty  negate    count     There are other ways advised in other answers  with static not method and lambda most likely being the best ideas  This concludes the tl dr section     However  if you want some deeper understanding of lambda type inference  I d like to explain it a bit more to depth  using examples  Look at these and try to figure out what happens   Object obj1                    String  isEmpty  Predicate lt String gt  p1           s - gt  s isEmpty    Function lt String  Boolean gt  f1   String  isEmpty  Object obj2                    p1  Function lt String  Boolean gt  f2    Function lt String  Boolean gt   obj2  Function lt String  Boolean gt  f3   p1  test  Predicate lt Integer gt  p2          s - gt  s isEmpty    Predicate lt Integer gt  p3          String  isEmpty     obj1 doesn t compile - lambdas need to infer a functional interface    with one abstract method  p1 and f1 work just fine  each inferring a different type obj2 casts a Predicate to Object - silly but valid f2 fails at runtime - you cannot cast Predicate to Function  it s no longer about inference f3 works - you call the predicate s method test that is defined by its lambda p2 doesn t compile - Integer doesn t have isEmpty method p3 doesn t compile either - there is no String  isEmpty static method with Integer argument   I hope this helps get some more insight into how type inferrence works

User · Answer

There is a way to compose a method reference that is the opposite of a current method reference  See  vlasec s answer below that shows how by explicitly casting the method reference to a Predicate and then converting it using the negate function  That is one way among a few other not too troublesome ways to do it   The opposite of this   Stream lt String gt  s        int emptyStrings   s filter String  isEmpty  count      is this   Stream lt String gt  s        int notEmptyStrings   s filter   Predicate lt String gt   String  isEmpty  negate    count     or this   Stream lt String gt  s        int notEmptyStrings   s filter  it - gt   it isEmpty     count      Personally  I prefer the later technique because I find it clearer to read it - gt   it isEmpty   than a long verbose explicit cast and then negate   One could also make a predicate and reuse it   Predicate lt String gt  notEmpty    String it  - gt   it isEmpty     Stream lt String gt  s        int notEmptyStrings   s filter notEmpty  count      Or  if having a collection or array  just use a for-loop which is simple  has less overhead  and  might be   faster   int notEmpty   0  for String s   list  if  s isEmpty    notEmpty       If you want to know what is faster  then use JMH http   openjdk java net projects code-tools jmh  and avoid hand benchmark code unless it avoids all JVM optimizations     see Java 8  performance of Streams vs Collections    I am getting flak for suggesting that the for-loop technique is faster  It eliminates a stream creation  it eliminates using another method call  negative function for predicate   and it eliminates a temporary accumulator list counter  So a few things that are saved by the last construct that might make it faster    I do think it is simpler and nicer though  even if not faster  If the job calls for a hammer and a nail  don t bring in a chainsaw and glue  I know some of you take issue with that   wish-list  I would like to see Java Stream functions evolve a bit now that Java users are more familiar with them  For example  the  count  method in Stream could accept a Predicate so that this can be done directly like this   Stream lt String gt  s        int notEmptyStrings   s count it - gt   it isEmpty      or  List lt String gt  list        int notEmptyStrings   lists count it - gt   it isEmpty

User · Answer

Another option is to utilize lambda casting in non-ambiguous contexts into one class   public static class Lambdas       public static  lt T gt  Predicate lt T gt  as Predicate lt T gt  predicate           return predicate             public static  lt T gt  Consumer lt T gt  as Consumer lt T gt  consumer           return consumer             public static  lt T gt  Supplier lt T gt  as Supplier lt T gt  supplier           return supplier             public static  lt T  R gt  Function lt T  R gt  as Function lt T  R gt  function           return function                 and then static import the utility class   stream filter as String  isEmpty  negate

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If you re using Spring Boot  2 0 0   you can use   import org springframework util StringUtils        filter StringUtils  hasLength        Which does  return  str    null  amp  amp   str isEmpty      So it will have the required negation effect for isEmpty

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In this case u could use the org apache commons lang3 StringUtilsand do  int nonEmptyStrings   s filter StringUtils  isNotEmpty  count

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I have written a complete utility class  inspired by Askar s proposal  that can take Java 8 lambda expression and turn them  if applicable  into any typed standard Java 8 lambda defined in the package java util function  You can for example do    asPredicate String  isEmpty  negate   asBiPredicate String  equals  negate     Because there would be numerous ambiguities if all the static methods would be named just as    I opted to call the method  as  followed by the returned type  This gives us full control of the lambda interpretation  Below is the first part of the  somewhat large  utility class revealing the pattern used    Have a look at the complete class here  at gist    public class FunctionCastUtil        public static  lt T  U gt  BiConsumer lt T  U gt  asBiConsumer BiConsumer lt T  U gt  biConsumer            return biConsumer             public static  lt T  U  R gt  BiFunction lt T  U  R gt  asBiFunction BiFunction lt T  U  R gt  biFunction            return biFunction              public static  lt T gt  BinaryOperator lt T gt  asBinaryOperator BinaryOperator lt T gt  binaryOperator            return binaryOperator                 and so on

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You can accomplish this as long emptyStrings   s filter s- gt  s isEmpty    count

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Building on other s answers and personal experience   Predicate lt String gt  blank   String  isEmpty  content stream           filter blank negate

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Shouldn t Predicate negate be what you are looking for

User · Answer

You can use Predicates from Eclipse Collections  MutableList lt String gt  strings   Lists mutable empty    int nonEmptyStrings   strings count Predicates not String  isEmpty      If you can t change the strings from List   List lt String gt  strings   new ArrayList lt  gt     int nonEmptyStrings   ListAdapter adapt strings  count Predicates not String  isEmpty      If you only need a negation of String isEmpty   you can also use StringPredicates notEmpty     Note  I am a contributor to Eclipse Collections

[java] How to negate a method reference predicate

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