Can a java lambda have more than 1 parameter

Question

In Java  is it possible to have a lambda accept multiple different types   I e  Single variable works       Function  lt Integer  Integer gt  adder   i - gt  i   1      System out println  adder apply  10      Varargs also work       Function  lt Integer     Integer gt  multiAdder   ints - gt            int sum   0          for  Integer i   ints                sum    i                    return sum                          System out println   multiAdder apply  new Integer      1  2  3  4         But I want something that can accept many different types of arguments  e g       Function  lt String  Integer  Double  Person  String gt  myLambda   a   b  c  d- gt          DO STUFF      return  done stuff           The main use is to have small inline functions inside functions for convenience    I ve looked around google and inspected Java s Function Package  but could not find  Is this possible

User · Answer

For this case you could use interfaces from default library (java 1.8):

java.util.function.BiConsumer
java.util.function.BiFunction

There is a small (not the best) example of default method in interface:

default BiFunction<File, String, String> getFolderFileReader() {
    return (directory, fileName) -> {
        try {
            return FileUtils.readFile(directory, fileName);
        } catch (IOException e) {
            LOG.error("Unable to read file {} in {}.", fileName, directory.getAbsolutePath(), e);
        }
        return "";
    };
}}

User · Answer

It s possible if you define such a functional interface with multiple type parameters  There is no such built in type   There are a few limited types with multiple parameters     FunctionalInterface interface Function6 lt One  Two  Three  Four  Five  Six gt        public Six apply One one  Two two  Three three  Four four  Five five      public static void main String   args  throws Exception       Function6 lt String  Integer  Double  Void  List lt Float gt   Character gt  func    a  b  c  d  e  - gt   z       I ve called it Function6 here  The name is at your discretion  just try not to clash with existing names in the Java libraries     There s also no way to define a variable number of type parameters  if that s what you were asking about     Some languages  like Scala  define a number of built in such types  with 1  2  3  4  5  6  etc  type parameters

User · Answer

For something with 2 parameters  you could use BiFunction  If you need more  you can define your own function interface  like so    FunctionalInterface public interface FourParameterFunction lt T  U  V  W  R gt        public R apply T t  U u  V v  W w       If there is more than one parameter  you need to put parentheses around the argument list  like so   FourParameterFunction lt String  Integer  Double  Person  String gt  myLambda    a  b  c  d  - gt           do something     return  done something

User · Answer

To make the use of lambda   There are three type of operation  1  Accept parameter --  Consumer  2  Test parameter return boolean --  Predicate 3  Manipulate parameter and return value --  Function  Java Functional interface upto two parameter  Single parameter interface Consumer Predicate Function  Two parameter interface BiConsumer BiPredicate BiFunction  For more than two  you have to create functional interface as follow Consumer type     FunctionalInterface public interface FiveParameterConsumer lt T  U  V  W  X gt        public void accept T t  U u  V v  W w  X x

User · Answer

You could also use jOOL library - https   github com jOOQ jOOL  It has already prepared function interfaces with different number of parameters  For instance  you could use org jooq lambda function Function3  etc from Function0 up to Function16

User · Answer

Some lambda function   import org junit Test  import java awt event ActionListener  import java util function Function   public class TestLambda     Test public void testLambda          System out println  quot test some lambda function quot                                                            1-any input   any output        lambda define      Runnable lambda1      - gt  System out println  quot no parameter quot          lambda execute      lambda1 run                                                             2-one input as ActionEvent    any output        lambda define      ActionListener lambda2    p  - gt  System out println  quot One parameter as action quot          lambda execute      lambda2 actionPerformed null                                                            3-one input   by output as Integer         lambda define      Function lt String  Integer gt  lambda3    p1  - gt            System out println  quot one parameters   quot    p1           return 10               lambda execute      lambda3 apply  quot test quot                                                             4-two input   any output       lambda define      TwoParameterFunctionWithoutReturn lt String  Integer gt  lambda4    p1  p2  - gt            System out println  quot two parameters   quot    p1    quot    quot    p2                lambda execute      lambda4 apply  quot param1 quot   10                                                            5-two input   by output as Integer        lambda define      TwoParameterFunctionByReturn lt Integer  Integer gt  lambda5    p1  p2  - gt            System out println  quot two parameters   quot    p1    quot    quot    p2           return p1   p2               lambda execute      lambda5 apply 10  20                                                            6-three input Integer Integer String    by output as Integer        lambda define      ThreeParameterFunctionByReturn lt Integer  Integer  Integer gt  lambda6    p1  p2  p3  - gt            System out println  quot three parameters   quot    p1    quot    quot    p2    quot    quot    p3           return p1   p2   p3               lambda execute      lambda6 apply 10  20  30         FunctionalInterface public interface TwoParameterFunctionWithoutReturn lt T  U gt        public void apply T t  U u       FunctionalInterface public interface TwoParameterFunctionByReturn lt T  U gt        public T apply T t  U u       FunctionalInterface public interface ThreeParameterFunctionByReturn lt M  N  O gt        public Integer apply M m  N n  O o

User · Answer

Another alternative  not sure if this applies to your particular problem but to some it may be applicable is to use UnaryOperator in java util function library  where it returns same type you specify  so you put all your variables in one class and is it as a parameter   public class FunctionsLibraryUse        public static void main String   args           UnaryOperator lt People gt  personsBirthday    p  - gt               System out println  it s     p getName       birthday                 p setAge p getAge     1               return p                     People mel   new People            mel setName  mel            mel setAge 27           mel   personsBirthday apply mel           System out println  he is now       mel getAge              class People      private String name      private int age      public String getName             return name            public void setName String name            this name   name            public int getAge             return age            public void setAge int age            this age   age            So the class you have  in this case Person  can have numerous instance variables and won t have to change the parameter of your lambda expression   For those interested  I ve written notes on how to use java util function library  http   sysdotoutdotprint com index php 2017 04 28 java-util-function-library

[java] Can a java lambda have more than 1 parameter?

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