How do I define a method which takes a lambda as a parameter in Java 8

Question

In Java 8  methods can be created as Lambda expressions and can be passed by reference  with a little work under the hood   There are plenty of examples online with lambdas being created and used with methods  but no examples of how to make a method taking a lambda as a parameter  What is the syntax for that   MyClass method  a  b  - gt  a b     class MyClass      How do I define this method    static int method Lambda l       return l 5  10

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There s a public Web-accessible version of the Lambda-enabled Java 8 JavaDocs  linked from http   lambdafaq org lambda-resources    This should obviously be a comment on Joachim Sauer s answer  but I can t get into my SO account with the reputation points I need to add a comment    The lambdafaq site  I maintain it  answers this and a lot of other Java-lambda questions   NB This answer was written before the Java 8 GA documentation became publicly available  I ve left in place  though  because the Lambda FAQ might still be useful to people learning about features introduced in Java 8

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Lambda is not a object but a Functional Interface  One can define as many as Functional Interfaces as they can using the  FuntionalInterface as an annotation   FuntionalInterface public interface SumLambdaExpression        public int do int a  int b      public class MyClass        public static void main String    args              SumLambdaExpression s    a b - gt a b            lambdaArgFunction s                public static void lambdaArgFunction SumLambdaExpression s              System out println  Output     s do 2 5               The Output will be as follows  Output   7   The Basic concept of a Lambda Expression is to define your own logic but already defined Arguments  So in the above code the you can change the definition of the do function from addition to any other definition  but your arguments are limited to 2

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Well  that s easy  The purpose of lambda expression is to implement Functional Interface  It is the interface with only one method  Here is awesone article about predefined and legacy functional interfaces   Anyway  if you want to implement your own functional interface  make it  Just for simple example   public interface MyFunctionalInterface       String makeIt String s       So let s make a class  where we will create a method  which accepts the type of MyFunctionalInterface    public class Main        static void printIt String s  MyFunctionalInterface f            System out println f makeIt s               public static void main String   args               The last thing you should do is to pass the implementation of the MyFunctionalInterface to the method we ve defined   public class Main        static void printIt String s  MyFunctionalInterface f            System out println f makeIt s               public static void main String   args            printIt  Java   s - gt  s     is Awesome              That s it

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There is flexibility in using lambda as parameter  It enables functional programming in java  The basic syntax is  param - gt  method body  Following is a way  you can define a method taking functional interface  lambda is used  as parameter  a  if you wish to define a method declared inside a functional interface  say  the functional interface is given as an argument parameter to a method called from main    FunctionalInterface interface FInterface      int callMeLambda String temp       class ConcreteClass               void funcUsesAnonymousOrLambda FInterface fi           System out println  quot    Executing method arg instantiated with Lambda    quot                        public static void main               calls a method having FInterface as an argument          funcUsesAnonymousOrLambda new FInterface                          int callMeLambda String temp     define callMeLambda     here                   return 0                                                     Can be replaced by Lambda below                   funcUsesAnonymousOrLambda   x  - gt                return 0     1                            FInterface fi    x  - gt    return 0     funcUsesAnonymousOrLambda fi   Here above it can be seen  how a lambda expression can be replaced with an interface   Above explains a particular usage of lambda expression  there are more  ref Java 8 lambda within a lambda can  39 t modify variable from outer lambda

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You can use functional interfaces as mentioned above  below are some of the examples  Function lt Integer  Integer gt  f1   num- gt  num 2 1   System out println f1 apply 10     Predicate lt Integer gt  f2  num- gt  num  gt  10   System out println f2 test 10    System out println f2 test 11     Supplier lt Integer gt  f3    - gt  100  System out println f3 get       Hope it helps

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Here s roughly how C  handles this problem  but expressed as Java code    Something like this could handle almost all your needs   import static org util function Functions     public class Test        public static void main String   args                Test invoke  a  b  - gt  a   b                     public static void invoke Func2 lt Integer  Integer  Integer gt  func                System out println func apply 5  6                package org util function   public interface Functions          Actions      public interface Action           public void apply               public interface Action1 lt T1 gt            public void apply T1 arg1              public interface Action2 lt T1  T2 gt            public void apply T1 arg1  T2 arg2              public interface Action3 lt T1  T2  T3 gt            public void apply T1 arg1  T2 arg2  T3 arg3              public interface Action4 lt T1  T2  T3  T4 gt            public void apply T1 arg1  T2 arg2  T3 arg3  T4 arg4              public interface Action5 lt T1  T2  T3  T4  T5 gt            public void apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5              public interface Action6 lt T1  T2  T3  T4  T5  T6 gt            public void apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5  T6 arg6              public interface Action7 lt T1  T2  T3  T4  T5  T6  T7 gt            public void apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5  T6 arg6  T7 arg7              public interface Action8 lt T1  T2  T3  T4  T5  T6  T7  T8 gt            public void apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5  T6 arg6  T7 arg7  T8 arg8                Functions      public interface Func lt TResult gt            public TResult apply               public interface Func1 lt T1  TResult gt            public TResult apply T1 arg1              public interface Func2 lt T1  T2  TResult gt            public TResult apply T1 arg1  T2 arg2              public interface Func3 lt T1  T2  T3  TResult gt            public TResult apply T1 arg1  T2 arg2  T3 arg3              public interface Func4 lt T1  T2  T3  T4  TResult gt            public TResult apply T1 arg1  T2 arg2  T3 arg3  T4 arg4              public interface Func5 lt T1  T2  T3  T4  T5  TResult gt            public TResult apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5              public interface Func6 lt T1  T2  T3  T4  T5  T6  TResult gt            public TResult apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5  T6 arg6              public interface Func7 lt T1  T2  T3  T4  T5  T6  T7  TResult gt            public TResult apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5  T6 arg6  T7 arg7              public interface Func8 lt T1  T2  T3  T4  T5  T6  T7  T8  TResult gt            public TResult apply T1 arg1  T2 arg2  T3 arg3  T4 arg4  T5 arg5  T6 arg6  T7 arg7  T8 arg8

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For functions that do not have more than 2 parameters  you can pass them without defining your own interface  For example   class Klass     static List lt String gt  foo Integer a  String b             class MyClass     static List lt String gt  method BiFunction lt Integer  String  List lt String gt  gt  fn       return fn apply 5   FooBar           List lt String gt  lStr   MyClass method  a  b  - gt  Klass foo  Integer  a   String  b      In BiFunction lt Integer  String  List lt String gt  gt   Integer and String are its parameters  and List lt String gt  is its return type    For a function with only one parameter  you can use Function lt T  R gt   where T is its parameter type  and R is its return value type   Refer to this page for all the interfaces that are already made available by Java

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Basically to pass a lamda expression as a parameter  we need a type in which we can hold it  Just as an integer value we hold in primitive int or Integer class  Java doesn t have a separate type for lamda expression instead it uses an interface as the type to hold the argument  But that interface should be a functional interface

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For anyone who is googling this  a good method would be to use java util function BiConsumer  ex   Import java util function Consumer public Class Main       public static void runLambda BiConsumer lt Integer  Integer gt  lambda            lambda accept 102  54             public static void main String   args            runLambda  int1  int2  - gt  System out println int1           int2            int1   int2             The outprint would be  166

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Do the following     You have declared method lambda l   All you want to do is create a Interface with the name lambda and declare one abstract method   public int add int a int b       method name does not matter here    So when u call MyClass method   a b - gt a b   This implementation   a b - gt a b will be injected to your interface add method  So whenever you call l add it is going to take this implementation and perform addition of a and b  and return l add 2 3  will return 5  - Basically this is what lambda does

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To use Lambda expression you need to either create your own functional interface or use Java functional interface for operation that require two integer and return as value  IntBinaryOperator Using user defined functional interface interface TwoArgInterface        public int operation int a  int b      public class MyClass        public static void main String javalatte                 this is lambda expression         TwoArgInterface plusOperation    a  b  - gt  a   b          System out println  quot Sum of 10 34    quot    plusOperation operation 10  34              Using Java functional interface import java util function IntBinaryOperator   public class MyClass1        static void main String javalatte                 this is lambda expression         IntBinaryOperator plusOperation    a  b  - gt  a   b          System out println  quot Sum of 10 34    quot    plusOperation applyAsInt 10  34

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Lambdas are purely a call-site construct  the recipient of the lambda does not need to know that a Lambda is involved  instead it accepts an Interface with the appropriate method   In other words  you define or use a functional interface  i e  an interface with a single method  that accepts and returns exactly what you want   For this Java 8 comes with a set of commonly-used interface types in java util function  thanks to Maurice Naftalin for the hint about the JavaDoc    For this specific use case there s java util function IntBinaryOperator with a single int applyAsInt int left  int right  method  so you could write your method like this   static int method IntBinaryOperator op       return op applyAsInt 5  10       But you can just as well define your own interface and use it like this   public interface TwoArgIntOperator       public int op int a  int b        elsewhere  static int method TwoArgIntOperator operator        return operator op 5  10       Using your own interface has the advantage that you can have names that more clearly indicate the intent

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To me  the solution that makes the most sense is to define a Callback interface    interface Callback       void call        and then to use it as parameter in the function you want to call    void somewhereInYourCode         method    - gt               You ve passed a lambda             method   is done  do whatever you want here             void method Callback callback           Do what you have to do                    Don t forget to notify the caller once you re done     callback call        Just a precision though  A lambda is not a special interface  class or anything else you could declare by yourself  Lambda is just the name given to the    - gt     special syntax  which allows better readability when passing single-method interfaces as parameter  It was designed to replace this    method new Callback          Override     public void call                Classic interface implementation  lot of useless boilerplate code             method   is done  do whatever you want here              So in the example above  Callback is not a lambda  it s just a regular interface   lambda is the name of the shortcut syntax you can use to implement it

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Lambda expression can be passed as a argument To pass a lambda expression as an argument the type of the parameter  which receives the lambda expression as an argument  must be of functional interface type   If there is a functional interface -   interface IMyFunc      boolean test int num       And there is a filter method which adds the int in the list only if it is greater than 5  Note here that filter method has funtional interface IMyFunc as one of the parameter  In that case lambda expression can be passed as an argument for the method parameter   public class LambdaDemo       public static List lt Integer gt  filter IMyFunc testNum  List lt Integer gt  listItems            List lt Integer gt  result   new ArrayList lt Integer gt             for Integer item  listItems                if testNum test item                     result add item                                   return result            public static void main String   args            List lt Integer gt  myList   new ArrayList lt Integer gt             myList add 1           myList add 4           myList add 6           myList add 7              calling filter method with a lambda expression            as one of the param         Collection lt Integer gt  values   filter n - gt  n  gt  5  myList            System out println  Filtered values     values

[java] How do I define a method which takes a lambda as a parameter in Java 8?

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