What are functional interfaces used for in Java 8

Question

I came across a new term in Java 8   quot functional interface quot   I could only find one use of it while working with lambda expressions  Java 8 provides some built-in functional interfaces and if we want to define any functional interface then we can make use of the  FunctionalInterface annotation  It will allow us to declare only a single method in the interface  For example   FunctionalInterface interface MathOperation       int operation int a  int b      How useful it is in Java 8 other than just working with lambda expressions   The question here is different from the one I asked  It is asking why we need functional interfaces while working with lambda expressions  My question is  What are the other uses of functional interfaces besides use with lambda expressions

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You can use lambda in Java 8  public static void main String   args        tentimes inputPrm -  gt  System out println inputPrm          tentimes System out  println       You can also replace lambda with static method reference    public static void tentimes Consumer myFunction        for  int i   0  i  lt  10  i            myFunction accept  hello        For further info about Java Lambdas and FunctionalInterfaces

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Functional Interfaces  An interface is called a functional interface if it has a single abstract method irrespective of the number of default or static methods  Functional Interface are use for lamda expression  Runnable  Callable  Comparable  Comparator are few examples of Functional Interface  KeyNotes   Annotation  FunctionalInterface is used Optional   It should have only 1 abstract method irrespective of number of default and static methods   Two abstract method gives compilation error Provider  FunctionalInterface annotation is used    This thread talks more in detail about what benefit functional Interface gives over anonymous class and how to use them

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Beside other answers  I think the main reason to  why using Functional Interface other than directly with lambda expressions  can be related to nature of Java language which is Object Oriented    The main attributes of Lambda expressions are  1  They can be passed around 2  and they can executed in future in specific time  several times   Now to support this feature in languages  some other languages deal simply with this matter    For instance in Java Script  a function  Anonymous function  or Function literals  can be addressed as a object  So  you can create them simply and also they can be assigned to a variable and so forth  For example   var myFunction   function                    alert myFunction          or via ES6  you can use an arrow function   const myFunction         gt        Up to now  Java language designers have not accepted to handle mentioned features via these manner  functional programming techniques   They believe that Java language is Object Oriented and therefore they should solve this problem via Object Oriented techniques  They don t want to miss simplicity and consistency of Java language    Therefore  they use interfaces  as when an object of an interface with just one method  I mean functional interface  is need you can replace it with a lambda expression  Such as   ActionListener listener   event - gt

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Functional Interface    Introduced in Java 8 Interface that contains a  single abstract  method    Example 1       interface CalcArea        --functional interface         double calcArea double rad                      Example 2   interface CalcGeometry      --functional interface     double calcArea double rad       default double calcPeri double rad            return 0 0                   Example 3   interface CalcGeometry       -- not functional interface     double calcArea double rad       double calcPeri double rad          Java8 annotation --  FunctionalInterface   Annotation check that interface contains only one abstract method  If not  raise error  Even though  FunctionalInterface missing  it is still functional interface  if having single abstract method   The annotation helps avoid mistakes  Functional interface may have additional static  amp  default methods  e g  Iterable lt    Comparable lt    Comparator lt      Applications of Functional Interface    Method references Lambda Expression Constructor references   To learn functional interfaces  learn first default methods in interface  and after learning functional interface  it will be easy to you to understand method reference and lambda expression

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The documentation makes indeed a difference between the purpose  An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface as defined by the Java Language Specification   and the use case  Note that instances of functional interfaces can be created with lambda expressions  method references  or constructor references   whose wording does not preclude other use cases in general  Since the primary purpose is to indicate a functional interface  your actual question boils down to    Are there other use cases for functional interfaces other than lambda expressions and method constructor references     Since functional interface is a Java language construct defined by the Java Language Specification  only that specification can answer that question  JLS   9 8  Functional Interfaces       In addition to the usual process of creating an interface instance by declaring and instantiating a class    15 9   instances of functional interfaces can be created with method reference expressions and lambda expressions    15 13    15 27    So the Java Language Specification doesn   t say otherwise  the only use case mentioned in that section is that of creating interface instances with method reference expressions and lambda expressions   This includes constructor references as they are noted as one form of method reference expression in the specification   So in one sentence  no  there is no other use case for it in Java  8

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A lambda expression can be assigned to a functional interface type  but so can method references  and anonymous classes   One nice thing about the specific functional interfaces in java util function is that they can be composed to create new functions  like Function andThen and Function compose  Predicate and  etc   due to the handy default methods they contain

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FunctionalInterface is a new annotation are released with Java 8 and provide target types for lambda expressions and it used on compilation time checking of your code   When you want to use it    1- Your interface must not have more than one abstract methods  otherwise compilation error will be given   1- Your interface Should be pure  which means functional interface is intended to be implemented by stateless classes  exmple of pure is Comparator interface because its not depend on the implementers state  in this case No compilation error will be given  but in many cases you will not be able to use lambda with this kind of interfaces   The java util function package contains various general purpose functional interfaces such as Predicate  Consumer  Function  and Supplier    Also please note that you can use lambdas without this annotation

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An interface with only one abstract method is called Functional Interface   It is not mandatory to use  FunctionalInterface  but it   s best practice to use it with functional interfaces to avoid addition of extra methods accidentally  If the interface is annotated with  FunctionalInterface annotation and we try to have more than one abstract method  it throws compiler error   package com akhi       FunctionalInterface     public interface FucnctionalDemo          void letsDoSomething            void letsGo           invalid because another abstract method does not allow       public String toString          valid because toString from Object        public boolean equals Object o     valid        public static int sum int a int b       valid because method static                          return a b                    public default int sub int a int b      valid because method default                       return a-b

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Not at all   Lambda expressions are the one and only point of that annotation

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FunctionalInterface annotation is useful for compilation time checking of your code  You cannot have more than one method besides static  default and abstract methods that override methods in Object in your  FunctionalInterface or any other interface used as a functional interface   But you can use lambdas without this annotation as well as you can override methods without  Override annotation   From docs     a functional interface has exactly one abstract method  Since default   methods have an implementation  they are not abstract  If an interface   declares an abstract method overriding one of the public methods of   java lang Object  that also does not count toward the interface s   abstract method count since any implementation of the interface will   have an implementation from java lang Object or elsewhere   This can be used in lambda expression   public interface Foo     public void doSomething        This cannot be used in lambda expression   public interface Foo     public void doSomething      public void doSomethingElse        But this will give compilation error    FunctionalInterface public interface Foo     public void doSomething      public void doSomethingElse           Invalid   FunctionalInterface  annotation  Foo is not a functional   interface

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As others have said  a functional interface is an interface which exposes one method  It may have more than one method  but all of the others must have a default implementation  The reason it s called a  functional interface  is because it effectively acts as a function  Since you can pass interfaces as parameters  it means that functions are now  first-class citizens  like in functional programming languages  This has many benefits  and you ll see them quite a lot when using the Stream API  Of course  lambda expressions are the main obvious use for them

[java] What are functional interfaces used for in Java 8?

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