Java 8 - Difference between Optional flatMap and Optional map

Question

What s the difference between these two methods  Optional flatMap   and Optional map     An example would be appreciated

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Optional map      Takes every element and if the value exists  it is passed to the function   Optional lt T gt  optionalValue        Optional lt Boolean gt  added   optionalValue map results  add     Now added has one of three values  true or false wrapped into an Optional   if optionalValue was present  or an empty Optional otherwise   If you don t need to process the result you can simply use ifPresent    it doesn t have return value   optionalValue ifPresent results  add       Optional flatMap      Works similar to the same method of streams  Flattens out the stream of streams  With the difference that if the value is presented it is applied to function  Otherwise  an empty optional is returned   You can use it for composing optional value functions calls    Suppose we have methods   public static Optional lt Double gt  inverse Double x        return x    0   Optional empty     Optional of 1   x      public static Optional lt Double gt  squareRoot Double x        return x  lt  0   Optional empty     Optional of Math sqrt x        Then you can compute the square root of the inverse  like   Optional lt Double gt  result   inverse -4 0  flatMap MyMath  squareRoot     or  if you prefer   Optional lt Double gt  result   Optional of -4 0  flatMap MyMath  inverse  flatMap MyMath  squareRoot     If either the inverse   or the squareRoot   returns Optional empty    the result is empty

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Use map if the function returns the object you need or flatMap if the function returns an Optional  For example   public static void main String   args      Optional lt String gt  s   Optional of  input      System out println s map Test  getOutput      System out println s flatMap Test  getOutputOpt       static String getOutput String input      return input    null   null    output for     input     static Optional lt String gt  getOutputOpt String input      return input    null   Optional empty     Optional of  output for     input       Both print statements print the same thing

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They both take a function from the type of the optional to something   map   applies the function  as is  on the optional you have   if  optional isEmpty    return Optional empty    else return Optional of f optional get        What happens if your function is a function from T - gt  Optional lt U gt   Your result is now an Optional lt Optional lt U gt  gt    That s what flatMap   is about  if your function already returns an Optional  flatMap   is a bit smarter and doesn t double wrap it  returning Optional lt U gt     It s the composition of two functional idioms  map and flatten

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You can refer below link to understand in detail  best explanation which I could find   https   www programmergirl com java-8-map-flatmap-difference  Both map and flatMap - accept Function  The return type of map   is a single value whereas flatMap  is returning stream of values  lt R gt  Stream lt R gt  map Function lt   super T    extends R gt  mapper    lt R gt  Stream lt R gt  flatMap Function lt   super T    extends Stream lt   extends R gt  gt  mapper

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Okay  You only need to use  flatMap  when you re facing nested Optionals  Here s the example   public class Person        private Optional lt Car gt  optionalCar       public Optional lt Car gt  getOptionalCar             return optionalCar           public class Car        private Optional lt Insurance gt  optionalInsurance       public Optional lt Insurance gt  getOptionalInsurance             return optionalInsurance           public class Insurance        private String name       public String getName             return name            public class Test           map cannot deal with nested Optionals     public Optional lt String gt  getCarInsuranceName Person person            return person getOptionalCar                    map Car  getOptionalInsurance       leads to a Optional lt Optional lt Insurance gt                   map Insurance  getName                         Like Stream  Optional map will return a value wrapped by a Optional  That s why we get a nested Optional -- Optional lt Optional lt Insurance gt   And at    we want to map it as an Insurance instance  that s how the tragedy happened  The root is nested Optionals  If we can get the core value regardless the shells  we ll get it done  That s what flatMap does   public Optional lt String gt  getCarInsuranceName Person person        return person getOptionalCar                     flatMap Car  getOptionalInsurance                    map Insurance  getName       In the end  I stronly recommed the Java 8 In Action to you if you d like to study Java8 Systematicly

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What helped me was a look at the source code of the two functions    Map - wraps the result in an Optional   public lt U gt  Optional lt U gt  map Function lt   super T    extends U gt  mapper        Objects requireNonNull mapper       if   isPresent            return empty        else           return Optional ofNullable mapper apply value       lt --- wraps in an optional           flatMap - returns the  raw  object  public lt U gt  Optional lt U gt  flatMap Function lt   super T  Optional lt U gt  gt  mapper        Objects requireNonNull mapper       if   isPresent            return empty        else           return Objects requireNonNull mapper apply value       lt ---  returns  raw  object

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Note - below is the illustration of map and flatmap function  otherwise Optional is primarily designed to be used as a return type only   As you already may know Optional is a kind of container which may or may not contain a single object  so it can be used wherever you anticipate a null value You may never see NPE if use Optional properly   For example if you have a method which expects a person object which may be nullable you may want to write the method something like this   void doSome Optional lt Person gt  person       and here you want to retrieve some property phone out of person     you may write something like this         Optional lt String gt  phone   person map  p - gt p getPhone       phone ifPresent  ph - gt dial ph      class Person    private String phone      setter  getters     Here you have returned a String type which is automatically wrapped in an Optional type   If person class looked like this  i e  phone is also Optional  class Person    private Optional lt String gt  phone      setter getter     In this case invoking map function will wrap the returned value in Optional and yield something like   Optional lt Optional lt String gt  gt     And you may want Optional lt String gt  instead  here comes flatMap  void doSome Optional lt Person gt  person     Optional lt String gt  phone   person flatMap  p - gt p getPhone       phone ifPresent  ph - gt dial ph        PS  Never call get method  if you need to  on an Optional without checking it with isPresent   unless you can t live without NullPointerExceptions

[java] Java 8 - Difference between Optional.flatMap and Optional.map

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