Functional style of Java 8 s Optional ifPresent and if-not-Present

Question

In Java 8  I want to do something to an Optional object if it is present  and do another thing if it is not present   if  opt isPresent        System out println  found      else     System out println  Not found        This is not a  functional style   though   Optional has an ifPresent   method  but I am unable to chain an orElse   method   Thus  I cannot write   opt ifPresent  x - gt  System out println  found     x       orElse  System out println  NOT FOUND       In reply to  assylias  I don t think Optional map   works for the following case   opt map  o - gt      System out println  while opt is present         o setProperty xxx     dao update o     return null     orElseGet     - gt      System out println  create new obj      dao save new obj     return null        In this case  when opt is present  I update its property and save to the database  When it is not available  I create a new obj and save to the database   Note in the two lambdas I have to return null   But when opt is present  both lambdas will be executed  obj will be updated  and a new object will be saved to the database   This is because of the return null in the first lambda  And orElseGet   will continue to execute

User · Answer

You cannot call orElse after ifPresent  the reason is  orElse is called on an optiional but ifPresent returns void  So the best approach to achieve is ifPresentOrElse  It could be like this  op ifPresentOrElse                value                   - gt    System out println                             quot Value is present  its   quot                             value                                       - gt    System out println                             quot Value is empty quot

User · Answer

If you are using Java 9   you can use ifPresentOrElse   method   opt ifPresentOrElse     value - gt  System out println  Found      value         - gt  System out println  Not found

User · Answer

Java 9 introduces   ifPresentOrElse if a value is present  performs the given action with the value  otherwise performs the given empty-based action   See excellent Optional in Java 8 cheat sheet   It provides all answers for most use cases   Short summary below  ifPresent   - do something when Optional is set  opt ifPresent x - gt  print x     opt ifPresent this  print     filter   - reject  filter out  certain Optional values   opt filter x - gt  x contains  ab    ifPresent this  print     map   - transform value if present  opt map String  trim  filter t - gt  t length    gt  1  ifPresent this  print     orElse   orElseGet   - turning empty Optional to default T  int len   opt map String  length  orElse -1   int len   opt      map String  length       orElseGet    - gt  slowDefault           orElseGet this  slowDefault    orElseThrow   - lazily throw exceptions on empty Optional  opt  filter s - gt   s isEmpty     map s - gt  s charAt 0    orElseThrow IllegalArgumentException  new

User · Answer

An alternative is   System out println opt map o - gt   Found                          orElse  Not found       I don t think it improves readability though   Or as Marko suggested  use a ternary operator   System out println opt isPresent      Found     Not found

User · Answer

Another solution could be following   This is how you use it       final Opt lt String gt  opt   Opt of  I m a cool text        opt ifPresent            apply s - gt  System out printf  Text is   s n   s            elseApply    - gt  System out println  no text available       Or in case you in case of the opposite use case is true       final Opt lt String gt  opt   Opt of  This is the text        opt ifNotPresent            apply    - gt  System out println  Not present             elseApply t - gt    do something here       This are the ingredients    Little modified Function interface  just for the  elseApply  method Optional enhancement A little bit of curring  -    The  cosmetically  enhanced Function interface    FunctionalInterface public interface Fkt lt T  R gt  extends Function lt T  R gt         default R elseApply final T t            return this apply t              And the Optional wrapper class for enhancement   public class Opt lt T gt         private final Optional lt T gt  optional       private Opt final Optional lt T gt  theOptional            this optional   theOptional             public static  lt T gt  Opt lt T gt  of final T value            return new Opt lt  gt  Optional of value               public static  lt T gt  Opt lt T gt  of final Optional lt T gt  optional            return new Opt lt  gt  optional              public static  lt T gt  Opt lt T gt  ofNullable final T value            return new Opt lt  gt  Optional ofNullable value               public static  lt T gt  Opt lt T gt  empty             return new Opt lt  gt  Optional empty                private final BiFunction lt Consumer lt T gt   Runnable  Void gt  ifPresent    present  notPresent  - gt            if  this optional isPresent                  present accept this optional get               else               notPresent run                      return null             private final BiFunction lt Runnable  Consumer lt T gt   Void gt  ifNotPresent    notPresent  present  - gt            if   this optional isPresent                  notPresent run              else               present accept this optional get                       return null              public Fkt lt Consumer lt T gt   Fkt lt Runnable  Void gt  gt  ifPresent             return Opt curry this ifPresent              public Fkt lt Runnable  Fkt lt Consumer lt T gt   Void gt  gt  ifNotPresent             return Opt curry this ifNotPresent              private static  lt X  Y  Z gt  Fkt lt X  Fkt lt Y  Z gt  gt  curry final BiFunction lt X  Y  Z gt  function            return  final X x  - gt   final Y y  - gt  function apply x  y             This should do the trick and could serve as a basic template how to deal with such requirements   The basic idea here is following  In a non functional style programming world you would probably implement a method taking two parameter where the first is a kind of runnable code which should be executed in case the value is available and the other parameter is the runnable code which should be run in case the value is not available  For the sake of better readability  you can use curring to split the function of two parameter in two functions of one parameter each  This is what I basically did here   Hint  Opt also provides the other use case where you want to execute a piece of code just in case the value is not available  This could be done also via Optional filter stuff but I found this much more readable   Hope that helps   Good programming  -

User · Answer

Supposing that you have a list and avoiding the isPresent   issue  related with optionals  you could use  iterator   hasNext   to check if not present

User · Answer

For me the answer of  Dane White is OK  first I did not like using Runnable but I could not find any alternatives  Here another implementation I preferred more  public class OptionalConsumer lt T gt        private Optional lt T gt  optional       private OptionalConsumer Optional lt T gt  optional            this optional   optional             public static  lt T gt  OptionalConsumer lt T gt  of Optional lt T gt  optional            return new OptionalConsumer lt  gt  optional              public OptionalConsumer lt T gt  ifPresent Consumer lt T gt  c            optional ifPresent c           return this             public OptionalConsumer lt T gt  ifNotPresent Runnable r            if   optional isPresent                  r run                      return this           Then  Optional lt Any gt  o   Optional of       OptionalConsumer of o  ifPresent s - gt  System out println  quot isPresent  quot    s                    ifNotPresent    - gt  System out println  quot   isPresent quot      Update 1  the above solution for the traditional way of development when you have the value and want to process it but what if I want to define the functionality and the execution will be then  check below enhancement  public class OptionalConsumer lt T gt  implements Consumer lt Optional lt T gt  gt    private final Consumer lt T gt  c  private final Runnable r   public OptionalConsumer Consumer lt T gt  c  Runnable r        super        this c   c      this r   r     public static  lt T gt  OptionalConsumer lt T gt  of Consumer lt T gt  c  Runnable r        return new OptionalConsumer c  r       Override public void accept Optional lt T gt  t        if  t isPresent              c accept t get               else           r run             Then could be used as  Consumer lt Optional lt Integer gt  gt  c   OptionalConsumer of      System out  println          - gt  System out println  quot Not fit quot       IntStream range 0  100       boxed        map i - gt  Optional of i       filter j - gt  j   2    0        forEach c    In this new code you have 3 things   can define the functionality before the existing of an object easy  not creating object reference for each Optional  only one  you have so less memory than less GC  it is implementing consumer for better usage with other components   By the way  now its name is more descriptive it is actually Consumer lt Optional lt   gt  gt

User · Answer

Another solution would be to use higher-order functions as follows  opt  lt Runnable gt map value - gt     - gt  System out println  Found     value       orElse    - gt  System out println  Not Found        run

User · Answer

There isn t a great way to do it out of the box  If you want to be using your cleaner syntax on a regular basis  then you can create a utility class to help out   public class OptionalEx       private boolean isPresent       private OptionalEx boolean isPresent            this isPresent   isPresent             public void orElse Runnable runner            if   isPresent                runner run                         public static  lt T gt  OptionalEx ifPresent Optional lt T gt  opt  Consumer lt   super T gt  consumer            if  opt isPresent                  consumer accept opt get                 return new OptionalEx true                     return new OptionalEx false             Then you can use a static import elsewhere to get syntax that is close to what you re after   import static com example OptionalEx ifPresent   ifPresent opt  x - gt  System out println  found     x        orElse    - gt  System out println  NOT FOUND

User · Answer

If you can use only Java 8 or lower   1  if you don t have spring-data the best way so far is   opt  lt Runnable gt map param - gt     - gt  System out println param          orElse    - gt  System out println  no-param-specified           run      Now I know it s not so readable and even hard to understand for someone  but looks fine for me personally and I don t see another nice fluent way for this case   2  if you re lucky enough and you can use spring-data the best way is Optionals ifPresentOrElse   Optionals ifPresentOrElse opt  System out  println           - gt  System out println  no-param-specified       If you can use Java 9  you should definitely go with   opt ifPresentOrElse System out  println           - gt  System out println  no-param-specified

User · Answer

The described behavior can be achieved by using Vavr  formerly known as Javaslang   an object-functional library for Java 8   that implements most of Scala constructs  being Scala a more expressive language with a way richer type system built on JVM   It is a very good library to add to your Java projects to write pure functional code  Vavr provides the Option monad that provides functions to work with the Option type such as   fold  to map the value of the option on both cases  defined empty  onEmpty  allows to execute a Runnable when option is empty peek  allows to consume the value of the option  when defined   and it is also Serializable on the contrary of Optional which means you can safely use it as method argument and instance member   Option follows the monad laws at difference to the Java s Optional  quot pseudo-monad quot  and provides a richer API  And of course you can make it from a Java s Optional  and the other way around   Option ofOptional javaOptional     Vavr is focused on interoperability  Going to the example     AWESOME Vavr functional collections  immutable for the gread good       fully convertible to Java s counterparts  final Map lt String  String gt  map   Map  quot key1 quot    quot value1 quot    quot key2 quot    quot value2 quot     final Option lt String gt  opt   map get  quot nonExistentKey quot       you re safe of null refs           final String result   opt fold             - gt   quot Not found    quot                     Option is None         val - gt   quot Found the value   quot    val        Option is Some val      Moreover  all Vavr types are convertible to its Java counterparts  for the sake of the example  Optional javaOptional   opt toJava    very easy    Of course the conversion also exists in the other way  Option option   Option ofOptional javaOptional   N B  Vavr offers a io vavr API class with a lot of convenient static methods    Further reading Null reference  the billion dollar mistake N B  This is only a very little example of what Vavr offers  pattern matching  streams a k a  lazy evaluated lists  monadic types  immutable collections

User · Answer

In case you want store the value   Pair of lt List lt  gt   List lt  gt  gt  output   opt map details - gt  Pair of details a  details b    orElseGet    - gt  Pair of Collections emptyList    Collections emptyList

[java] Functional style of Java 8's Optional.ifPresent and if-not-Present?

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