How to execute logic on Optional if not present

Question

I want to replace the following code using java8 Optional   public Obj getObjectFromDB         Obj obj   dao find        if  obj    null            obj setAvailable true         else           logger fatal  Object not available               return obj      The following pseudocode does not work as there is no orElseRun method  but anyways it illustrates my purpose   public Optional lt Obj gt  getObjectFromDB         Optional lt Obj gt  obj   dao find        return obj ifPresent obj setAvailable true   orElseRun logger fatal  Object not available

User · Answer

For Java 8 Spring offers ifPresentOrElse from  Utility methods to work with Optionals  to achieve what you want  Example would be   import static org springframework data util Optionals ifPresentOrElse       ifPresentOrElse dao find    obj - gt  obj setAvailable true      - gt  logger fatal  Object not available

User · Answer

ifPresentOrElse can handle cases of nullpointers as well  Easy approach      Optional ofNullable null               ifPresentOrElse name - gt  System out println  my name is    name                         - gt System out println  no name or was a null pointer

User · Answer

First of all  your dao find   should either return an Optional lt Obj gt  or you will have to create one   e g   Optional lt Obj gt    dao find      or you can do it yourself like   Optional lt Obj gt    Optional ofNullable dao find       this one will return Optional lt Obj gt  if present or Optional empty   if not present   So now let s get to the solution   public Obj getObjectFromDB        return Optional ofNullable dao find    flatMap ob - gt                ob setAvailable true               return Optional of ob                  orElseGet    - gt                logger fatal  Object not available                return null                      This is the one liner you re looking for

User · Answer

I don t think you can do it in a single statement  Better do   if   obj isPresent          logger fatal            else       obj get   setAvailable true     return obj

User · Answer

I was able to came up with a couple of  one line  solutions  for example       obj map o - gt   Runnable     - gt  o setAvailable true           orElse    - gt  logger fatal  Object not available            run      or      obj map o - gt   Consumer lt Object gt   c - gt  o setAvailable true           orElse o - gt  logger fatal  Object not available            accept null     or      obj map o - gt   Supplier lt Object gt      - gt                o setAvailable true               return null         orElse       - gt                logger fatal  Object not available               return null         get      It doesn t look very nice  something like orElseRun would be much better  but I think that option with Runnable is acceptable if you really want one line solution

User · Answer

I suppose you cannot change the dao find   method to return an instance of Optional lt Obj gt   so you have to create the appropriate one yourself   The following code should help you out  I ve create the class OptionalAction  which provides the if-else mechanism for you   public class OptionalTest     public static Optional lt DbObject gt  getObjectFromDb              doa find       DbObject v   find            create appropriate Optional     Optional lt DbObject gt  object   Optional ofNullable v            formatter off     OptionalAction      ifPresent object       then o - gt  o setAvailable true        elseDo o - gt  System out println  Fatal  Object not available              formatter on     return object         public static void main String   args          Optional lt DbObject gt  object   getObjectFromDb        if  object isPresent          System out println object get         else       System out println  There is no object               find may return null   public static DbObject find           return  Math random    gt  0 5    null   new DbObject           static class DbObject         private boolean available   false       public boolean isAvailable               return available             public void setAvailable boolean available              this available   available              Override     public String toString               return  DbObject  available     available                     static class OptionalAction         public static  lt T gt  IfAction lt T gt  ifPresent Optional lt T gt  optional              return new IfAction lt  gt  optional              private static class IfAction lt T gt              private final Optional lt T gt  optional         public IfAction Optional lt T gt  optional                  this optional   optional                 public ElseAction lt T gt  then Consumer lt   super T gt  consumer                  if  optional isPresent              consumer accept optional get             return new ElseAction lt  gt  optional                      private static class ElseAction lt T gt              private final Optional lt T gt  optional         public ElseAction Optional lt T gt  optional                  this optional   optional                 public void elseDo Consumer lt   super T gt  consumer                  if   optional isPresent              consumer accept null

User · Answer

You will have to split this into multiple statements  Here is one way to do that   if   obj isPresent        logger fatal  Object not available       obj ifPresent o - gt  o setAvailable true    return obj    Another way  possibly over-engineered  is to use map   if   obj isPresent        logger fatal  Object not available       return obj map o - gt   o setAvailable true   return o       If obj setAvailable conveniently returns obj  then you can simply the second example to   if   obj isPresent        logger fatal  Object not available       return obj map o - gt  o setAvailable true

User · Answer

For those of you who want to execute a side-effect only if an optional is absent  i e  an equivalent of ifAbsent   or ifNotPresent   here is a slight modification to the great answers already provided   myOptional ifPresentOrElse x - gt         - gt         logic goes here

User · Answer

You need Optional isPresent   and orElse    Your snippet won t work because it doesn t return anything if not present    The point of Optional is to return it from the method

User · Answer

With Java 8 Optional it can be done with       Optional lt Obj gt  obj   dao find         obj map obj setAvailable true   orElseGet    - gt            logger fatal  Object not available            return null

User · Answer

There is an  orElseRun method  but it is called  orElseGet  The main problem with your pseudocode is that  isPresent doesn t return an Optional lt  gt   But  map returns an Optional lt  gt  which has the orElseRun method  If you really want to do this in one statement this is possible  public Optional lt Obj gt  getObjectFromDB         return dao find            map  obj - gt                 obj setAvailable true               return Optional of obj                         orElseGet     - gt                logger fatal  quot Object not available quot                 return Optional empty               But this is even clunkier than what you had before

User · Answer

With Java 9 or higher  ifPresentOrElse is most likely what you want   Optional lt  gt  opt   dao find     opt ifPresentOrElse obj - gt  obj setAvailable true                          - gt  logger error            Currying using vavr or alike might get even neater code  but I haven t tried yet

[java] How to execute logic on Optional if not present?

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