What s the difference between a Future and a Promise

Question

What s the difference between Future and Promise  They both act like a placeholder for future results  but where is the main difference

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Future and Promise are proxy object for unknown result Promise completes a Future  Future - read consumer of unknown result  Promise - write producer of unknown result      Future has a reference to Promise Future - gt  Promise  As a producer I promise something and responsible for it As a consumer who retrieved a promise I expect to have a result in future As for Java CompletableFutures it is a Promise because you can set the result and also it implements Future

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For client code  Promise is for observing or attaching callback when a result is available  whereas Future is to wait for result and then continue  Theoretically anything which is possible to do with futures what can done with promises  but due to the style difference  the resultant API for promises in different languages make chaining easier

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I will give an example of what is Promise and how its value could be set at any time  in opposite to Future  which value is only readable   Suppose you have a mom and you ask her for money      Now   you trick your mom into creating you a promise of eventual    donation  she gives you that promise object  but she is not really    in rush to fulfill it yet  Supplier lt Integer gt  momsPurse     - gt             try               Thread sleep 1000    mom is busy           catch  InterruptedException e                                     return 100            ExecutorService ex   Executors newFixedThreadPool 10    CompletableFuture lt Integer gt  promise     CompletableFuture supplyAsync momsPurse  ex       You are happy  you run to thank you your mom  promise thenAccept u- gt System out println  Thank you mom for      u         But your father interferes and generally aborts mom s plans and     completes the promise  sets its value   with far lesser contribution     as fathers do  very resolutely  while mom is slowly opening her purse      remember the Thread sleep         promise complete 10      Output of that is   Thank you mom for  10   Mom s promise was created   but waited for some  completion  event   CompletableFuture lt Integer gt  promise      You created such event  accepting her promise and announcing your plans to thank your mom   promise thenAccept      At this moment mom started open her purse   but very slow     and father interfered much faster and completed the promise instead of your mom   promise complete 10     Have you noticed an executor that I wrote explicitly   Interestingly  if you use a default implicit executor instead  commonPool  and father is not at home  but only mom with her  slow purse   then her promise will only complete  if the program lives longer than mom needs to get money from the purse   The default executor acts kind of like a  daemon  and does not wait for all promises to be fulfilled  I have not found a good description of this fact

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I am aware that there s already an accepted answer but would like to add my two cents nevertheless   TLDR  Future and Promise are the two sides of an asynchronous operation  consumer caller vs  producer implementor   As a caller of an asynchronous API method  you will get a Future as a handle to the computation s result  You can e g  call get   on it to wait for the computation to complete and retrieve the result   Now think of how this API method is actually implemented  The implementor must return a Future immediately  They are responsible for completing that future as soon as the computation is done  which they will know because it is implementing the dispatch logic  -    They will use a Promise CompletableFuture to do just that  Construct and return the CompletableFuture immediately  and call complete T result  once the computation is done

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According to this discussion  Promise has finally been called CompletableFuture for inclusion in Java 8  and its javadoc explains      A Future that may be explicitly completed  setting its value and status   and may be used as a CompletionStage  supporting dependent functions and actions that trigger upon its completion      An example is also given on the list   f then  s - gt  aStringFunction s   thenAsync s - gt          Note that the final API is slightly different but allows similar asynchronous execution   CompletableFuture lt String gt  f        f thenApply this  modifyString  thenAccept System out  println

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No set method in Future interface  only get method  so it is read-only  About CompletableFuture  this article maybe helpful  completablefuture

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I m not completely happy with the answers so far  so here is my attempt      I think that  Kevin Wright s comment   You can make a Promise and it s up to you to keep it  When someone else makes you a promise you must wait to see if they honour it in the Future   summarizes it pretty well  but some explanation can be useful   Futures and promises are pretty similar concepts  the difference is that a future is a read-only container for a result that does not yet exist  while a promise can be written  normally only once   The Java 8 CompletableFuture  and the Guava SettableFuture can be thought of as promises  because their value can be set   completed    but they also implement the Future interface  therefore there is no difference for the client    The result of the future will be set by  someone else  - by the result of an asynchronous computation  Note how FutureTask - a classic future - must be initialized with a Callable or Runnable  there is no no-argument constructor  and both Future and FutureTask are read-only from the outside  the set methods of FutureTask are protected   The value will be set to the result of the computation from the inside   On the other hand  the result of a promise can be set by  you   or in fact by anybody  anytime because it has a public setter method  Both CompletableFuture and SettableFuture can be created without any task  and their value can be set at any time  You send a promise to the client code  and fulfill it later as you wish   Note that CompletableFuture is not a  pure  promise  it can be initialized with a task just like FutureTask  and its most useful feature is the unrelated chaining of processing steps   Also note that a promise does not have to be a subtype of future and it does not have to be the same object  In Scala a Future object is created by an asynchronous computation or by a different Promise object  In C   the situation is similar  the promise object is used by the producer and the future object by the consumer  The advantage of this separation is that the client cannot set the value of the future   Both Spring and EJB 3 1 have an AsyncResult class  which is similar to the Scala C   promises  AsyncResult does implement Future but this is not the real future  asynchronous methods in Spring EJB return a different  read-only Future object through some background magic  and this second  real  future can be used by the client to access the result

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In this example you can take a look at how Promises can be used in Java for creating asynchronous sequences of calls  doSomeProcess        whenResult result - gt  System out println String format  quot Result of some process is   s  quot   result         whenException e - gt  System out println String format  quot Exception after some process is   s  quot   e getMessage           map String  toLowerCase       mapEx  result  e  - gt  e    null   String format  quot The mapped result is   s  quot   result    e getMessage         whenResult s - gt  System out println s

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Not sure if this can be an answer but as I see what others have said for someone it may look like you need two separate abstractions for both of these concepts so that one of them  Future  is just a read-only view of the other  Promise      but actually this is not needed   For example take a look at how promises are defined in javascript   https   promisesaplus com   https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Promise  The focus is on the composability using the then method like   asyncOp1    then function op1Result        do something   return asyncOp2        then function op2Result        do something more   return asyncOp3        then function op3Result        do something even more   return syncOp4 op3Result           then function result     console log result       catch function error     console log error        which makes asynchronous computation to look like synchronous   try     op1Result   syncOp1         do something   op1Result   syncOp2         do something more   op3Result   syncOp3         do something even more   syncOp4 op3Result           console log result     catch error      console log error       which is pretty cool   Not as cool as async-await but async-await just removes the boilerplate     then function result        from it    And actually their abstraction is pretty good as the promise constructor   new Promise  function resolve  reject       do it           allows you to provide two callbacks which can be used to either complete the Promise successfully or with an error  So that only the code that constructs the Promise can complete it and the code that receives an already constructed Promise object has the read-only view   With inheritance the above can be achieved if resolve and reject are protected methods

[java] What's the difference between a Future and a Promise?

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