What is a callback in java

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Possible Duplicate    What is a callback function        I have read the wikipedia definition of a callback but I still didn t get it  Can anyone explain me what a callback is  especially the following line      In computer programming  a callback is a reference to executable code  or a piece of executable code  that is passed as an argument to other code  This allows a lower-level software layer to call a subroutine  or function  defined in a higher-level layer

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Strictly speaking  the concept of a callback function does not exist in Java  because in Java there are no functions  only methods  and you cannot pass a method around  you can only pass objects and interfaces   So  whoever has a reference to that object or interface may invoke any of its methods  not just one method that you might wish them to    However  this is all fine and well  and we often speak of callback objects and callback interfaces  and when there is only one method in that object or interface  we may even speak of a callback method or even a callback function  we humans tend to thrive in inaccurate communication     Actually  perhaps the best approach is to just speak of  a callback  without adding any qualifications  this way  you cannot possibly go wrong    See next sentence     One of the most famous examples of using a callback in Java is when you call an ArrayList object to sort itself  and you supply a comparator which knows how to compare the objects contained within the list     Your code is the high-level layer  which calls the lower-level layer  the standard java runtime list object  supplying it with an interface to an object which is in your  high level  layer  The list will then be  calling back  your object to do the part of the job that it does not know how to do  namely to compare elements of the list  So  in this scenario the comparator can be thought of as a callback object

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In Java  callback methods are mainly used to address the  Observer Pattern   which is closely related to  Asynchronous Programming    Although callbacks are also used to simulate passing methods as a parameter  like what is done in functional programming languages

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A callback is commonly used in asynchronous programming  so you could create a method which handles the response from a web service  When you call the web service  you could pass the method to it so that when the web service responds  it call s the method you told it     it  calls back    In Java this can commonly be done through implementing an interface and passing an object  or an anonymous inner class  that implements it  You find this often with transactions and threading - such as the Futures API   http   docs oracle com javase 1 5 0 docs api java util concurrent Future html

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A callback is some code that you pass to a given method  so that it can be called at a later time   In Java one obvious example is java util Comparator  You do not usually use a Comparator directly  rather  you pass it to some code that calls the Comparator at a later time   Example   class CodedString implements Comparable lt CodedString gt        private int code      private String text                 Override     public boolean equals                member-wise equality             Override     public int hashCode                member-wise equality              Override     public boolean compareTo CodedString cs               Compare using  code  first  then             text  if both codes are equal                public void sortCodedStringsByText List lt CodedString gt  codedStrings        Comparator lt CodedString gt  comparatorByText   new Comparator lt CodedString gt               Override         public int compare CodedString cs1  CodedString cs2                   Compare cs1 and cs2 using just the  text  field                         Here we pass the comparatorByText callback to Collections sort             Collections sort      will then call this callback whenever it        needs to compare two items from the list being sorted         As a result  we will get the list sorted by just the  text  field         If we do not pass a callback  Collections sort will use the default        comparison for the class  first by  code   then by  text        Collections sort codedStrings  comparatorByText

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Maybe an example would help   Your app wants to download a file from some remote computer and then write to to a local disk   The remote computer is the other side of a dial-up modem and a satellite link  The latency and transfer time will be huge and you have other things to do   So  you have a function method that will write a buffer to disk   You pass a pointer to this method to your network API  together with the remote URI and other stuff   This network call returns  immediately  and you can do your other stuff   30 seconds later  the first buffer from the remote computer arrives at the network layer   The network layer then calls the function that you passed during the setup and so the buffer gets written to disk - the network layer has  called back    Note that  in this example  the callback would happen on a network layer thread than the originating thread  but that does not matter - the buffer still gets written to the disk

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Callbacks are most easily described in terms of the telephone system  A function call is analogous to calling someone on a telephone  asking her a question  getting an answer  and hanging up  adding a callback changes the analogy so that after asking her a question  you also give her your name and number so she can call you back with the answer  Paul Jakubik  Callback Implementations in C

[java] What is a callback in java

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