Java Delegates

Question

Does the Java language have delegate features  similar to how C  has support for delegates

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No  but it has similar behavior  internally   In C  delegates are used to creates a separate entry point and they work much like a function pointer   In java there is no thing as function pointer  on a upper look  but internally Java needs to do the same thing in order to achieve these objectives   For example  creating threads in Java requires a class extending Thread or implementing Runnable  because a class object variable can be used a memory location pointer

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While it is nowhere nearly as clean  but you could implement something like C  delegates using a Java Proxy

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I know this post is old  but Java 8 has added lambdas  and the concept of a functional interface  which is any interface with only one method   Together these offer similar functionality to C  delegates   See here for more info  or just google Java Lambdas  http   cr openjdk java net  briangoetz lambda lambda-state-final html

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Have you read this       Delegates are a useful construct in event-based systems  Essentially   Delegates are objects that encode a method dispatch on a specified   object  This document shows how java inner classes provide a more   generic solution to such problems       What is a Delegate  Really it is very similar to a pointer to member   function as used in C    But a delegate contains the target object   alongwith the method to be invoked  Ideally it would be nice to be   able to say       obj registerHandler ano methodOne           and that the method methodOne would be called on ano when some specific event was received       This is what the Delegate structure achieves       Java Inner Classes       It has been argued that Java provides this   functionality via anonymous inner classes and thus does not need the additional   Delegate construct    obj registerHandler new Handler             public void handleIt Event ev                methodOne ev                             At first glance this seems correct but at the same time a nuisance    Because for many event processing examples the simplicity of the   Delegates syntax is very attractive       General Handler       However  if event-based programming is used in a more   pervasive manner  say  for example  as a part of a general   asynchronous programming environment  there is more at stake       In such a general situation  it is not sufficient to include only the   target method and target object instance  In general there may be   other parameters required  that are determined within the context when   the event handler is registered       In this more general situation  the java approach can provide a very   elegant solution  particularly when combined with use of final   variables    void processState final T1 p1  final T2 dispatch       final int a1   someCalculation       m obj registerHandler new Handler         public void handleIt Event ev         dispatch methodOne a1  ev  p1                       final   final   final       Got your attention       Note that the final variables are accessible from within the anonymous   class method definitions  Be sure to study this code carefully to   understand the ramifications  This is potentially a very powerful   technique  For example  it can be used to good effect when registering   handlers in MiniDOM and in more general situations       By contrast  the Delegate construct does not provide a solution for   this more general requirement  and as such should be rejected as an   idiom on which designs can be based

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Depending precisely what you mean  you can achieve a similar effect  passing around a method  using the Strategy Pattern   Instead of a line like this declaring a named method signature      C  public delegate void SomeFunction      declare an interface      Java public interface ISomeBehaviour      void SomeFunction        For concrete implementations of the method  define a class that implements the behaviour      Java public class TypeABehaviour implements ISomeBehaviour      public void SomeFunction              TypeA behaviour         public class TypeBBehaviour implements ISomeBehaviour      public void SomeFunction              TypeB behaviour          Then wherever you would have had a SomeFunction delegate in C   use an ISomeBehaviour reference instead      C  SomeFunction doSomething   SomeMethod  doSomething    doSomething   SomeOtherMethod  doSomething        Java ISomeBehaviour someBehaviour   new TypeABehaviour    someBehaviour SomeFunction    someBehaviour   new TypeBBehaviour    someBehaviour SomeFunction      With anonymous inner classes  you can even avoid declaring separate named classes and almost treat them like real delegate functions      Java public void SomeMethod ISomeBehaviour pSomeBehaviour                   SomeMethod new ISomeBehaviour          Override    public void SomeFunction              your implementation            This should probably only be used when the implementation is very specific to the current context and wouldn t benefit from being reused   And then of course in Java 8  these do become basically lambda expressions      Java 8 SomeMethod    - gt       your implementation

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The code described  offers many of the advantages of C  delegates  Methods  either static or dynamic  can be treated in a uniform manner  The complexity in calling methods through reflection is reduced and the code is reusable  in the sense of requiring no additional classes in the user code  Note we are calling an alternate convenience version of invoke  where a method with one parameter can be called without creating an object array Java code below     class Class1           public void show String s    System out println s                class Class2           public void display String s    System out println s                   allows static method as well     class Class3           public static void staticDisplay String s    System out println s                public class TestDelegate            public static final Class   OUTPUT ARGS     String class            public final Delegator DO SHOW   new Delegator OUTPUT ARGS Void TYPE            public void main String   args                 Delegate   items   new Delegate 3                items 0    DO SHOW  build new Class1    show                items 1    DO SHOW build  new Class2    display                items 2    DO SHOW build Class3 class   staticDisplay                 for int i   0  i  lt  items length  i                      items i  invoke  Hello World

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With safety-mirror on the classpath you get something similar to C  s delegates and events  Examples from the project s README  Delegates in Java    Delegate With1Param lt String  String gt  greetingsDelegate   new Delegate With1Param lt  gt       greetingsDelegate add str - gt   quot Hello  quot    str     greetingsDelegate add str - gt   quot Goodbye  quot    str      DelegateInvocationResult lt String gt  invocationResult      greetingsDelegate invokeAndAggregateExceptions  quot Sir quot       invocationResult getFunctionInvocationResults   forEach funInvRes - gt      System out println funInvRes getResult          prints   quot Hello sir quot  and  quot Goodbye Sir quot   Events     Create a private Delegate  Make sure it is private so only  you  can invoke it    private static Delegate With0Params lt String gt  trimDelegate   new Delegate With0Params lt  gt          Create a public Event using the delegate you just created    public static Event With0Params lt String gt  trimEvent  new Event With0Params lt  gt  trimDelegate   See also this SO answer

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Java doesn t have delegates and is proud of it     From what I read here I found in essence 2 ways to fake delegates  1  reflection  2  inner class  Reflections are slooooow  Inner class does not cover the simplest use-case  sort function  Do not want to go into details  but the solution with inner class basically is to create a wrapper class for an array of integers to be sorted in ascending order and an class for an array of integers to be sorted in descending order

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I have implemented callback delegate support in Java using reflection   Details and working source are available on my website   How It Works  There is a principle class named Callback with a nested class named WithParms   The API which needs the callback will take a Callback object as a parameter and  if neccessary  create a Callback WithParms as a method variable   Since a great many of the applications of this object will be recursive  this works very cleanly   With performance still a high priority to me  I didn t want to be required to create a throwaway object array to hold the parameters for every invocation - after all in a large data structure there could be thousands of elements  and in a message processing scenario we could end up processing thousands of data structures a second   In order to be threadsafe the parameter array needs to exist uniquely for each invocation of the API method  and for efficiency the same one should be used for every invocation of the callback  I needed a second object which would be cheap to create in order to bind the callback with a parameter array for invocation   But  in some scenarios  the invoker would already have a the parameter array for other reasons   For these two reasons  the parameter array does not belong in the Callback object   Also the choice of invocation  passing the parameters as an array or as individual objects  belongs in the hands of the API using the callback enabling it to use whichever invocation is  best suited to its inner workings   The WithParms nested class  then  is optional and serves two purposes  it contains the parameter object array needed for the callback invocations  and it provides 10 overloaded invoke   methods  with from 1 to 10 parameters  which load the parameter array and then invoke the callback target   What follows is an example using a callback to process the files in a directory tree   This is an initial validation pass which just counts the files to process and ensure none exceed a predetermined maximum size  In this case we just create the callback inline with the API invocation   However  we reflect the target method out as a static value so that the reflection is not done every time   static private final Method             COUNT  Callback getMethod Xxx class  callback count  true File class File class         IoUtil processDirectory root new Callback this COUNT  selector         private void callback count File dir  File fil        if fil  null                                                                                   file is null for processing a directory         fileTotal            if fil length   gt fileSizeLimit                throw new Abort  Failed   File size exceeds maximum of   TextUtil formatNumber fileSizeLimit    bytes    fil                               progress  Counting  dir fileTotal           IoUtil processDirectory            Process a directory using callbacks   To interrupt  the callback must throw an  unchecked  exception     Subdirectories are processed only if the selector is null or selects the directories  and are done    after the files in any given directory   When the callback is invoked for a directory  the file    argument is null      lt p gt     The callback signature is      lt pre gt     void callback File dir  File ent   lt  pre gt      lt p gt      return          The number of files processed      static public int processDirectory File dir  Callback cbk  FileSelector sel        return  processDirectory dir new Callback WithParms cbk 2  sel          static private int  processDirectory File dir  Callback WithParms cbk  FileSelector sel        int                                 cnt 0       if  dir isDirectory              if sel  null    sel accept dir     cbk invoke dir getParent   dir   cnt                    else           cbk invoke dir  Object   null            File   lst  sel  null   dir listFiles     dir listFiles sel            if lst  null                for int xa 0  xa lt lst length  xa                      File ent lst xa                   if  ent isDirectory                          cbk invoke dir ent                       lst xa  null                      cnt                                                        for int xa 0  xa lt lst length  xa                      File ent lst xa                   if ent  null    cnt   processDirectory ent cbk sel                                                   return cnt          This example illustrates the beauty of this approach - the application specific logic is abstracted into the callback  and the drudgery of recursively walking a directory tree is tucked nicely away in a completely reusable static utility method   And we don t have to repeatedly pay the price of defining and implementing an interface for every new use  Of course  the argument for an interface is that it is far more explicit about what to implement  it s enforced  not simply documented  - but in practice I have not found it to be a problem to get the callback definition right   Defining and implementing an interface is not really so bad  unless you re distributing applets  as I am  where avoiding creating extra classes actually matters   but where this really shines is when you have multiple callbacks in a single class  Not only is being forced to push them each into a separate inner class added overhead in the deployed application  but it s downright tedious to program and all that boiler-plate code is really just  noise

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Yes  amp  No  but delegate pattern in Java could be thought of this way  This video tutorial is about data exchange between activity - fragments  and it has great essence of delegate sorta pattern using interfaces

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It doesn t have an explicit delegate keyword as C   but you can achieve similar in Java 8 by using a functional interface  i e  any interface with exactly one method  and lambda   private interface SingleFunc       void printMe       public static void main String   args        SingleFunc sf      - gt            System out println  Hello  I am a simple single func                SingleFunc sfComplex      - gt            System out println  Hello  I am a COMPLEX single func                delegate sf       delegate sfComplex      private static void delegate SingleFunc f        f printMe        Every new object of type SingleFunc must implement printMe    so it is safe to pass it to another method  e g  delegate SingleFunc   to call the printMe   method

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Short story                                        no   Introduction The newest version of the Microsoft Visual J   development environment supports a language construct called delegates or bound method references  This construct  and the new keywords delegate and multicast introduced to support it  are not a part of the JavaTM programming language  which is specified by the Java Language Specification and amended by the Inner Classes Specification included in the documentation for the JDKTM 1 1 software  It is unlikely that the Java programming language will ever include this construct  Sun already carefully considered adopting it in 1996  to the extent of building and discarding working prototypes  Our conclusion was that bound method references are unnecessary and detrimental to the language  This decision was made in consultation with Borland International  who had previous experience with bound method references in Delphi Object Pascal  We believe bound method references are unnecessary because another design alternative  inner classes  provides equal or superior functionality  In particular  inner classes fully support the requirements of user-interface event handling  and have been used to implement a user-interface API at least as comprehensive as the Windows Foundation Classes  We believe bound method references are harmful because they detract from the simplicity of the Java programming language and the pervasively object-oriented character of the APIs  Bound method references also introduce irregularity into the language syntax and scoping rules  Finally  they dilute the investment in VM technologies because VMs are required to handle additional and disparate types of references and method linkage efficiently

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No  Java doesn t have that amazing feature  But you could create it manually using the observer pattern  Here is an example  Write C  delegate in java

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No  but they re fakeable using proxies and reflection     public static class TestClass         public String knockKnock               return  who s there                   private final TestClass testInstance   new TestClass        Test public void   can delegate a single method interface to an instance   throws Exception         Delegator lt TestClass  Callable lt String gt  gt  knockKnockDelegator   Delegator ofMethod  knockKnock                                                                       of TestClass class                                                                      to Callable class         Callable lt String gt  callable   knockKnockDelegator delegateTo testInstance         assertThat callable call    is  who s there            The nice thing about this idiom is that you can verify that the delegated-to method exists  and has the required signature  at the point where you create the delegator  although not at compile-time  unfortunately  although a FindBugs plug-in might help here   then use it safely to delegate to various instances   See the karg code on github for more tests and implementation

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Not really  no  You may be able to achieve the same effect by using reflection to get Method objects you can then invoke  and the other way is to create an interface with a single  invoke  or  execute  method  and then instantiate them to call the method your interested in  i e  using an anonymous inner class   You might also find this article interesting   useful   A Java Programmer Looks at C  Delegates   blueskyprojects com

[java] Java Delegates?

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