Function Pointers in Java

Question

This may be something common and trivial  but I seem to be having trouble finding a concrete answer  In C  there is a concept of delegates  which relates strongly to the idea of function pointers from C    Is there a similar functionality in Java  Given that pointers are somewhat absent  what is the best way about this  And to be clear  we re talking first class here

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No  functions are not first class objects in java  You can do the same thing by implementing a handler class - this is how callbacks are implemented in the Swing etc   There are however proposals for closures  the official name for what you re talking about  in future versions of java - Javaworld has an interesting article

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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  We have 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 did 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 it s 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

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To achieve similar functionality you could use anonymous inner classes   If you were to define a interface Foo   interface Foo       Object myFunc Object arg       Create a method bar which will receive a  function pointer  as an argument   public void bar Foo foo                     Object object   foo myFunc argValue                    Finally call the method as follows   bar new Foo         public Object myFunc Object arg               Function code

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This brings to mind Steve Yegge s Execution in the Kingdom of Nouns  It basically states that Java needs an object for every action  and therefore does not have  verb-only  entities like function pointers

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You can use reflection to do it   Pass as parameter the object and the method name  as a string  and then invoke the method  For example   Object methodCaller Object theObject  String methodName       return theObject getClass   getMethod methodName  invoke theObject         Catch the exceptions     And then use it as in   String theDescription   methodCaller object1   toString    Class theClass   methodCaller object2   getClass      Of course  check all exceptions and add the needed casts

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There is no such thing in Java  You will need to wrap your function into some object and pass the reference to that object in order to pass the reference to the method on that object   Syntactically  this can be eased to a certain extent by using anonymous classes defined in-place or anonymous classes defined as member variables of the class    Example   class MyComponent extends JPanel       private JButton button      public MyComponent             button   new JButton  click me            button addActionListener buttonAction           add button              private ActionListener buttonAction   new ActionListener             public void actionPerformed ActionEvent e                   handle the event                   note how the handler instance can access                 members of the surrounding class             button setText  you clicked me

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You can substitue a function pointer with an interface  Lets say you want to run through a collection and do something with each element   public interface IFunction     public void execute Object o       This is the interface we could pass to some say CollectionUtils2 doFunc Collection c  IFunction f    public static void doFunc Collection c  IFunction f       for  Object o   c          f execute o            As an example say we have a collection of numbers and you would like to add 1 to every element   CollectionUtils2 doFunc List numbers  new IFunction         public void execute Object o           Integer anInt    Integer  o         anInt

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Java8 has introduced lambdas and method references  So if your function matches a functional interface  you can create your own  you can use a method reference in this case   Java provides a set of common functional interfaces  whereas you could do the following   public class Test      public void test1 Integer i        public void test2 Integer i        public void consumer Consumer lt Integer gt  a         a accept 10           public void provideConsumer          consumer this  test1        method reference      consumer x - gt  test2 x       lambda

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Check the closures how they have been implemented in the lambdaj library  They actually have a behavior very similar to C  delegates   http   code google com p lambdaj wiki Closures

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Relative to most people here I am new to java but since I haven t seen a similar suggestion I have another alternative to suggest  Im not sure if its a good practice or not  or even suggested before and I just didn t get it  I just like it since I think its self descriptive      Just to merge functions in a common name    public class CustomFunction    public CustomFunction            Actual functions    public class Function1 extends CustomFunction   public Function1      public void execute      something here          public class Function2 extends CustomFunction   public Function2      public void execute      something here                   in Main class    CustomFunction functionpointer   null    then depending on the application  assign   functionpointer   new Function1     functionpointer   new Function2      etc    and call by   functionpointer execute

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The Java idiom for function-pointer-like functionality is an an anonymous class implementing an interface  e g   Collections sort list  new Comparator lt MyClass gt         public int compare MyClass a  MyClass b                   compare objects             Update  the above is necessary in Java versions prior to Java 8  Now we have much nicer alternatives  namely lambdas   list sort  a  b  - gt  a isGreaterThan b      and method references   list sort MyClass  isGreaterThan

[java] Function Pointers in Java

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