Java equivalent to C extension methods

Question

I am looking to implement a functionality in a list of object as I would in C  using an extension method   Something like this   List lt DataObject gt  list         List initialization  list getData id     How do I do that in Java

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Another option is to use ForwardingXXX classes from google-guava library

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Project Lombok provides an annotation  ExtensionMethod that can be used to achieve the functionality you are asking for

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There is no extension methods in Java  but you can have it by manifold as below  You define  quot echo quot  method for strings by below sample   Extension public class MyStringExtension       public static void echo  This String thiz            System out println thiz            And after that  you can use this method  echo  for strings anywhere like   quot Hello World quot  echo        prints  quot Hello World quot   quot Welcome quot  echo            prints  quot Welcome quot  String name    quot Jonn quot   name echo                 prints  quot John quot     You can also  of course  have parameters like   Extension public class MyStringExtension       public static void echo  This String thiz  String suffix            System out println thiz    quot   quot    suffix            And use like this   quot Hello World quot  echo  quot programming quot        prints  quot Hello World programming quot   quot Welcome quot  echo  quot 2021 quot                   prints  quot Welcome 2021 quot  String name    quot Jonn quot   name echo  quot Conor quot                       prints  quot John Conor quot   You can take a look at this sample also  Manifold-sample

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We can simulate the implementation of C  extension methods in Java by using the default method implementation available since Java 8  We start by defining an interface that will allow us to access the support object via a base   method  like so   public interface Extension lt T gt         default T base             return null            We return null since interfaces cannot have state  but this has to be fixed later via a proxy   The developer of extensions would have to extend this interface by a new interface containing extension methods  Let s say we want to add a forEach consumer on List interface   public interface ListExtension lt T gt  extends Extension lt List lt T gt  gt         default void foreach Consumer lt T gt  consumer            for  T item   base                  consumer accept item                        Because we extend the Extension interface  we can call base   method inside our extension method to access the support object we attach to   The Extension interface must have a factory method which will create an extension of a given support object   public interface Extension lt T gt                  static  lt E extends Extension lt T gt   T gt  E create Class lt E gt  type  T instance            if  type isInterface                  ExtensionHandler lt T gt  handler   new ExtensionHandler lt T gt  instance               List lt Class lt   gt  gt  interfaces   new ArrayList lt Class lt   gt  gt                 interfaces add type               Class lt   gt  baseType   type getSuperclass                while  baseType    null  amp  amp  baseType isInterface                      interfaces add baseType                   baseType   baseType getSuperclass                              Object proxy   Proxy newProxyInstance                      Extension class getClassLoader                        interfaces toArray new Class lt   gt  interfaces size                          handler               return type cast proxy             else               return null                      We create a proxy that implements the extension interface and all the interface implemented by the type of the support object  The invocation handler given to the proxy would dispatch all the calls to the support object  except for the  base  method  which must return the support object  otherwise its default implementation is returning null   public class ExtensionHandler lt T gt  implements InvocationHandler        private T instance       private ExtensionHandler T instance            this instance   instance              Override     public Object invoke Object proxy  Method method  Object   args              throws Throwable           if   base  equals method getName                     amp  amp  method getParameterCount      0                return instance            else               Class lt   gt  type   method getDeclaringClass                MethodHandles Lookup lookup   MethodHandles lookup                    in type               Field allowedModesField   lookup getClass   getDeclaredField  allowedModes                makeFieldModifiable allowedModesField               allowedModesField set lookup  -1               return lookup                  unreflectSpecial method  type                   bindTo proxy                   invokeWithArguments args                        private static void makeFieldModifiable Field field  throws Exception           field setAccessible true           Field modifiersField   Field class getDeclaredField  modifiers            modifiersField setAccessible true           modifiersField                  setInt field  field getModifiers    amp   Modifier FINAL              Then  we can use the Extension create   method to attach the interface containing the extension method to the support object  The result is an object which can be casted to the extension interface by which we can still access the support object calling the base   method  Having the reference casted to the extension interface  we now can safely call the extension methods that can have access to the support object  so that now we can attach new methods to the existing object  but not to its defining type   public class Program        public static void main String   args            List lt String gt  list   Arrays asList  a    b    c            ListExtension lt String gt  listExtension   Extension create ListExtension class  list           listExtension foreach System out  println              So  this is a way we can simulate the ability to extend objects in Java by adding new contracts to them  which allow us to call additional methods on the given objects   Below you may find the code of the Extension interface   import java lang reflect Field  import java lang reflect InvocationHandler  import java lang reflect Method  import java lang reflect Modifier  import java lang reflect Proxy  import java util ArrayList  import java util List   public interface Extension lt T gt         public class ExtensionHandler lt T gt  implements InvocationHandler            private T instance           private ExtensionHandler T instance                this instance   instance                      Override         public Object invoke Object proxy  Method method  Object   args                  throws Throwable               if   base  equals method getName                         amp  amp  method getParameterCount      0                    return instance                else                   Class lt   gt  type   method getDeclaringClass                    MethodHandles Lookup lookup   MethodHandles lookup                        in type                   Field allowedModesField   lookup getClass   getDeclaredField  allowedModes                    makeFieldModifiable allowedModesField                   allowedModesField set lookup  -1                   return lookup                      unreflectSpecial method  type                       bindTo proxy                       invokeWithArguments args                                    private static void makeFieldModifiable Field field  throws Exception               field setAccessible true               Field modifiersField   Field class getDeclaredField  modifiers                modifiersField setAccessible true               modifiersField setInt field  field getModifiers    amp   Modifier FINAL                         default T base             return null             static  lt E extends Extension lt T gt   T gt  E create Class lt E gt  type  T instance            if  type isInterface                  ExtensionHandler lt T gt  handler   new ExtensionHandler lt T gt  instance               List lt Class lt   gt  gt  interfaces   new ArrayList lt Class lt   gt  gt                 interfaces add type               Class lt   gt  baseType   type getSuperclass                while  baseType    null  amp  amp  baseType isInterface                      interfaces add baseType                   baseType   baseType getSuperclass                              Object proxy   Proxy newProxyInstance                      Extension class getClassLoader                        interfaces toArray new Class lt   gt  interfaces size                          handler               return type cast proxy             else               return null

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The XTend language nbsp  mdash  which is a super-set of Java  and compiles to Java source code1 nbsp  mdash  supports this

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Java does not have such feature   Instead you can either create regular subclass of your list implementation or create anonymous inner class   List lt String gt  list   new ArrayList lt String gt         public String getData            return        add your implementation here             The problem is to call this method  You can do it  in place    new ArrayList lt String gt         public String getData            return        add your implementation here          getData

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Java does not support extension methods   Instead  you can make a regular static method  or write your own class

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It looks like there is some small chance that Defender Methods  i e  default methods  might make it into Java 8  However  as far as I understand them  they only allow the author of an interface to retroactively extend it  not arbitrary users   Defender Methods   Interface Injection would then be able to fully implement C -style extension methods  but AFAICS  Interface Injection isn t even on the Java 8 road-map yet

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You can create a C  like extension helper method by  RE  implementing the Collections interface and adding- example for Java Collection   public class RockCollection lt T extends Comparable lt T gt  gt  implements Collection lt T gt    private Collection lt T gt   list   new ArrayList lt T gt                   Custom extension methods             public T doSomething           do some stuff     return  list        proper examples public T find Predicate lt T gt  predicate        return  list stream                filter predicate               findFirst                get       public List lt T gt  findAll Predicate lt T gt  predicate        return  list stream                filter predicate               collect Collectors  lt T gt toList        public String join String joiner        StringBuilder aggregate   new StringBuilder           list forEach  item - gt          aggregate append item toString     joiner             return aggregate toString   substring 0  aggregate length   - 1      public List lt T gt  reverse         List lt T gt  listToReverse    List lt T gt   list      Collections reverse listToReverse       return listToReverse     public List lt T gt  sort Comparator lt T gt  sortComparer        List lt T gt  listToReverse    List lt T gt   list      Collections sort listToReverse  sortComparer       return listToReverse     public int sum         List lt T gt  list    List lt T gt   list      int total   0      for  T aList   list            total    Integer parseInt aList toString               return total     public List lt T gt  minus RockCollection lt T gt  listToMinus        List lt T gt  list    List lt T gt   list      int total   0      listToMinus forEach list  remove       return list     public Double average         List lt T gt  list    List lt T gt   list      Double total   0 0      for  T aList   list            total    Double parseDouble aList toString               return total   list size       public T first         return  list stream   findFirst   get                   collect Collectors  lt T gt toList       public T last         List lt T gt  list    List lt T gt   list      return list get  list size   - 1                                                        Re-implement existing methods  Override public int size         return  list size        Override public boolean isEmpty         return  list    null     list size      0

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Manifold provides Java with C -style extension methods and several other features   Unlike other tools  Manifold has no limitations and does not suffer from issues with generics  lambdas  IDE etc   Manifold provides several other features such as F -style custom types  TypeScript-style structural interfaces  and Javascript-style expando types   Additionally  IntelliJ provides comprehensive support for Manifold via the Manifold plugin   Manifold is an open source project available on github

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Extension methods are not just static method and not just convenience syntax sugar  in fact they are quite powerful tool  The main thing there is ability to override different methods based on different generic   s parameters instantiation  This is similar to Haskell   s type classes  and in fact  it looks like they are in C  to support C    s Monads  i e  LINQ   Even dropping LINQ syntax  I still don   t know any way to implement similar interfaces in Java    And I don   t think it is possible to implement them in Java  because of Java   s type erasure semantics of generics parameters

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Bit late to the party on this question  but in case anyone finds it useful I just created a subclass   public class ArrayList2 lt T gt  extends ArrayList lt T gt         private static final long serialVersionUID   1L       public T getLast                 if  this isEmpty                          return null                    else                              return this get this size   - 1

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Technically C  Extension have no equivalent in Java  But if you do want to implement such functions for a cleaner code and maintainability  you have to use Manifold framework   package extensions java lang String   import manifold ext api      Extension public class MyStringExtension      public static void print  This String thiz        System out println thiz           Extension   public static String lineSeparator         return System lineSeparator

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Java 8 now supports default methods  which are similar to C  s extension methods

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One could be use the decorator object-oriented design pattern  An example of this pattern being used in Java s standard library would be the DataOutputStream   Here s some code for augmenting the functionality of a List   public class ListDecorator lt E gt  implements List lt E gt        public final List lt E gt  wrapee       public ListDecorator List lt E gt  wrapee                this wrapee   wrapee                implementation of all the list s methods here         public  lt R gt  ListDecorator lt R gt  map Transform lt E R gt  transformer                ArrayList lt R gt  result   new ArrayList lt R gt  size             for  E element   this                        R transformed   transformer transform element               result add transformed                     return new ListDecorator lt R gt  result             P S  I m a big fan of Kotlin  It has extension methods and also runs on the JVM

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