Callback functions in Java

Question

Is there a way to pass a call back function in a Java method   The behavior I m trying to mimic is a  Net Delegate being passed to a function   I ve seen people suggesting creating a separate object but that seems overkill  however I am aware that sometimes overkill is the only way to do things

User · Answer

I found the idea of implementing using the reflect library interesting and came up with this which I think works quite well. The only down side is losing the compile time check that you are passing valid parameters.

public class CallBack {
    private String methodName;
    private Object scope;

    public CallBack(Object scope, String methodName) {
        this.methodName = methodName;
        this.scope = scope;
    }

    public Object invoke(Object... parameters) throws InvocationTargetException, IllegalAccessException, NoSuchMethodException {
        Method method = scope.getClass().getMethod(methodName, getParameterClasses(parameters));
        return method.invoke(scope, parameters);
    }

    private Class[] getParameterClasses(Object... parameters) {
        Class[] classes = new Class[parameters.length];
        for (int i=0; i < classes.length; i++) {
            classes[i] = parameters[i].getClass();
        }
        return classes;
    }
}

You use it like this

public class CallBackTest {
    @Test
    public void testCallBack() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        TestClass testClass = new TestClass();
        CallBack callBack = new CallBack(testClass, "hello");
        callBack.invoke();
        callBack.invoke("Fred");
    }

    public class TestClass {
        public void hello() {
            System.out.println("Hello World");
        }

        public void hello(String name) {
            System.out.println("Hello " + name);
        }
    }
}

User · Answer

You also can do theCallback using the Delegate pattern   Callback java  public interface Callback       void onItemSelected int position       PagerActivity java  public class PagerActivity implements Callback        CustomPagerAdapter mPagerAdapter       public PagerActivity             mPagerAdapter   new CustomPagerAdapter this               Override     public void onItemSelected int position               Do something         System out println  Item     postion     selected             CustomPagerAdapter java  public class CustomPagerAdapter       private static final int DEFAULT POSITION   1      public CustomPagerAdapter Callback callback            callback onItemSelected DEFAULT POSITION

User · Answer

I tried using java lang reflect to implement  callback   here s a sample   package StackOverflowQ443708 JavaCallBackTest   import java lang reflect    import java util concurrent     class MyTimer       ExecutorService EXE             Executors newCachedThreadPool             Executors newSingleThreadExecutor          public static void PrintLine                  System out println   --------------------------------------------------------------------------------               public void SetTimer  final int timeout  final Object obj  final String methodName  final Object    args                SetTimer  timeout  obj  false  methodName  args             public void SetTimer  final int timeout  final Object obj  final boolean isStatic  final String methodName  final Object    args                Class lt   gt    argTypes   null          if  args    null                        argTypes   new Class lt   gt   args length               for  int i 0  i lt args length  i                                  argTypes i    args i  getClass                                      SetTimer  timeout  obj  isStatic  methodName  argTypes  args             public void SetTimer  final int timeout  final Object obj  final String methodName  final Class lt   gt    argTypes  final Object    args                SetTimer  timeout  obj  false  methodName  argTypes  args             public void SetTimer  final int timeout  final Object obj  final boolean isStatic  final String methodName  final Class lt   gt    argTypes  final Object    args                EXE execute               new Runnable                                 public void run                                          Class lt   gt  c                      Method method                      try                                               if  isStatic  c    Class lt   gt  obj                          else c   obj getClass                              System out println   Wait for     timeout     seconds to invoke     c getSimpleName              methodName                                 TimeUnit SECONDS sleep  timeout                           System out println                             System out println   invoking     c getSimpleName              methodName                                    PrintLine                             method   c getDeclaredMethod  methodName  argTypes                           method invoke  obj  args                                             catch  Exception e                                                e printStackTrace                                              finally                                               PrintLine                                                                                public void ShutdownTimer                  EXE shutdown              public class CallBackTest       public void onUserTimeout                  System out println   onUserTimeout              public void onTestEnd                  System out println   onTestEnd              public void NullParameterTest  String sParam  int iParam                System out println   NullParameterTest  String parameter     sParam      int parameter     iParam             public static void main  String   args                CallBackTest test   new CallBackTest             MyTimer timer   new MyTimer              timer SetTimer   int  Math random    10   test   onUserTimeout            timer SetTimer   int  Math random    10   test   onTestEnd            timer SetTimer   int  Math random    10   test   A-Method-Which-Is-Not-Exists          java lang NoSuchMethodException          timer SetTimer   int  Math random    10   System out   println    this is an argument of System out println   which is called by timer            timer SetTimer   int  Math random    10   System class  true   currentTimeMillis            timer SetTimer   int  Math random    10   System class  true   currentTimeMillis    Should-Not-Pass-Arguments          java lang NoSuchMethodException          timer SetTimer   int  Math random    10   String class  true   format     d  X   100  200      java lang NoSuchMethodException         timer SetTimer   int  Math random    10   String class  true   format     d  X   new Object   100  200             timer SetTimer   int  Math random    10   test   NullParameterTest   new Class lt   gt    String class  int class   null  888            timer ShutdownTimer

User · Answer

A method is not  yet  a first-class object in Java  you can t pass a function pointer as a callback  Instead  create an object  which usually implements an interface  that contains the method you need and pass that   Proposals for closures in Java mdash which would provide the behavior you are looking for mdash have been made  but none will be included in the upcoming Java 7 release

User · Answer

Create an Interface  and Create the Same Interface Property in Callback Class   interface dataFetchDelegate       void didFetchdata String data       callback class public class BackendManager     public dataFetchDelegate Delegate      public void getData              Do something  Http calls  Any other work        Delegate didFetchdata  this is callbackdata              Now in the class where you want to get called back implement the above Created Interface  and Also Pass  this  Object Reference of your class to be called back   public class Main implements dataFetchDelegate              public static void main  String   args                 new Main   getDatafromBackend               public void getDatafromBackend             BackendManager inc   new BackendManager              Pass this object as reference in this Scenario this is Main Object                     inc Delegate   this            make call         inc getData                 This method is called after task Code Completion     public void didFetchdata String callbackData               TODO Auto-generated method stub         System out println callbackData

User · Answer

yet i see there is most preferred way which was what i was looking for   it s basically derived from these answers but i had to manipulate it to more more redundant and efficient   and i think everybody looking for what i come up with  To the point    first make an Interface that simple  public interface myCallback       void onSuccess        void onError String err       now to make this callback run when ever you wish to do to handle the results - more likely after async call and you wanna run some stuff which depends on these reuslts     import the Interface class here  public class App        public static void main String   args               call your method         doSomething  list your Params   new myCallback                 Override             public void onSuccess                        no errors                 System out println  Done                                Override             public void onError String err                       error happen                 System out println err                                        private void doSomething String param     some params                                myCallback callback               now call onSuccess whenever you want if results are ready         if results success              callback onSuccess            else             callback onError someError              doSomething is the function that takes some time you wanna add a callback to it to notify you when the results came  add the call back interface as a parameter to this method  hope my point is clear  enjoy

User · Answer

A method is not  yet  a first-class object in Java  you can t pass a function pointer as a callback  Instead  create an object  which usually implements an interface  that contains the method you need and pass that   Proposals for closures in Java mdash which would provide the behavior you are looking for mdash have been made  but none will be included in the upcoming Java 7 release

User · Answer

Since Java 8  there are lambda and method references   Oracle Docs  Lambda Expressions Oracle Docs  Method References  For example  if you want a functional interface A - gt  B such as  import java util function Function   public MyClass       public static String applyFunction String name  Function lt String String gt  function           return function apply name            then you can call it like so MyClass applyFunction  quot 42 quot   str - gt   quot the answer is   quot    str      returns  quot the answer is  42 quot   Also you can pass class method  Say you have   Value    lombok public class PrefixAppender       private String prefix       public String addPrefix String suffix           return prefix   quot   quot  suffix           Then you can do  PrefixAppender prefixAppender  new PrefixAppender  quot prefix quot    MyClass applyFunction  quot some text quot   prefixAppender  addPrefix      returns  quot prefix some text quot   Note  Here I used the functional interface Function lt A B gt   but there are many others in the package java util function  Most notable ones are  Supplier  void - gt  A Consumer  A - gt  void BiConsumer   A B  - gt  void Function  A - gt  B BiFunction   A B  - gt  C  and many others that specialize on some of the input output type  Then  if it doesn t provide the one you need  you can create your own functional interface like so   FunctionalInterface interface Function3 lt In1  In2  In3  Out gt        In1 In2 In3  - gt  Out     public Out apply In1 in1  In2 in2  In3 in3      Example of use  String computeAnswer Function3 lt String  Integer  Integer  String gt  f       return f apply  quot 6x9  quot   6  9      computeAnswer  question  a  b  - gt  question    quot 42 quot        quot 6 9 42 quot   And you can also do that with thrown exception   FunctionalInterface interface FallibleFunction lt In  Out  Ex extends Exception gt        Out get In input  throws Ex    public  lt Ex extends IOException gt  String yo FallibleFunction lt Integer  String  Ex gt  f  throws Ex       return f get 42

User · Answer

Simpliest and easiest way is by creating a reusable model and trigger     https   onecompiler com java 3wejrcby2 fbclid IwAR0dHbGDChRUJoCZ3CIDW-JQu7Dz3iYGNGYjxYVCPCWfEqQDogFGTwuOuO8

User · Answer

For simplicity  you can use a Runnable   private void runCallback Runnable callback           Run callback     callback run        Usage   runCallback new Runnable          Override     public void run                    Running callback

User · Answer

When I need this kind of functionality in Java  I usually use the Observer pattern   It does imply an extra object  but I think it s a clean way to go  and is a widely understood pattern  which helps with code readability

User · Answer

public class HelloWorldAnonymousClasses          this is an interface with only one method     interface HelloWorld           public void printSomething String something                this is a simple function called from main       public void sayHello            this is an object with interface reference followed by the definition of the interface itself          new HelloWorld                 public void printSomething String something                    System out println  Hello     something                           printSomething  Abhi            imagine this as an object which is calling the function printSomething               public static void main String    args            HelloWorldAnonymousClasses myApp                   new HelloWorldAnonymousClasses            myApp sayHello              Output is  Hello Abhi    Basically if you want to make the object of an interface it is not possible  because interface cannot have objects   The option is to let some class implement the interface and then call that function using the object of that class  But this approach is really verbose   Alternatively  write new HelloWorld     oberserve this is an interface not a class  and then follow it up with the defination of the interface methods itself    This defination is in reality the anonymous class   Then you get the object reference through which you can call the method itself

User · Answer

I found the idea of implementing using the reflect library interesting and came up with this which I think works quite well. The only down side is losing the compile time check that you are passing valid parameters.

public class CallBack {
    private String methodName;
    private Object scope;

    public CallBack(Object scope, String methodName) {
        this.methodName = methodName;
        this.scope = scope;
    }

    public Object invoke(Object... parameters) throws InvocationTargetException, IllegalAccessException, NoSuchMethodException {
        Method method = scope.getClass().getMethod(methodName, getParameterClasses(parameters));
        return method.invoke(scope, parameters);
    }

    private Class[] getParameterClasses(Object... parameters) {
        Class[] classes = new Class[parameters.length];
        for (int i=0; i < classes.length; i++) {
            classes[i] = parameters[i].getClass();
        }
        return classes;
    }
}

You use it like this

public class CallBackTest {
    @Test
    public void testCallBack() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        TestClass testClass = new TestClass();
        CallBack callBack = new CallBack(testClass, "hello");
        callBack.invoke();
        callBack.invoke("Fred");
    }

    public class TestClass {
        public void hello() {
            System.out.println("Hello World");
        }

        public void hello(String name) {
            System.out.println("Hello " + name);
        }
    }
}

User · Answer

it s a bit old  but nevertheless    I found the answer of Peter Wilkinson nice except for the fact that it does not work for primitive types like int Integer   The problem is the  getClass   for the parameters i   which returns for instance java lang Integer  which on the other hand will not be correctly interpreted by getMethod methodName parameters     Java s fault       I combined it with the suggestion of Daniel Spiewak  in his answer to this   steps to success included  catching NoSuchMethodException -  getMethods   -  looking for the matching one by method getName   -  and then explicitly looping through the list of parameters and applying Daniels solution  such identifying the type matches and the signature matches

User · Answer

I tried using java lang reflect to implement  callback   here s a sample   package StackOverflowQ443708 JavaCallBackTest   import java lang reflect    import java util concurrent     class MyTimer       ExecutorService EXE             Executors newCachedThreadPool             Executors newSingleThreadExecutor          public static void PrintLine                  System out println   --------------------------------------------------------------------------------               public void SetTimer  final int timeout  final Object obj  final String methodName  final Object    args                SetTimer  timeout  obj  false  methodName  args             public void SetTimer  final int timeout  final Object obj  final boolean isStatic  final String methodName  final Object    args                Class lt   gt    argTypes   null          if  args    null                        argTypes   new Class lt   gt   args length               for  int i 0  i lt args length  i                                  argTypes i    args i  getClass                                      SetTimer  timeout  obj  isStatic  methodName  argTypes  args             public void SetTimer  final int timeout  final Object obj  final String methodName  final Class lt   gt    argTypes  final Object    args                SetTimer  timeout  obj  false  methodName  argTypes  args             public void SetTimer  final int timeout  final Object obj  final boolean isStatic  final String methodName  final Class lt   gt    argTypes  final Object    args                EXE execute               new Runnable                                 public void run                                          Class lt   gt  c                      Method method                      try                                               if  isStatic  c    Class lt   gt  obj                          else c   obj getClass                              System out println   Wait for     timeout     seconds to invoke     c getSimpleName              methodName                                 TimeUnit SECONDS sleep  timeout                           System out println                             System out println   invoking     c getSimpleName              methodName                                    PrintLine                             method   c getDeclaredMethod  methodName  argTypes                           method invoke  obj  args                                             catch  Exception e                                                e printStackTrace                                              finally                                               PrintLine                                                                                public void ShutdownTimer                  EXE shutdown              public class CallBackTest       public void onUserTimeout                  System out println   onUserTimeout              public void onTestEnd                  System out println   onTestEnd              public void NullParameterTest  String sParam  int iParam                System out println   NullParameterTest  String parameter     sParam      int parameter     iParam             public static void main  String   args                CallBackTest test   new CallBackTest             MyTimer timer   new MyTimer              timer SetTimer   int  Math random    10   test   onUserTimeout            timer SetTimer   int  Math random    10   test   onTestEnd            timer SetTimer   int  Math random    10   test   A-Method-Which-Is-Not-Exists          java lang NoSuchMethodException          timer SetTimer   int  Math random    10   System out   println    this is an argument of System out println   which is called by timer            timer SetTimer   int  Math random    10   System class  true   currentTimeMillis            timer SetTimer   int  Math random    10   System class  true   currentTimeMillis    Should-Not-Pass-Arguments          java lang NoSuchMethodException          timer SetTimer   int  Math random    10   String class  true   format     d  X   100  200      java lang NoSuchMethodException         timer SetTimer   int  Math random    10   String class  true   format     d  X   new Object   100  200             timer SetTimer   int  Math random    10   test   NullParameterTest   new Class lt   gt    String class  int class   null  888            timer ShutdownTimer

User · Answer

When I need this kind of functionality in Java  I usually use the Observer pattern   It does imply an extra object  but I think it s a clean way to go  and is a widely understood pattern  which helps with code readability

User · Answer

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

User · Answer

Since Java 8  there are lambda and method references   Oracle Docs  Lambda Expressions Oracle Docs  Method References  For example  if you want a functional interface A - gt  B such as  import java util function Function   public MyClass       public static String applyFunction String name  Function lt String String gt  function           return function apply name            then you can call it like so MyClass applyFunction  quot 42 quot   str - gt   quot the answer is   quot    str      returns  quot the answer is  42 quot   Also you can pass class method  Say you have   Value    lombok public class PrefixAppender       private String prefix       public String addPrefix String suffix           return prefix   quot   quot  suffix           Then you can do  PrefixAppender prefixAppender  new PrefixAppender  quot prefix quot    MyClass applyFunction  quot some text quot   prefixAppender  addPrefix      returns  quot prefix some text quot   Note  Here I used the functional interface Function lt A B gt   but there are many others in the package java util function  Most notable ones are  Supplier  void - gt  A Consumer  A - gt  void BiConsumer   A B  - gt  void Function  A - gt  B BiFunction   A B  - gt  C  and many others that specialize on some of the input output type  Then  if it doesn t provide the one you need  you can create your own functional interface like so   FunctionalInterface interface Function3 lt In1  In2  In3  Out gt        In1 In2 In3  - gt  Out     public Out apply In1 in1  In2 in2  In3 in3      Example of use  String computeAnswer Function3 lt String  Integer  Integer  String gt  f       return f apply  quot 6x9  quot   6  9      computeAnswer  question  a  b  - gt  question    quot 42 quot        quot 6 9 42 quot   And you can also do that with thrown exception   FunctionalInterface interface FallibleFunction lt In  Out  Ex extends Exception gt        Out get In input  throws Ex    public  lt Ex extends IOException gt  String yo FallibleFunction lt Integer  String  Ex gt  f  throws Ex       return f get 42

User · Answer

A little nitpicking      I ve seem people suggesting creating a   separate object but that seems   overkill   Passing a callback includes creating a separate object in pretty much any OO language  so it can hardly be considered overkill  What you probably mean is that in Java  it requires you to create a separate class  which is more verbose  and more resource-intensive  than in languages with explicit first-class functions or closures  However  anonymous classes at least reduce the verbosity and can be used inline

User · Answer

When I need this kind of functionality in Java  I usually use the Observer pattern   It does imply an extra object  but I think it s a clean way to go  and is a widely understood pattern  which helps with code readability

User · Answer

I think using an abstract class is more elegant  like this      Something java  public abstract class Something          public abstract void test                public void usingCallback             System out println  This is before callback method            test            System out println  This is after callback method                CallbackTest java  public class CallbackTest extends Something        Override     public void test             System out println  This is inside CallbackTest                public static void main String   args            CallbackTest myTest   new CallbackTest            myTest usingCallback                    Output  This is before callback method This is inside CallbackTest  This is after callback method

User · Answer

it s a bit old  but nevertheless    I found the answer of Peter Wilkinson nice except for the fact that it does not work for primitive types like int Integer   The problem is the  getClass   for the parameters i   which returns for instance java lang Integer  which on the other hand will not be correctly interpreted by getMethod methodName parameters     Java s fault       I combined it with the suggestion of Daniel Spiewak  in his answer to this   steps to success included  catching NoSuchMethodException -  getMethods   -  looking for the matching one by method getName   -  and then explicitly looping through the list of parameters and applying Daniels solution  such identifying the type matches and the signature matches

User · Answer

A little nitpicking      I ve seem people suggesting creating a   separate object but that seems   overkill   Passing a callback includes creating a separate object in pretty much any OO language  so it can hardly be considered overkill  What you probably mean is that in Java  it requires you to create a separate class  which is more verbose  and more resource-intensive  than in languages with explicit first-class functions or closures  However  anonymous classes at least reduce the verbosity and can be used inline

User · Answer

A method is not  yet  a first-class object in Java  you can t pass a function pointer as a callback  Instead  create an object  which usually implements an interface  that contains the method you need and pass that   Proposals for closures in Java mdash which would provide the behavior you are looking for mdash have been made  but none will be included in the upcoming Java 7 release

User · Answer

I think using an abstract class is more elegant  like this      Something java  public abstract class Something          public abstract void test                public void usingCallback             System out println  This is before callback method            test            System out println  This is after callback method                CallbackTest java  public class CallbackTest extends Something        Override     public void test             System out println  This is inside CallbackTest                public static void main String   args            CallbackTest myTest   new CallbackTest            myTest usingCallback                    Output  This is before callback method This is inside CallbackTest  This is after callback method

User · Answer

I ve recently started doing something like this   public class Main        FunctionalInterface     public interface NotDotNetDelegate           int doSomething int a  int b              public static void main String   args               in java 8  lambdas           System out println functionThatTakesDelegate  a  b  - gt   return a b     10  20                public static int functionThatTakesDelegate NotDotNetDelegate del  int a  int b                           return del doSomething a  b

User · Answer

I ve recently started doing something like this   public class Main        FunctionalInterface     public interface NotDotNetDelegate           int doSomething int a  int b              public static void main String   args               in java 8  lambdas           System out println functionThatTakesDelegate  a  b  - gt   return a b     10  20                public static int functionThatTakesDelegate NotDotNetDelegate del  int a  int b                           return del doSomething a  b

User · Answer

If you mean somthing like  NET anonymous delegate  I think Java s anonymous class can be used as well    public class Main        public interface Visitor          int doJob int a  int b               public static void main String   args            Visitor adder   new Visitor                public int doJob int a  int b                    return a   b                                    Visitor multiplier   new Visitor                public int doJob int a  int b                    return a b                                    System out println adder doJob 10  20            System out println multiplier doJob 10  20

User · Answer

Simpliest and easiest way is by creating a reusable model and trigger     https   onecompiler com java 3wejrcby2 fbclid IwAR0dHbGDChRUJoCZ3CIDW-JQu7Dz3iYGNGYjxYVCPCWfEqQDogFGTwuOuO8

User · Answer

When I need this kind of functionality in Java  I usually use the Observer pattern   It does imply an extra object  but I think it s a clean way to go  and is a widely understood pattern  which helps with code readability

User · Answer

A method is not  yet  a first-class object in Java  you can t pass a function pointer as a callback  Instead  create an object  which usually implements an interface  that contains the method you need and pass that   Proposals for closures in Java mdash which would provide the behavior you are looking for mdash have been made  but none will be included in the upcoming Java 7 release

User · Answer

public class HelloWorldAnonymousClasses          this is an interface with only one method     interface HelloWorld           public void printSomething String something                this is a simple function called from main       public void sayHello            this is an object with interface reference followed by the definition of the interface itself          new HelloWorld                 public void printSomething String something                    System out println  Hello     something                           printSomething  Abhi            imagine this as an object which is calling the function printSomething               public static void main String    args            HelloWorldAnonymousClasses myApp                   new HelloWorldAnonymousClasses            myApp sayHello              Output is  Hello Abhi    Basically if you want to make the object of an interface it is not possible  because interface cannot have objects   The option is to let some class implement the interface and then call that function using the object of that class  But this approach is really verbose   Alternatively  write new HelloWorld     oberserve this is an interface not a class  and then follow it up with the defination of the interface methods itself    This defination is in reality the anonymous class   Then you get the object reference through which you can call the method itself

User · Answer

If you mean somthing like  NET anonymous delegate  I think Java s anonymous class can be used as well    public class Main        public interface Visitor          int doJob int a  int b               public static void main String   args            Visitor adder   new Visitor                public int doJob int a  int b                    return a   b                                    Visitor multiplier   new Visitor                public int doJob int a  int b                    return a b                                    System out println adder doJob 10  20            System out println multiplier doJob 10  20

User · Answer

This is very easy in Java 8 with lambdas   public interface Callback       void callback       public class Main       public static void main String   args            methodThatExpectsACallback    - gt  System out println  I am the callback                private static void methodThatExpectsACallback Callback callback           System out println  I am the method             callback callback

User · Answer

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

User · Answer

If you mean somthing like  NET anonymous delegate  I think Java s anonymous class can be used as well    public class Main        public interface Visitor          int doJob int a  int b               public static void main String   args            Visitor adder   new Visitor                public int doJob int a  int b                    return a   b                                    Visitor multiplier   new Visitor                public int doJob int a  int b                    return a b                                    System out println adder doJob 10  20            System out println multiplier doJob 10  20

User · Answer

Create an Interface  and Create the Same Interface Property in Callback Class   interface dataFetchDelegate       void didFetchdata String data       callback class public class BackendManager     public dataFetchDelegate Delegate      public void getData              Do something  Http calls  Any other work        Delegate didFetchdata  this is callbackdata              Now in the class where you want to get called back implement the above Created Interface  and Also Pass  this  Object Reference of your class to be called back   public class Main implements dataFetchDelegate              public static void main  String   args                 new Main   getDatafromBackend               public void getDatafromBackend             BackendManager inc   new BackendManager              Pass this object as reference in this Scenario this is Main Object                     inc Delegate   this            make call         inc getData                 This method is called after task Code Completion     public void didFetchdata String callbackData               TODO Auto-generated method stub         System out println callbackData

User · Answer

For simplicity  you can use a Runnable   private void runCallback Runnable callback           Run callback     callback run        Usage   runCallback new Runnable          Override     public void run                    Running callback

User · Answer

This is very easy in Java 8 with lambdas   public interface Callback       void callback       public class Main       public static void main String   args            methodThatExpectsACallback    - gt  System out println  I am the callback                private static void methodThatExpectsACallback Callback callback           System out println  I am the method             callback callback

User · Answer

yet i see there is most preferred way which was what i was looking for   it s basically derived from these answers but i had to manipulate it to more more redundant and efficient   and i think everybody looking for what i come up with  To the point    first make an Interface that simple  public interface myCallback       void onSuccess        void onError String err       now to make this callback run when ever you wish to do to handle the results - more likely after async call and you wanna run some stuff which depends on these reuslts     import the Interface class here  public class App        public static void main String   args               call your method         doSomething  list your Params   new myCallback                 Override             public void onSuccess                        no errors                 System out println  Done                                Override             public void onError String err                       error happen                 System out println err                                        private void doSomething String param     some params                                myCallback callback               now call onSuccess whenever you want if results are ready         if results success              callback onSuccess            else             callback onError someError              doSomething is the function that takes some time you wanna add a callback to it to notify you when the results came  add the call back interface as a parameter to this method  hope my point is clear  enjoy

User · Answer

You also can do theCallback using the Delegate pattern   Callback java  public interface Callback       void onItemSelected int position       PagerActivity java  public class PagerActivity implements Callback        CustomPagerAdapter mPagerAdapter       public PagerActivity             mPagerAdapter   new CustomPagerAdapter this               Override     public void onItemSelected int position               Do something         System out println  Item     postion     selected             CustomPagerAdapter java  public class CustomPagerAdapter       private static final int DEFAULT POSITION   1      public CustomPagerAdapter Callback callback            callback onItemSelected DEFAULT POSITION

User · Answer

If you mean somthing like  NET anonymous delegate  I think Java s anonymous class can be used as well    public class Main        public interface Visitor          int doJob int a  int b               public static void main String   args            Visitor adder   new Visitor                public int doJob int a  int b                    return a   b                                    Visitor multiplier   new Visitor                public int doJob int a  int b                    return a b                                    System out println adder doJob 10  20            System out println multiplier doJob 10  20

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