Java Pass Method as Parameter

Question

I am looking for a way to pass a method by reference  I understand that Java does not pass methods as parameters  however  I would like to get an alternative    I ve been told interfaces are the alternative to passing methods as parameters but I don t understand how an interface can act as a method by reference  If I understand correctly an interface is simply an abstract set of methods that are not defined  I don t want to send an interface that needs to be defined every time because several different methods could call the same method with the same parameters   What I would like to accomplish is something similar to this   public void setAllComponents Component   myComponentArray  Method myMethod        for  Component leaf   myComponentArray            if  leaf instanceof Container      recursive call if Container             Container node    Container  leaf              setAllComponents node getComponents    myMethod               end if node         myMethod leaf           end looping through components     invoked such as   setAllComponents this getComponents    changeColor     setAllComponents this getComponents    changeSize

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Use the java lang reflect Method object and call invoke

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First define an Interface with the method you want to pass as a parameter  public interface Callable     public void call int param       Implement a class with the method  class Test implements Callable     public void call int param        System out println  param               Invoke like that  Callable cmd   new Test      This allows you to pass cmd as parameter and invoke the method call defined in the interface  public invoke  Callable callable       callable call  5

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Use the Observer pattern  sometimes also called Listener pattern    interface ComponentDelegate       void doSomething Component component      public void setAllComponents Component   myComponentArray  ComponentDelegate delegate                   delegate doSomething leaf      setAllComponents this getComponents    new ComponentDelegate                                                 void doSomething Component component                                                    changeColor component      or do directly what you want                                                                                            new ComponentDelegate      declares an anonymous type implementing the interface

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Since Java 8 there is a Function lt T  R gt  interface  docs   which has method   R apply T t     You can use it to pass functions as parameters to other functions  T is the input type of the function  R is the return type   In your example you need to pass a function that takes Component type as an input and returns nothing - Void  In this case Function lt T  R gt  is not the best choice  since there is no autoboxing of Void type  The interface you are looking for is called Consumer lt T gt   docs  with method   void accept T t     It would look like this   public void setAllComponents Component   myComponentArray  Consumer lt Component gt  myMethod        for  Component leaf   myComponentArray            if  leaf instanceof Container                 Container node    Container  leaf              setAllComponents node getComponents    myMethod                      myMethod accept leaf              And you would call it using method references   setAllComponents this getComponents    this  changeColor   setAllComponents this getComponents    this  changeSize      Assuming that you have defined changeColor   and changeSize   methods in the same class     If your method happens to accept more than one parameter  you can use BiFunction lt T  U  R gt  - T and U being types of input parameters and R being return type  There is also BiConsumer lt T  U gt   two arguments  no return type   Unfortunately for 3 and more input parameters  you have to create an interface by yourself  For example   public interface Function4 lt A  B  C  D  R gt         R apply A a  B b  C c  D d

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If you don t need these methods to return something  you could make them return Runnable objects   private Runnable methodName  final int arg        return  new Runnable             public void run                  do stuff with arg                       Then use it like   private void otherMethodName  Runnable arg       arg run

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Here is a basic example   public class TestMethodPassing       private static void println                 System out println  Do println               private static void print                 System out print  Do print               private static void performTask BasicFunctionalInterface functionalInterface                functionalInterface performTask                FunctionalInterface     interface BasicFunctionalInterface               void performTask               public static void main String   arguments                performTask TestMethodPassing  println           performTask TestMethodPassing  print             Output   Do println Do print

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I did not found any solution here that show how to pass method with parameters bound to it as a parameter of a method  Bellow is example of how you can pass a method with parameter values already bound to it     Step 1  Create two interfaces one with return type  another without  Java has similar interfaces but they are of little practical use because they do not support Exception throwing          public interface Do       void run   throws Exception              public interface Return           R run   throws Exception            Example of how we use both interfaces to wrap method call in transaction  Note that we pass method with actual parameters            example - when passed method does not return any value     public void tx final Do func  throws Exception           connectionScope beginTransaction            try               func run                connectionScope commit              catch  Exception e                connectionScope rollback                throw e            finally               connectionScope close                           Invoke code above by      tx    -  api delete 6       Another example shows how to pass a method that actually returns something            public  R tx final Return func  throws Exception       R r null      connectionScope beginTransaction        try                   r func run                    connectionScope commit                  catch  Exception e                    connectionScope rollback                    throw e                finally                   connectionScope close                          return r                      Invoke code above by          Object x  tx    -  api get id

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I m not a java expert but I solve your problem like this    FunctionalInterface public interface AutoCompleteCallable lt T gt      String call T model  throws Exception      I define the parameter in my special Interface  public  lt T gt  void initialize List lt T gt  entries  AutoCompleteCallable getSearchText             call here String value   getSearchText call item           Finally  I implement getSearchText method while calling initialize method   initialize getMessageContactModelList    new AutoCompleteCallable                Override           public String call Object model  throws Exception               return  custom string      xxxModel model getTitle

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Java do have a mechanism to pass name and call it  It is part of the reflection mechanism  Your function should take additional parameter of class Method   public void YouMethod       Method methodToCall  Object objWithAllMethodsToBeCalled        Object retobj   methodToCall invoke objWithAllMethodsToBeCalled  arglist

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I appreciate the answers above but I was able to achieve the same behavior using the method below  an idea borrowed from Javascript callbacks  I m open to correction though so far so good  in production    The idea is to use the return type of the function in the signature  meaning that the yield has to be static   Below is a function that runs a process with a timeout   public static void timeoutFunction String fnReturnVal         Object p   null     whatever object you need here      String threadSleeptime   null       Config config       try           config   ConfigReader getConfigProperties            threadSleeptime   config getThreadSleepTime           catch  Exception e            log error e           log error              log error  Defaulting thread sleep time to 105000 miliseconds             log error              threadSleeptime    100000              ExecutorService executor   Executors newCachedThreadPool        Callable lt Object gt  task   new Callable lt Object gt              public Object call                    Do job here using --- fnReturnVal --- and return appropriate value             return null                       Future lt Object gt  future   executor submit task        try           p   future get Integer parseInt threadSleeptime   TimeUnit MILLISECONDS         catch  Exception e            log error e      The function timed out after      threadSleeptime                      miliseconds before a response was received           finally              if task has started then don t stop it         future cancel false            private static String returnString         return  hello      public static void main String   args        timeoutFunction returnString

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Example of solution with reflection  passed method must be public  import java lang reflect Method  import java lang reflect InvocationTargetException   public class Program       int i       public static void main String   args            Program   obj   new Program         some object          try               Method method   obj getClass   getMethod  target                repeatMethod  5  obj  method                       catch   NoSuchMethodException   IllegalAccessException   InvocationTargetException e                System out println  e                          static void repeatMethod  int times  Object object  Method method          throws IllegalAccessException  InvocationTargetException            for  int i 0  i lt times  i                method invoke object             public void target                       public is necessary         System out println  target         i

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I didn t find any example explicit enough for me on how to use java util function Function for simple method as parameter function  Here is a simple example   import java util function Function   public class Foo      private Foo String parameter        System out println  I m a Foo     parameter          public static Foo method final String parameter        return new Foo parameter          private static Function parametrisedMethod Function lt String  Foo gt  function        return function         public static void main String   args        parametrisedMethod Foo  method  apply  from a method            Basically you have a Foo object with a default constructor  A method that will be called as a parameter from the parametrisedMethod which is of type Function lt String  Foo gt     Function lt String  Foo gt  means that the function takes a String as parameter and return a Foo  The Foo  Method correspond to a lambda like x - gt  Foo method x   parametrisedMethod Foo  method  could be seen as x - gt  parametrisedMethod Foo method x   The  apply  from a method   is basically to do parametrisedMethod Foo method  from a method      Which will then return in the output    gt  gt  I m a Foo from a method   The example should be running as is  you can then try more complicated stuff from the above answers with different classes and interfaces

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Last time I checked  Java is not capable of natively doing what you want  you have to use  work-arounds  to get around such limitations  As far as I see it  interfaces ARE an alternative  but not a good alternative  Perhaps whoever told you that was meaning something like this   public interface ComponentMethod     public abstract void PerfromMethod Container c      public class ChangeColor implements ComponentMethod      Override   public void PerfromMethod Container c           do color change stuff        public class ChangeSize implements ComponentMethod      Override   public void PerfromMethod Container c           do color change stuff        public void setAllComponents Component   myComponentArray  ComponentMethod myMethod        for  Component leaf   myComponentArray            if  leaf instanceof Container      recursive call if Container             Container node    Container  leaf              setAllComponents node getComponents    myMethod               end if node         myMethod PerfromMethod leaf           end looping through components     Which you d then invoke with   setAllComponents this getComponents    new ChangeColor     setAllComponents this getComponents    new ChangeSize

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While this is not yet valid for Java 7 and below  I believe that we should look to the future and at least recognize the changes to come in new versions such as Java 8    Namely  this new version brings lambdas and method references to Java  along with new APIs  which are another valid solution to this problem  While they still require an interface no new objects are created  and extra classfiles need not pollute output directories due to different handling by the JVM    Both flavors lambda and method reference  require an interface available with a single method whose signature is used   public interface NewVersionTest      String returnAString Object oIn  String str       Names of methods will not matter from here on  Where a lambda is accepted  a method reference is as well  For example  to use our signature here   public static void printOutput NewVersionTest t  Object o  String s       System out println t returnAString o  s        This is just a simple interface invocation  up until the lambda1 gets passed   public static void main String   args       printOutput   Object oIn  String sIn  - gt            System out println  Lambda reached             return  lambda return                    This will output   Lambda reached  lambda return   Method references are similar  Given   public class HelperClass      public static String testOtherSig Object o  String s           return  real static method             and main   public static void main String   args       printOutput HelperClass  testOtherSig       the output would be real static method  Method references can be static  instance  non-static with arbitrary instances  and even constructors  For the constructor something akin to ClassName  new would be used   1 This is not considered a lambda by some  as it has side effects  It does illustrate  however  the use of one in a more straightforward-to-visualize fashion

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In Java 8  you can now pass a method more easily using Lambda Expressions and Method References  First  some background  a functional interface is an interface that has one and only one abstract method  although it can contain any number of default methods  new in Java 8  and static methods  A lambda expression can quickly implement the abstract method  without all the unnecessary syntax needed if you don t use a lambda expression   Without lambda expressions   obj aMethod new AFunctionalInterface          Override     public boolean anotherMethod int i                return i    982             With lambda expressions   obj aMethod i - gt  i    982       Here is an excerpt from the Java tutorial on Lambda Expressions      Syntax of Lambda Expressions      A lambda expression consists of the following          A comma-separated list of formal parameters enclosed in parentheses  The CheckPerson test method contains one parameter  p    which represents an instance of the Person class Note  You   can omit the data type of the parameters in a lambda expression  In   addition  you can omit the parentheses if there is only one parameter    For example  the following lambda expression is also valid   p - gt  p getGender      Person Sex MALE       amp  amp  p getAge    gt   18      amp  amp  p getAge    lt   25    The arrow token  - gt    A body  which consists of a single expression or a statement block  This example uses the following expression   p getGender      Person Sex MALE       amp  amp  p getAge    gt   18      amp  amp  p getAge    lt   25       If you specify a single expression  then the Java runtime evaluates the expression and then returns its value  Alternatively    you can use a return statement   p - gt        return p getGender      Person Sex MALE          amp  amp  p getAge    gt   18          amp  amp  p getAge    lt   25          A return statement is not an expression  in a lambda expression  you must enclose statements in braces       However  you do not have   to enclose a void method invocation in braces  For example  the   following is a valid lambda expression   email - gt  System out println email           Note that a lambda expression looks a lot like a method declaration    you can consider lambda expressions as anonymous methods   methods   without a name      Here is how you can  pass a method  using a lambda expression   interface I       public void myMethod Component component      class A       public void changeColor Component component               code here            public void changeSize Component component               code here             class B       public void setAllComponents Component   myComponentArray  I myMethodsInterface            for Component leaf   myComponentArray                if leaf instanceof Container       recursive call if Container                 Container node    Container leaf                  setAllComponents node getComponents    myMethodInterface                    end if node             myMethodsInterface myMethod leaf                end looping through components             class C       A a   new A        B b   new B         public C             b setAllComponents this getComponents    component - gt  a changeColor component            b setAllComponents this getComponents    component - gt  a changeSize component              Class C can be shortened even a bit further by the use of method references like so   class C       A a   new A        B b   new B         public C             b setAllComponents this getComponents    a  changeColor           b setAllComponents this getComponents    a  changeSize

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Edit  as of Java 8  lambda expressions are a nice solution as other answers have pointed out  The answer below was written for Java 7 and earlier       Take a look at the command pattern      NOTE  code not tested  but I believe this is valid java    public class CommandExample        public interface Command                public void execute Object data              public class PrintCommand implements Command                public void execute Object data                         System out println data toString                              public static void callCommand Command command  Object data                 command execute data              public static void main String    args                 callCommand new PrintCommand     hello world              Edit  as Pete Kirkham points out  there s another way of doing this using a Visitor  The visitor approach is a little more involved - your nodes all need to be visitor-aware with an acceptVisitor   method - but if you need to traverse a more complex object graph then it s worth examining

[java] Java Pass Method as Parameter

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