How to pass a function as a parameter in Java

Question

In Java  how can one pass a function as an argument of another function

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Lambda Expressions  To add on to jk  s excellent answer  you can now pass a method more easily using Lambda Expressions  in Java 8   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   Note  this uses a new standard functional interface  java util function IntConsumer   class A       public static void methodToPass int i                do stuff             import java util function IntConsumer   class B       public void dansMethod int i  IntConsumer aMethod               you can now call the passed method by saying aMethod accept i   and it         will be the equivalent of saying A methodToPass i                 class C       B b   new B         public C             b dansMethod 100  j - gt  A methodToPass j        Lambda Expression here             The above example can be shortened even more using the    operator   public C         b dansMethod 100  A  methodToPass

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You could use Java reflection to do this   The method would be represented as an instance of java lang reflect Method   import java lang reflect Method   public class Demo        public static void main String   args  throws Exception          Class   parameterTypes   new Class 1           parameterTypes 0    String class          Method method1   Demo class getMethod  method1   parameterTypes            Demo demo   new Demo            demo method2 demo  method1   Hello World               public void method1 String message            System out println message              public void method2 Object object  Method method  String message  throws Exception           Object   parameters   new Object 1           parameters 0    message          method invoke object  parameters

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Java does not  yet  support closures  But there are other languages like Scala and Groovy which run in the JVM and do support closures

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I used the command pattern that  jk  mentioned  adding a return type   public interface Callable lt I  O gt         public O call I input

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Thanks to Java 8 you don t need to do the steps below to pass a function to a method  that s what lambdas are for  see Oracle s Lambda Expression tutorial  The rest of this post describes what we used to have to do in the bad old days in order to implement this functionality   Typically you declare your method as taking some interface with a single method  then you pass in an object that implements that interface  An example is in commons-collections  where you have interfaces for Closure  Transformer  and Predicate  and methods that you pass implementations of those into  Guava is the new improved commons-collections  you can find equivalent interfaces there   So for instance  commons-collections has org apache commons collections CollectionUtils  which has lots of static methods that take objects passed in  to pick one at random  there s one called exists with this signature   static boolean exists java util Collection collection  Predicate predicate     It takes an object that implements the interface Predicate  which means it has to have a method on it that takes some Object and returns a boolean   So I can call it like this   CollectionUtils exists someCollection  new Predicate         public boolean evaluate Object object             return   a  equals object toString                 and it returns true or false depending on whether someCollection contains an object that the predicate returns true for   Anyway  this is just an example  and commons-collections is outdated  I just forget the equivalent in Guava

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Java 8 and above  Using Java 8  lambda expressions  if you have a class or interface with only a single abstract method  sometimes called a SAM type   for example   public interface MyInterface       String doSomething int param1  String param2       then anywhere where MyInterface is used  you can substitute a lambda expression   class MyClass       public MyInterface myInterface    p1  p2  - gt    return p2   p1         For example  you can create a new thread very quickly   new Thread    - gt  someMethod    start      And use the method reference syntax to make it even cleaner   new Thread this  someMethod  start      Without lambda expressions  these last two examples would look like   new Thread new Runnable     someMethod       start      Before Java 8  A common pattern would be to  wrap  it within an interface  like Callable  for example  then you pass in a Callable   public T myMethod Callable lt T gt  func        return func call        This pattern is known as the Command Pattern   Keep in mind you would be best off creating an interface for your particular usage  If you chose to go with callable  then you d replace T above with whatever type of return value you expect  such as String   In response to your comment below you could say   public int methodToPass                 do something    public void dansMethod int i  Callable lt Integer gt  myFunc              do something     then call it  perhaps using an anonymous inner class   dansMethod 100  new Callable lt Integer gt         public Integer call             return methodToPass               Keep in mind this is not a  trick   It s just java s basic conceptual equivalent to function pointers

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I know this is a rather old post but I have another slightly simpler solution  You could create another class within and make it abstract  Next make an Abstract method name it whatever you like  In the original class make a method that takes the new class as a parameter  in this method call the abstract method  It will look something like this   public class Demo        public Demo                     public void view Action a           a preform                          The Action Class is for making the Demo        View Custom Code             public abstract class Action            public Action                             abstract void preform              Now you can do something like this to call a method from within the class        Demo d   new Demo  Action a   new Action           Override     void preform               Custom Method Code Goes Here                 d view a    Like I said I know its old but this way I think is a little easier  Hope it helps

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Java supports closures just fine  It just doesn t support functions  so the syntax you re used to for closures is much more awkward and bulky  you have to wrap everything up in a class with a method  For example   public Runnable foo final int x      return new Runnable         public void run           System out println x                  Will return a Runnable object whose run   method  closes over  the x passed in  just like in any language that supports first-class functions and closures

[java] How to pass a function as a parameter in Java?

Lambda Expressions

Syntax of Lambda Expressions

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