How can I write an anonymous function in Java

Question

Is it even possible

User · Answer

Anonymous inner classes implementing or extending the interface of an existing type has been done in other answers, although it is worth noting that multiple methods can be implemented (often with JavaBean-style events, for instance).

A little recognised feature is that although anonymous inner classes don't have a name, they do have a type. New methods can be added to the interface. These methods can only be invoked in limited cases. Chiefly directly on the new expression itself and within the class (including instance initialisers). It might confuse beginners, but it can be "interesting" for recursion.

private static String pretty(Node node) {
    return "Node: " + new Object() {
        String print(Node cur) {
            return cur.isTerminal() ?
                cur.name() :
                ("("+print(cur.left())+":"+print(cur.right())+")");
        }
    }.print(node);
}

(I originally wrote this using node rather than cur in the print method. Say NO to capturing "implicitly final" locals?)

User · Answer

Here s an example of an anonymous inner class   System out println new Object          Override public String toString             return  Hello world                 prints  Hello world     This is not very useful as it is  but it shows how to create an instance of an anonymous inner class that extends Object and  Override its toString   method   See also   JLS 15 9 5 Anonymous Class Declarations     Anonymous inner classes are very handy when you need to implement an interface which may not be highly reusable  and therefore not worth refactoring to its own named class   An instructive example is using a custom java util Comparator lt T gt  for sorting   Here s an example of how you can sort a String   based on String length     import java util           String   arr      xxx    cd    ab    z     Arrays sort arr  new Comparator lt String gt           Override public int compare String s1  String s2            return s1 length   - s2 length                         System out println Arrays toString arr       prints   z  cd  ab  xxx     Note the comparison-by-subtraction trick used here  It should be said that this technique is broken in general  it s only applicable when you can guarantee that it will not overflow  such is the case with String lengths    See also   Java Integer  what is faster comparison or subtraction    Comparison-by-subtraction is broken in general  Create a Sorted Hash in Java with a Custom Comparator How are Anonymous  inner  classes used in Java

User · Answer

Yes if you are using latest java which is version 8  Java8 make it possible to define anonymous functions which was impossible in previous versions   Lets take example from java docs to get know how we can declare anonymous functions  classes     The following example  HelloWorldAnonymousClasses  uses anonymous   classes in the initialization statements of the local variables   frenchGreeting and spanishGreeting  but uses a local class for the   initialization of the variable englishGreeting    public class HelloWorldAnonymousClasses        interface HelloWorld           public void greet            public void greetSomeone String someone              public void sayHello              class EnglishGreeting implements HelloWorld               String name    world               public void greet                     greetSomeone  world                              public void greetSomeone String someone                    name   someone                  System out println  Hello     name                                    HelloWorld englishGreeting   new EnglishGreeting             HelloWorld frenchGreeting   new HelloWorld                 String name    tout le monde               public void greet                     greetSomeone  tout le monde                              public void greetSomeone String someone                    name   someone                  System out println  Salut     name                                     HelloWorld spanishGreeting   new HelloWorld                 String name    mundo               public void greet                     greetSomeone  mundo                              public void greetSomeone String someone                    name   someone                  System out println  Hola      name                                    englishGreeting greet            frenchGreeting greetSomeone  Fred            spanishGreeting greet               public static void main String    args            HelloWorldAnonymousClasses myApp               new HelloWorldAnonymousClasses            myApp sayHello                          Syntax of Anonymous Classes  Consider the instantiation of the frenchGreeting object       HelloWorld frenchGreeting   new HelloWorld             String name    tout le monde           public void greet                 greetSomeone  tout le monde                      public void greetSomeone String someone                name   someone              System out println  Salut     name                      The anonymous class expression consists of the following    The new operator The name of an interface to implement or a class to extend  In this example  the anonymous class is implementing the interface HelloWorld  Parentheses that contain the arguments to a constructor  just like a normal class instance creation expression  Note  When you implement an interface  there is no constructor  so you use an empty pair of parentheses  as in this example  A body  which is a class declaration body  More specifically  in the body  method declarations are allowed but statements are not

User · Answer

if you mean an anonymous function  and are using a version of Java before Java 8  then in a word  no   Read about lambda expressions if you use Java 8    However  you can implement an interface with a function like so    Comparator lt String gt  c   new Comparator lt String gt          int compare String s  String s2               and you can use this with inner classes to get an almost-anonymous function

User · Answer

With the introduction of lambda expression in Java 8 you can now have anonymous methods   Say I have a class Alpha and I want to filter Alphas on a specific condition  To do this you can use a Predicate lt Alpha gt   This is a functional interface which has a method test that accepts an Alpha and returns a boolean   Assuming that the filter method has this signature   List lt Alpha gt  filter Predicate lt Alpha gt  filterPredicate    With the old anonymous class solution you would need to something like   filter new Predicate lt Alpha gt         boolean test Alpha alpha          return alpha centauri  gt  1             With the Java 8 lambdas you can do   filter alpha - gt  alpha centauri  gt  1     For more detailed information see the Lambda Expressions tutorial

[java] How can I write an anonymous function in Java?

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