Callback functions in Java

Is there a way to do pass a call back function in a Java method?

The bahaviour I'm trying to mimic is a .Net Delegate being passed to a function.

I've seem people suggesting creating a separate object but that seems overkill, however I am aware that sometimes overkill is the only way to do things.

From stackoverflow

• You can use an interface. See here: http://www.javaworld.com/javaworld/javatips/jw-javatip10.html

  Alex

• 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));
  
      }
  }
  

  Charlie Martin : This is the canonical method since Java 1.0.

  Omar Kooheji : I've been usign this, it's slioghtly more verbose than what I'd like, but it works.

• 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—which would provide the behavior you are looking for—have been made, but none will be included in the upcoming Java 7 release.

  Jonas Kölker : "A method is not (yet) a first-class object in Java" -- well, there's the method class[1], of which you can certainly pass around instances. It's not the clean, idiomatic, OO code that you'd expect from java, but it might be expedient. Certainly something to consider, though. [1] http://java.sun.com/j2se/1.4.2/docs/api/java/lang/reflect/Method.html

• 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.

• 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.

  Omar Kooheji : Yes that is what i meant. With 30 or so events you end up with 30 classes.

• 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