This is a lightning talk I gave at an internal event called Techquilla in ThoughtWorks Pune office. More details: http://www.wikyblog.com/AmanKing/Multimethods

1. Multimethodsan introduction Aman King king@thoughtworks.com

2. What is it?

3. Multiple dispatch

4. Basically: a language feature

5. Before the details…

6. Some Java code snippets…

7. public class MultimethodsDemo { static class Walk { } static class Dance { } static class Attack { } static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } } public static void main(String[] args) { Robot robot = new Robot();System.out.println( robot.doThis( new Walk() ));System.out.println( robot.doThis( new Dance() ));System.out.println( robot.doThis( new Attack() )); }}

8. I am walking...Let's boogie woogie...Exception in thread "main" java.lang.RuntimeException: I don't believe in violence! at MultimethodsDemo$Robot.doThis(MultimethodsDemo.java:10) at MultimethodsDemo.main(MultimethodsDemo.java:18)

9. Polymorphism

10. Overloading

11. public class MultimethodsDemo { static class Walk { } static class Dance { } static class Attack { } static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } } public static void main(String[] args) { Robot robot = new Robot();System.out.println( robot.doThis( new Walk() ));System.out.println( robot.doThis( new Dance() ));System.out.println( robot.doThis( new Attack() )); }}

12. import java.util.*;public class MultimethodsDemo {static interface Command {} static class Walk implements Command { } static class Dance implements Command { } static class Attack implements Command { } static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } } public static void main(String[] args) { Robot robot = new Robot(); List<Command> commands = Arrays.asList( new Walk(), new Dance(), new Attack() );for ( Command command : commands ) {System.out.println( robot.doThis(command) ); } }}

13. MultimethodsDemo.java:20: cannot find symbolsymbol : method doThis(MultimethodsDemo.Command)location: class MultimethodsDemo.RobotSystem.out.println( robot.doThis(command) ); ^1 error

14. Solution?

15. Satisfy the compiler!

16. import java.util.*;public class MultimethodsDemo { static interface Command {} static class Walk implements Command {} static class Dance implements Command {} static class Attack implements Command {} static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); }public String doThis(Command command) { throw new RuntimeException("Unknown command: " + command.getClass()); } } public static void main(String[] args) { Robot robot = new Robot(); List<Command> commands = Arrays.asList( new Walk(), new Dance(), new Attack() ); for ( Command command : commands ) {System.out.println( robot.doThis(command) ); } }}

17. Made the compiler happy but…

18. Exception in thread "main" java.lang.RuntimeException: Unknown command: class MultimethodsDemo$Walk at MultimethodsDemo$Robot.doThis(MultimethodsDemo.java:15) at MultimethodsDemo.main(MultimethodsDemo.java:23)

19. Early Binding(compile-time)

20. Now what?

21. Forcing correct overloadingatcompile-time…

22. // … static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } public String doThis(Command command) { if (command instanceof Walk) { return doThis( (Walk) command ); } if (command instanceof Dance) { return doThis( (Dance) command ); } if (command instanceof Attack) { return doThis( (Attack) command ); } throw new RuntimeException("Unknown command: " +command.getClass()); } } // ...

23. or use Visitor pattern.

24. So what went wrong?

25. Java does early binding for overloading polymorphism

26. But remember…

27. Java does late binding (runtime)for overriding polymorphism

28. public class MultimethodsDemo { static class Robot { public String name() { return "normal robot"; } } static class AwesomeRobot extends Robot { public String name() { return "awesome robot!!!"; } } public static void main(String[] args) {Robot robot = new Robot();Robot awesomeRobot = new AwesomeRobot();System.out.println( robot.name() ); // normal robotSystem.out.println( awesomeRobot.name() ); // awesome robot!!! }}

29. Now think ofrobot.doThis(command)asdoThis(robot, command)

30. C# 3.0 : Extension Methodsnamespace CustomExtensions{ public static class CustomExtensionsClass { public static string Times(thisobject input, int times) { return "don't care!"; } public static string Times(thisstring input, int times) { string result = "";for(inti = 0; i < times; i++) result += input; return result; } public static intTimes(thisint input, int times) { return input * times; } }}// …using CustomExtensions;varaStringObject = "aman"; aStringObject.Times(2); // "amanaman"varanIntObject = 3; anIntObject.Times(2); // 6// …

31. In these examples, polymorphism is based on the runtime type of a single argument: the 1st one (implicitly passed)

32. In other words…

33. Java and C# 3.0support Single Dispatch

34. not Double Dispatch,not Multiple Dispatch.

35. So what is Multiple Dispatch?

36. Selection aka dispatchof a method based on the runtime type of multiple arguments

38. Clojure

39. Perl

40. Groovy

42. // …Robot robot = new Robot();ICommandcommand = newWalk();robot.DoThisCommand( command ); // "I am walking... "robot.DoThis( command ); // Exception: "Unknown command:Walk"// …

43. So what is interesting about Multiple Dispatch /Multimethods?

44. For starters…

45. Thinking about…

46. OO as syntactic sugarfor method dispatch?

47. Design patterns as a way to overcome lack of language features?Visitor for Double DispatchStrategy for Closures/Lambdas

48. Do we really need more than Single Dispatch or at best Double Dispatch?

50. http://www.c2.com/cgi/wiki?MultiMethods

51. http://en.wikipedia.org/wiki/Extension_method