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Syntax of closure

Posted by forax on August 31, 2006 at 6:37 AM PDT

In forum (fora) and blogs, i have often seen that closures are compared to anonymous classes
in order to reject the proposed syntax of closure.

In this entry, i try to show up why the closure syntax
is such as it is by explaining the differences with the
anonymous class syntax.

Let me introduce an example that sort a list of integers
in reverse order.
There are two codes, one written with an anonymous class
and the other written with a closure using the closure convertion.

The code using a closure:

List<Integer> list=Arrays.asList(666,7,12);
Collections.sort(list,int(Integer i1,Integer i2) {
  if (i1<i2)
    return 1;
   if (i2<i1)
     return -1;
   return 0;

And the code using an anonymous class:

List<Integer> list=Arrays.asList(666,7,12);
Collections.sort(list,new Comparator<Integer>(){
  public int compareTo(Integer i1,Integer i2) {
    if (i1<i2)
      return 1;
      if (i2<i1)
        return -1;
      return 0;

I've found five differences and all have a reason so
i don't see why people want to change the proposed syntax.

A step by step comparison

  1. An anonymous class starts with new,
    because it always create a new object, closure can reuse
    the same singleton so no "new".

  2. an anonymous class takes a type (a class, an abstract
    class or an interface), closure doesn't need to specify
    a type because the compiler infers the type.
    Due to an implementation detail, closure convertion can
    only occur with an interface.

  3. an anonymous class must parameterize its constructor
    (Comparator). Closure can infer the
    value of the type variable. Because of the backward
    compatibility of generics,
    the compiler must be able to distinguish a raw type from a
    parametrized type so the type variable of an anonymous class
    can't be infered.

  4. an anonymous class can take arguments,
    a closure can't because as state above,
    a closure can only implements an interface.

  5. an anonymous class need to specify access modifier (public)
    and the method name. The closure have only one public method
    so it's neither necessary to name it
    nor to specify a modifier.

I hope this entry will reconcile the grimpies with the proposed

Wow, this is my fourth entry about closure,
it's time for me to register with CA (Closure Anonymous) :)

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