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;
  else
   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;
    else
      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<Integer>). 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 syntax.
Wow, this is my fourth entry about closure, it's time for me to register with CA (Closure Anonymous) :)