Does Java need closures?

Question

I've been reading a lot lately about the next release of Java possibly supporting closures. I feel like I have a pretty firm grasp on what closures are, but I can't think of a solid example of how they would make an Object-Oriented language "better". Can anyone give me a specific use-case where a closure would be needed (or even preferred)?

Answer 1

Java Closure Examples

Answer 2

You might want to look at Groovy, a language that's mostly compatible with Java, and runs on the JRE, but supports Closures.

Answer 3

I've been reading a lot lately about the next release of Java possibly supporting closures. I feel like I have a pretty firm grasp on what closures are, but I can't think of a solid example of how they would make an Object-Oriented language "better."

Well, most people who use the term "closure" actually mean "function object", and in this sense, function objects make it possible to write simpler code in certain circumstances such as when you need custom comparators in a sort function.

For example, in Python:

def reversecmp(x, y):
   return y - x

a = [4, 2, 5, 9, 11]
a.sort(cmp=reversecmp)

This sorts the list a in reverse order by passing the custom comparison functoin reversecmp. The addition of the lambda operator makes things even more compact:

a = [4, 2, 5, 9, 11]
a.sort(cmp=lambda x, y : y - x)

Java does not have function objects, so it uses "functor classes" to simulate them. In Java you do the equivalent operation by implementing a custom version of the Comparator class, and passing that to the sort function:

class ReverseComparator implements Comparator {
   public compare(Object x, Object y) {
      return (Integer) y - (Integer) x;
   }

...

List<Integer> a = Arrays.asList(4, 2, 5, 9, 11);
Collections.sort(a, new ReverseComparator());

As you can see, it gives the same effect as closures, but is clumsier and more verbose. However, the addition of anonymous inner classes obviates most of the pain:

List<Integer> a = Arrays.asList(4, 2, 5, 9, 11);
Comparator reverse = new Comparator() {
   public Compare(Object x, Object y) {
       return (Integer) y - (Integer) x;
   }
}
Collections.sort(a, reverse);

So I would say that the combination of functor classes + anonymous inner classes in Java is sufficient to compensate for the lack of true function objects, making their addition unnecessary.

Answer 4

What about readability and maintainability...one-liner closures are harder to understand and debug, imo Software has looong life and you can get people with rudimentary knowledge of the language to maintain it...So spread out logic better than one-liners for easy maintenance...You generally don't have a software star looking after software after its release...

Answer 5

There are some very useful 'higher order functions' which can do operations on lists using closures. Higher order functions are functions having 'function objects' as parameters.

E.g. it is a very common operation to apply some transformation to every element in a list. This higher order function is commonly called 'map' or 'collect'. (See the *. spread operator of Groovy).

For example to square each element in a list without closures you would probably write:

List<Integer> squareInts(List<Integer> is){
   List<Integer> result = new ArrayList<Integer>(is.size());
   for (Integer i:is)
      result.add(i*i);
   return result;
}

Using closures and map and the proposed syntax, you could write it like that:

is.map({Integer i => i*i})

(There is a possible performance problem here regarding boxing of primitive types.)

As explained by Pop Catalin there is another higher order function called 'select' or 'filter': It can be used to get all the elements in a list complying to some condition. For example:

Instead of:

void onlyStringsWithMoreThan4Chars(List<String> strings){
   List<String> result = new ArrayList<String>(str.size()); // should be enough
   for (String str:strings)
      if (str.length() > 4) result.add(str);
   return result;
}

Instead you could write something like

strings.select({String str => str.length() > 4});

using the proposal.

You might look at the Groovy syntax, which is an extension of the Java language to support closures right now. See the chapter on collections of the Groovy User Guide for more examples what to do with closures.

A remark:

There is perhaps some clarification needed regarding the term 'closure'. What I've shown above are strictly spoken no closures. They are just 'function objects'. A closure is everything which can capture - or 'close over' - the (lexical) context of the code surrounding it. In that sense there are closures in Java right now, i.e. anonymous classes:

Runnable createStringPrintingRunnable(final String str){
    return new Runnable(){
       public void run(){
          System.out.println(str); // this accesses a variable from an outer scope
       }
    };
}

Answer 6

I suppose for supporting core functional programming concepts, you need closures. Makes the code more elegant and composable with the support for closures. Also, I like the idea of passing around lines of code as parameters to functions.