Groovy closures

Java does not have closures, but till sometime back it was a highly debated topic on various Java forums. Groovy is a language which compiles to Java bytecode and does have closures.

Closures are very prominent in many programming languages, such as Groovy, Python, Ruby, Javascript, and probably many more.

So what are closures? A closure is a block of code which can be passed to a function. Closures in Groovy are normal objects, which do not have any visible state.

Closures are most often used in iteration logic and resource handling.

Whenever we iterate across a collection, we normally do so to perform some logic on all or some members of the collection. Closures make it easy to represent that logic in a very concise and readable way.

Closures can also be used for resource handling. Very often we open a file resource do something with the file (read from it, write to it, or both), and then close the file. The first and last parts, that is opening and closing the file are really plumbing code. The interesting part is what we do with it. Using closures it is possible to create objects which will take care of the plumbing and will perform some operations on the file, such that the code which represents the operations are given to it in a closure.

Here is an example. Let's assume we have a list of Toy objects, where each toy object has a color which can be accessed with the getColor() method. I want all the blue toys and my sister wants all the pink toys.

Here is code to get toys for me using regular Java syntax:

Color myFavColor = Color.BLUE;
List toysIWant = new ArrayList()
Iterator iter = toys.iterator();
while(iter.hasNext()) {
  Toy toy = (Toy)iter.next();
  if(toy.getColor().equals(myFavColor)) {
    toysIWant.add(toy);
  }
}

Here is code to get toys for my sister using closures:

def mySistersFavColor = Color.PINK
def toysMySisterWants = toys.grep() {
  it.getColor() == mySistersFavColor
}

What really happened in the second code snippet was, the list object has a grep method which takes a closure. It calls that closure for every item in the list and passes that item to the closure as the variable 'it'. If the expression in the closure evaluates to true, then the item is added to an internal list. This list is returned to the caller when grep returns.

You might be thinking that this can be done in Java using anonymous inner classes. That is partially right. But there are two very important differences. Anonymous inner classes cannot refer to non static local variables in the scope they are declared in, which means they could not have referred to the variables 'myFavCOlor', and 'mySistersFavColor'. Another difference is that anonymous inner classes are very verbose. This may not seem important, but many people have said that it is something that makes anonymous inner classes unusable.

Resources:

• A very good blog post by Martin Fowler on closures
• A long but very complete explanation of closures by Neal Gafter
• A nice blog post on uses of blocks and closures
• A podcast on Groovy closures (accompanying code snippet)