65. Introducing Closures

In the last tutorial we looked at methods and this prepares us as we start to look at closures. If you take a look at the following code you’re likely to quickly see what the printer() method does:

def printer() {
    println 'Hello, world'
}

def cls = this.&printer
cls()

So what about def cls = this.&printer? Well, .& is the Method Reference operator and it returns a reference to the printer() method. I use this in the script as I need to reference the current instance in which I’m running - remember that Groovy wraps the script in a class.

Once I have the reference I can then call the closure with cls().

Note the following:

1. When I define the closure (def cls = this.&printer) I don’t put the parentheses after the method name (printer)
2. When I call the closure I pass in the parameters

Say I set this up a little differently and create a Test class with two printer methods - one that takes a parameter and one that doesn’t:

class Test {
    static printer() {
        println 'Hello, world'
    }

    static printer(name) {
        println "Hello, $name"
    }
}

def cls = Test.&printer
cls()
cls('Newman')

You’ll see if you run that last example that the call to the closure (cls) will result in the associated method being called depending on the parameters I provide.

Anonymous Functions

In the first examples of this chapter I used the Method Reference operator to point to an existing method that I had defined in a class. Closures can also be defined using anonymous functions. This lets us create a function at the point we define the variable.

In the next example I create an anonymous function using the regular block syntax ({..}) and store the function reference in cls:

def cls = { println 'Hello, world' }
cls()

That’s pretty nifty! We can define a function when needed and store it in a variable. This variable can then be passed to methods and other closures as a parameter.

The ‘it’ parameter

Anonymous functions get a single parameter named it by default. That means that you can use it as a parameter inside your function and it will contain the parameter passed in the call to the closure.

Let’s write a version of the printer method that uses the it parameter:

def cls = { println "Hello, $it" }
cls('Jerry')