What is a closure?

Closures are such an unfamiliar concept to begin with that it can be hard to grasp initially. Closures have characteristics that make them look like a method in so far as we can pass parameters to them and they can return a value. However, unlike methods, closures are anonymous. A closure is just a snippet of code that can be assigned to a variable and executed later:

def flintstones = ["Fred","Barney"]
def greeter = { println "Hello, ${it}" }
flintstones.each( greeter )
greeter "Wilma"
greeter = { }
flintstones.each( greeter )
greeter "Wilma"

Because closures are anonymous, they can easily be lost or overwritten. In the preceding example, we defined a variable greeter to contain a closure that prints a greeting. After greeter is overwritten with an empty closure, any reference to the original closure is lost.

Tip

It's important to remember that greeter is not the closure. It is a variable that contains a closure, so it can be supplanted at any time.

Given that greeter is a...