Understanding Groovy Closures: Definition, Usage, and List Operations

This article continues from the previous posts "From Java to Groovy Evolution" and "Groovy List", focusing on Groovy closures.

In Groovy, a closure is a block of code that can access variables and methods from its surrounding scope; it can be assigned to a variable and passed around like any other object.

Example of a simple closure:

def c = { println "hello" }
c()

Closures have an implicit parameter named it, but you can also define explicit parameters. The last expression in a closure is its return value, though you may use the return keyword for clarity.

def square = { it * it }
assert square(3) == 9

def lengthThan = { String s, int i -> return s.size() > i }
assert lengthThan("FunTester", 4) == true
assert lengthThan("Fun", 6) == false

You can pass closures to methods. The following log method accepts a Closure, measures execution time, and prints the result:

def log(Closure c) {
    println "Calling closure"
    def start = System.currentTimeMillis()
    println "Result: " + c()
    def end = System.currentTimeMillis()
    println "Log success (time: ${end - start} ms)"
}
log({ return "Groovy is best language!" })
log { return "Groovy is best language!" }

Using each(), a closure can be applied to every element of a list:

names.each({ String name -> println name })
names.each { String name -> println name }
names.each { println it }

The findAll() method filters a collection based on a closure that returns a boolean, producing a new list of matching elements. Groovy also provides find(), every(), and any() for other common collection operations. def shortNames = names.findAll { it.size() <= 3 } Finally, the article shows how to rewrite a simple Java class in Groovy using closures and collection methods:

def names = ["Ted", "Fred", "Jed", "Ned"]
println names

def shortNames = names.findAll { it.size() <= 3 }
println shortNames.size()
shortNames.each { println it }

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