We already know that parentheses around method parameters are optional, so the previous call to each can also be considered equivalent to:

flintstones.each ({ println "Hello, ${it}")

Groovy has special handling for methods whose last parameter is a closure. When invoking these methods, the closure can be defined anonymously after the method call parentheses. So, yet another legitimate way to call the preceding line is:

flintstones.each() { println "hello, ${it}" }

The general convention is not to use parentheses unless there are parameters in addition to the closure:

given:
    def flintstones = ["Fred", "Barney", "Wilma"]
when: "we call findIndexOf passing int and a Closure"
    def result = flintstones.findIndexOf(0) { it == 'Wilma'}
then:
    result == 2

The signature of the GDK findIndexOf method is:

int findIndexOf(int, Closure)

We can define our own methods that accept closures as parameters. The simplest case is a method that accepts only a single closure as...