// long form
String sayHello(String name){
    "Hello, ${name ? name : 'stranger'}."
}

// elvis
String sayHello(String name){
    "Hello, ${name ?: 'stranger'}."
}

Notice that the "elvis" format omits the "true" term because the original comparison value is to be used in the "true" case. If name is Groovy true, then it will be returned as the value of the expression.

def results = []
(1..4).each{
    def what = (it%2) ? 'odd' :  'even'
    results << what
}
assert results == ['odd', 'even', 'odd', 'even']

Here, the if-condition (in (parentheses)) is slightly more complex than just testing for existence/Groovy-Truth.