Potential Groovy Puzzler

MultiAssignAndNoArgAutoCalls.groovy

/* 
 * Groovy has a controversial feature to call a single-arg method w/ null when no arg is passed.
 * Equally surprising, if the first arg is an array, Groovy auto-passes an empty array instead
 *
 * Groovy has a useful feature called multiassignment. It's innocuous looking but not very forgiving;
 * if used on a null or empty array, it will throw NPE and IndexOutOfBoundsExceptions respectively 
 */

String m1(String arg) {
    def a = arg
    "$a"
}

String m2(String arg) {
    def (a) = arg
    "$a"
}

assert m1() == 'null' // true
assert m2() == 'null' /* fails: throws NPE: parens interpreted as multi-assignment, Groovy
                                attempts to call getAt() on the null argument */

/*
 * What happens when we have an array/varargs as the first argument?
 */
                                
String m3(String... args) {
    def a = args
    "$a"
}

String m4(String... args) {
    def (a) = args
    "$a"
}
                               
assert m2() == 'null' // false: returns '[]' -- default no-arg passed value for array param
assert m3() == 'null' /* fails: throws ArrayIndexOutofBoundsException: 
                                the use of parens attempts to multiassign the first
                                element of the array to a, and since the array is
                                empty, the indexing attempt produces a runtime error */