I just found that code:
[1,2] [4, 4]
is completely valid in Groovy but can't find what does such expression evaluates to, for me it returns null in all possible cases:
groovy:000> [1, 2] []
===> []
groovy:000> [1, 2] [4]
===> null
groovy:000> [1, 2] [4,5]
===> [null, null]
So basically the question is what does the expression:
a = list1 list2
mean in Groovy?
In groovy, the [] operator is just a shorthand for getAt(), so in this case it's calling the method List.getAt(Collection).
The behavior is to return a list containing all the elements whose index is listed in the collection. So for [1,2][4,5], it's returning a list with elements 4 and 5, which both happen to be out of range, so null.
Here are some examples that illustrate it a little better:
assert ['a', 'b', 'c', 'd', 'e'][1, 3] == ['b', 'd']
assert [0, 1, 2, 3, 4][4..0] == [4, 3, 2, 1, 0]
def firstAndThird = [4,3,2,1][0,2] doesn't make too much sense, but def firstAndThird = someList[0,2] does a bit more :)foo.bar 1 means foo.bar(1), but foo.bar [1] does not mean foo.bar([1]). And foo.bar 1, 2 is legal syntax, but foo.bar [1], 2 yields a syntax error :). I think what is confusing is the parentheses-less syntax; it has many corner-cases that makes its usage ugly.