Immutable Maps (and Lists)

//source: http://royontechnology.blogspot.co.at/2010/06/creating-collection-with-single-element.html

//note the different constructor args; both return immutables

assert [2] == Collections.singletonMap(1, 1).collect {k,v ->
    return k+v
}

assert [3, 7] == Collections.unmodifiableMap([1:2, 3:4]).collect {k,v ->
    return k+v
}