We can use the flatten method in Groovy to flatten a collection that contains other collections into a single collection with all elements. We can pass a closure as extra argument to the flatten method to transform each element that is flattened. The argument of the closure is the element from the original collection.

In the following example we first use the flatten method without a closure argument. Then we pass a closure argument and transform the element:

def list = [1, [2, 3], [[4]]]

// Simple flatten the nested collections.
assert list.flatten() == [1, 2, 3, 4]

// We can use a closure to transform
// the elements in the resulting collection.
assert list.flatten { it * 2 } == [2, 4, 6, 8]

Written with Groovy 2.5.7.