Map Functions

Map functions transform elements in the Stream from one form to another.

At it’s simplest level, it looks like the following:

def stream = Stream.from( 1..3 )
                   .map { num -> "Item $num" }

assert stream.collect() == [ 'Item 1', 'Item 2', 'Item 3' ]

You can also pass the index into the map call by calling mapWithIndex:

def stream = Stream.from( 1..3 )
                   .mapWithIndex { num, idx -> "Item $num @ $idx" }

assert stream.collect() == [ 'Item 1 @ 0', 'Item 2 @ 1', 'Item 3 @ 2' ]

There also exists a flatMap variant, so you can return a Collection of values and each of these will be iterated first, before the underlying Stream is queried again for the next sequence in the list:

def stream = Stream.from( 1..3 )
                   .flatMap { num -> [ num, num ] }

assert stream.collect() == [ 1, 1, 2, 2, 3, 3 ]

And of course, this has a flatMapWithIndex variant.

def stream = Stream.from( 1..3 )
                   .flatMapWithIndex { num, idx -> [ num ] * ( idx + 1 ) }

assert stream.collect() == [ 1, 2, 2, 3, 3, 3 ]