How to implement a reverse mapping Java using lambdas?

Question

As far as I know, in Java 8, mapping works as follows

for example:

List<String> objList = Arrays.asList("1", "2", "3", "4", "5", "6", "7");
 objList.stream().map( i -> i ). forEach( i -> System.out.print(" " + i));

output:

1 2 3 4 5 6 7

but my question is: how I can do the reverse mapping? like

so output will be this

 7 6 5 4 3 2 1 

Or it is not possible to do so?

Answer 1

In this example I accumulate the list into a LinkedList.

The LinkedList implements Deque (Double ended queue) which allows me to append the items in a last-in-first-out (LIFO) order.

I perform the accumulation in the collect method using LinkedList::addFirst which places the items read off the stream at the head of the list.

public static void main(final String... args) {

    final List<String> objList = Arrays.asList("1", "2", "3", "4", "5", "6", "7");


    final List<String> reversed = objList.stream()
        .collect(LinkedList::new, LinkedList::addFirst, LinkedList::addAll);

    //[7, 6, 5, 4, 3, 2, 1]  
    System.out.println(reversed);

}

The solution below is OK but I don't like having to close over the original list.

public static void main(final String... args) {

    final List<String> objList = Arrays.asList("1", "2", "3", "4", "5", "6", "7");

     final List<String> reversed = IntStream.range(-(objList.size() - 1), 1)
        .mapToObj(i -> objList.get(-i))
        .collect(Collectors.toList());

    //[7, 6, 5, 4, 3, 2, 1]  
    System.out.println(reversed);

}