Java lambda on One-Dim array

If you do not have a restriction to only use sort, I have a contrived way to do an in place reversal using lambdas:

import java.util.Arrays;
import java.util.Collections;
import java.util.stream.IntStream;

public class ReverseArr {

    public static void main(String[] args) {
    Integer[] arr = { 4, 5, 61, 3, 9, 3, 1, -4, 7, 2, -8, 6, -3 };

    IntStream.rangeClosed(0, arr.length - 1)
             .boxed()
             .sorted(Collections.reverseOrder())
             .forEach(e -> {
                 if ((arr.length - e) - e > 1) {
                 int temp = arr[e];
                 arr[e] = arr[arr.length - e - 1];
                 arr[arr.length - e - 1] = temp;
                 }
             });

    Arrays.stream(arr)
          .forEach(e -> System.out.print(e+ " "));

    }

}

As the comment mentions, we have to do the reversal based on the indices. So I just generated those values and switch the values of arr using those indices through lambdas.

A key part is this condition: (arr.length - e) - e > 1. What it means is to stop reversing the elements once we get to the middle (the middle is when the difference is 1 if the array has an even number of elements and it is 0 if the array has an odd number of elements. This way we do not undo our reversal.

Output:

-3 6 -8 2 7 -4 1 3 9 3 61 5 4 

To reverse 1-D array

• Using Java 8 lambda

   IntStream.rangeClosed(1, arr.length)
         .mapToObj(i -> arr[arr.length - i])
         .toArray();
  

• Using Collections.reverse() to reverse array

   Collections.reverse(arr);