I have observed that your code prints the reference of the unsorted array, whereas it should assign the sorted array to a new variable and print that instead. Furthermore, the comparator that you have defined is not used and is not required for the code's functionality.

The original array can't be changed Java streams. Java streams work with unchangeable data and make new ones instead of modifying the original. So, if you sort with Java streams, you need to create a new sorted array. If you want to change the original array, use the usual Arrays.sort() method to sort it.

Here are both methods fixed and working, plus more:

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

class Scratch {
  public static int[] UNSORTED_ARR = {9, 21, 5, 8};

  public static void main(String[] args) {
    sort1();
    sort2();
    sort3();
    sort4();
    sort5();
  }

  /**
   * Sort using comparator and streams
   */
  private static void sort1() {
    System.out.println("Sort using comparator and streams");
    Comparator<Integer> comparator = Integer::compareTo;
    var sortedStream = Arrays.stream(UNSORTED_ARR).boxed().sorted(comparator).toArray();
    System.out.println(Arrays.toString(sortedStream));
  }

  /**
   * Sort using sorted() method and streams
   */
  private static void sort2() {
    System.out.println("Sort using sorted() method and streams");
    var sortedStream = Arrays.stream(UNSORTED_ARR).sorted().toArray();
    System.out.println(Arrays.toString(sortedStream));
  }


  /**
   * Sort using sorted() method and collect list with streams
   */
  private static void sort3() {
    System.out.println("Sort using sorted() method and collect list with streams");
    List<Integer> sortedList = Arrays.stream(UNSORTED_ARR)
        .sorted()
        .boxed()
        .collect(Collectors.toList());
    System.out.println(sortedList);
  }

  /**
   * Sort using reverseOrder() method and streams
   */
  private static void sort4() {
    System.out.println("Reverse sort using reverseOrder() method and streams");
    int[] sortedArr = Arrays.stream(UNSORTED_ARR)
        .boxed()
        .sorted(Comparator.reverseOrder())
        .mapToInt(Integer::intValue)
        .toArray();
    System.out.println(Arrays.toString(sortedArr));
  }


  /**
   * Sort using Arrays.sort()
   * This modifies the original array and not using streams
   */
  private static void sort5() {
    System.out.println("Sort using Arrays.sort()");
    Arrays.sort(UNSORTED_ARR);
    System.out.println(Arrays.toString(UNSORTED_ARR));
  }
}

Output:

Sort using comparator and streams
[5, 8, 9, 21]
Sort using sorted() method and streams
[5, 8, 9, 21]
Sort using sorted() method and collect list with streams
[5, 8, 9, 21]
Reverse sort using reverseOrder() method and streams
[21, 9, 8, 5]
Sort using Arrays.sort()
[5, 8, 9, 21]

Every time you create a Stream, another object is created. Therefore you have to assign the result of sorted stream to a variable and transform it to a data structure like a List.

class Scratch {
   public static void main(String[] args) {
       int [] arr3 = { 9, 21, 5, 8 };
       List<Integer> sorted = Arrays.stream(arr3).boxed().sorted().toList();
       System.out.println(Arrays.toString(sorted));
   }
}

In this case it is not necessary to define a comparator because when you call sorted the default comparator is the comparator of the data type that is Integer.

       Comparator<Integer> cc = (e1,e2) -> {
            return e1.compareTo(e2);
       };