• java.util.Arrays import is not needed in reference solution.
• Please select "Java 17" in the language version dropdown.
• random field can be final.
• Please return the input string or an empty string in the solution setup to make it compile.

Fixed these things you pointed out in editing this kumite. Thanks!

• Test classes and methods can be package private in JUnit5.
• There seems to be something odd with the description in the diff view. But that could also just be a visual bug.
• If you need a stream of codepoints, you can use String.chars().
• Random tests only create lowercase letter inputs.
  For random inputs you could use e.g.

random.ints(random.nextLong(50), 32, 128).collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append).toString();

• Please stick to Java naming conventions which is camelCase() for methods.
• top level structures cannot have the modifier static (see Solution setup)
• There is exactly one random test which is not executed because of missing @Test annotation.

Duplicate: https://www.codewars.com/kata/595970246c9b8fa0a8000086/