How to work with ArrayList correctly?

Question

Java

Hello. I'm learning Java by solving problems on Codewars.
First, I solve the problem I'm interested in in JavaScript, then in Java, thereby learning methods in Java.
For example:

function findChildren(dancingBrigade) {
    return dancingBrigade
        .toLowerCase()
        .split('')
        .sort()
        .map((currentValue, index, array) => {
            const previousValue = array[index - 1];

            if (index === 0 || currentValue !== previousValue) {
                return currentValue.toUpperCase();
            }

            return currentValue;
        })
        .join('');
}

const s = 'beeeEBb'; // BbbEeee
const s2 = 'uwwWUueEe'; // EeeUuuWww

console.log(findChildren(s)); // beeeEBb
// console.log(findChildren(s2)); // EeeUuuWww

Java

package main.java.kyu6;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class WhereIsMyParent {

    static int index = 0;

    static String findChildren(final String text) {

        ArrayList<Character> arrayList = new ArrayList<>();

        for (int i = 0; i < text.length(); i++) {
            Character currentValue = Character.toLowerCase(text.charAt(i));
//            System.out.println(text.charAt(i));

            arrayList.add(currentValue);
        }

        System.out.println(arrayList);

        System.out.println(Collections.sort(arrayList));

        List<Character> result = arrayList.stream().map((currentValue) -> {
            Character previousValue = arrayList.get(index - 1);

            index++;

            return currentValue;

        }).collect(Collectors.toList());

//        System.out.println("result " + result);

        return "";
    }

    public static void main(String[] args) {
        String s = "beeeEBb"; // "EeeeBbb"
        String s2 = "uwwWUueEe"; // "WwwUuuEee"

        System.out.println(findChildren(s)); // "EeeeBbb"
//        System.out.println(findChildren(s2)); // "WwwUuuEee"
    }
}

Once again I will clarify that the question is not how to solve the problem, but in the specific moments described below.
Initially, I created an ArrayList collection in a loop and filled it. In this case, I'm parsing exactly the work with ArrayList.
Next, using the Collections helper class, I want to sort the collection in alphabetical order:
System.out.println(Collections.sort(arrayList));
First question, why doesn't it sort?
I will also clarify that here it is precisely the desire to practice functional programming in Java.
Next, using the Stream API, using the map method, I want to loop through the collection. The second question, did I understand correctly that there is only currentValue in the parameters? Accordingly, to get the index, I need to separately declare a variable above?
Next in the condition I want to check:
If (currentValue !== previousValue) {}
Accordingly, I need to get the previousValue:

Character previousValue = arrayList.get(index - 1);

The third question, as far as I understand, if the element at the specified index does not exist, not null will be returned, but an error. How to get the previousValue correctly?

I ask you not to judge strictly, but, if possible, to prompt about the above issues.

Answer 1

0. It's the 21st year in the yard. No need to use old versions of java and declare a type for local variables.
var arrayList = new ArrayList<Character>>();
1. To make an array out of a string, use the .split function: var array = someString.toLowerCase().split("")
2. System.out.println(Collections.sort(arrayList));.sort is void. It doesn't return anything. Here you will have a compilation error.
3.

{
            Character previousValue = arrayList.get(index - 1);

            index++;

            return currentValue;

        }

no one does that at all. The main problem is the use of global variables inside the lambda. Consider that there is no way you can get previousValue (not in StreamAPI). Come up with another solution.