It is often better practice to CREATE a NEW OBJECT and RETURN it from the method.
In this case, the original object is not changed.

The normal way to return an object is with a return statement. 
On the calling method side, the return value must of course be stored in a variable if it is to be used later in the program.

import java.util.ArrayList;

public class Example {
    public static void main(String[] parameters){
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(5);
        numbers.add(1);
        numbers.add(8);

        System.out.println("List before: " + numbers);
        System.out.println("New list: " + increasebyOne(numbers));
        System.out.println("List after: " + numbers);   
    }

    public static ArrayList<Integer> increasebyOne(ArrayList<Integer> numbers) {
        ArrayList<Integer> newList = new ArrayList<>();
        for (int element : numbers) {
            newList.add(element + 1);
        }
        return newList;
    }
}

Program outputs:
List before: [5, 1, 8]
New list: [6, 2, 9]
List after: [5, 1, 8]
 

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Of course, the method can also create an object from scratch, and return it.

The example creates a NEW LIST with the number of EMPTY STRINGS specified by the given parameter:

import java.util.ArrayList;

public class Example {
    public static void main(String[] parameters){
        ArrayList<String> strings = createList(10);
        System.out.println("List size:" + strings.size());  
    }

    public static ArrayList<String> createList(int size) {
        ArrayList<String> list = new ArrayList<>();
        for (int i=0; i<size; i++) {
            list.add("");
        }
        return list;
    }
}

Program outputs:
List size: 10