Objects as parameters -> passed as a REFERENCE


so
a method can change/MODIFY INPLACE the input list it receives as a parameter


import java.util.ArrayList;

public class Example {
    public static void main(String[] parameters){
        ArrayList<Integer> numbers = new ArrayList<>();
        for (int i=1; i<5; i++) {
            numbers.add(i);
        }
        System.out.println(numbers);
        increase(numbers);
        System.out.println(numbers);
    }

    public static void increase(ArrayList<Integer> list) {
        for (int i=0; i<list.size(); i++) {
            list.set(i, list.get(i) + 1);
        }
    }
}

Program outputs:
[1, 2, 3, 4]
[2, 3, 4, 5]