import java.util.ArrayList;
import java.util.List;

class Main {
    public static void main(String[] args) {
        var original = new ArrayList<>(List.of(1, 2, 3));
        // alias is the SAME list, not a copy
        var alias = original;
        alias.add(4);
        System.out.println(original);
    }
}

two references, one object