Immutable Lists (List.of and List.copyOf)

/*
 * Problem : Process an array.
 * Input   : n, then n integers.
 * Output  : The list and a modify test.
 * Approach: Read-only collections.
 * Time    : O(n log n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(int[] a) {
        // Step 1: Make a sorted set of the different numbers. TreeSet keeps them sorted.
        java.util.TreeSet<Integer> set = new java.util.TreeSet<>();
        for (int value : a) {
            set.add(value);
        }

        // Step 2: Before Java 9 we need two steps: make a list, then wrap it as unmodifiable.
        java.util.List<Integer> list = java.util.Collections.unmodifiableList(new java.util.ArrayList<>(set));

        // Step 3: Try to change the list. An unmodifiable list throws UnsupportedOperationException.
        String modify;
        try {
            list.add(0);
            modify = "succeeded";
        } catch (UnsupportedOperationException error) {
            modify = "failed";
        }

        // Step 4: Print the result.
        StringBuilder text = new StringBuilder();
        for (int value : list) {
            text.append(text.length() == 0 ? "" : " ").append(value);
        }
        return new String[] {"list: " + text, "modify: " + modify};
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        System.out.println(String.join("\n", solve(a)));
    }
}