Counting Set Wrapper (Composition)

/*
 * Problem : Run the commands.
 * Input   : Command lines.
 * Output  : One line per answer.
 * Approach: Wrapper class
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The wrapper HAS a set inside. It does not extend the set, so its own counting cannot be counted twice.
        static class CountingSet {
            private final java.util.Set<Integer> inner = new java.util.HashSet<Integer>();
            private int attempts = 0;

            // Step 2: Every add is one attempt, whether or not the number is new.
            void add(int value) {
                attempts++;
                inner.add(value);
            }

            int size() {
                return inner.size();
            }

            int attempts() {
                return attempts;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        CountingSet set = new CountingSet();
        java.util.List<String> out = new java.util.ArrayList<String>();
        for (String line : lines) {
            String[] part = line.split(" ");
            if (part[0].equals("add")) {
                set.add(Integer.parseInt(part[1]));
            } else if (part[0].equals("addall")) {
                for (int i = 1; i < part.length; i++) {
                    set.add(Integer.parseInt(part[i]));
                }
            } else if (part[0].equals("size")) {
                out.add(String.valueOf(set.size()));
            } else {
                out.add(String.valueOf(set.attempts()));
            }
        }
        return out.toArray(new String[0]);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(String.join("\n", solve(lines)));
    }
}