URL Shortener Service

/*
 * Problem : Process lines.
 * Input   : Several command lines.
 * Output  : One result per command.
 * Approach: Two maps in a class.
 * Time    : O(n)
 * Space   : O(urls)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The service keeps two maps: url to code, and code to a record with the url and the visit count.
        static class Shortener {
            private final java.util.Map<String, String> codeOf = new java.util.HashMap<String, String>();
            private final java.util.Map<String, Object[]> records = new java.util.HashMap<String, Object[]>();

            String shorten(String url) {
                String code = codeOf.get(url);
                if (code == null) {
                    code = "c" + (codeOf.size() + 1);
                    codeOf.put(url, code);
                    records.put(code, new Object[] {url, 0});
                }
                return code;
            }

            // Step 2: resolve() counts a visit. stats() only reads the count.
            String resolve(String code) {
                Object[] record = records.get(code);
                if (record == null) {
                    return "unknown";
                }
                record[1] = (Integer) record[1] + 1;
                return (String) record[0];
            }

            String stats(String code) {
                Object[] record = records.get(code);
                return record == null ? "unknown" : String.valueOf(record[1]);
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        Shortener service = new Shortener();
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] part = lines[i].split(" ");
            if (part[0].equals("SHORTEN")) {
                output[i] = service.shorten(part[1]);
            } else if (part[0].equals("RESOLVE")) {
                output[i] = service.resolve(part[1]);
            } else {
                output[i] = service.stats(part[1]);
            }
        }
        return output;
    }

    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)));
    }
}