Proxy Pattern: Caching Proxy

/*
 * Problem : Process lines.
 * Input   : Several lines.
 * Output  : One answer per command.
 * Approach: A proxy in front of a service.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: Both the real service and the proxy implement the same interface. The caller cannot tell them apart.
        interface Service {
            int compute(String key);
        }

        static class RealService implements Service {
            int calls = 0;

            @Override
            public int compute(String key) {
                calls++;
                int sum = 0;
                for (char c : key.toCharArray()) {
                    sum += c;
                }
                return sum % 1000;
            }
        }

        // Step 2: The proxy keeps a map of earlier answers and calls the real service only for new keys.
        static class CachingProxy implements Service {
            private final Service real;
            private final java.util.Map<String, Integer> cache = new java.util.HashMap<>();
            int hits = 0;
            boolean lastWasHit = false;

            CachingProxy(Service real) {
                this.real = real;
            }

            @Override
            public int compute(String key) {
                lastWasHit = cache.containsKey(key);
                if (lastWasHit) {
                    hits++;
                    return cache.get(key);
                }
                int value = real.compute(key);
                cache.put(key, value);
                return value;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        RealService real = new RealService();
        CachingProxy proxy = new CachingProxy(real);
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] part = lines[i].split(" ");
            // Step 3: The main loop talks only to the proxy.
            if (part[0].equals("get")) {
                int value = proxy.compute(part[1]);
                output[i] = part[1] + ": " + value + (proxy.lastWasHit ? " (hit)" : " (miss)");
            } else {
                output[i] = "service calls=" + real.calls + " cache hits=" + proxy.hits;
            }
        }
        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)));
    }
}