Test with a Fake Clock

/*
 * Problem : Process lines.
 * Input   : Several lines.
 * Output  : One greeting per line.
 * Approach: Fake clock.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The clock is an interface. A real program passes the system clock, a test passes a fake one.
        interface Clock {
            int hour();
        }

        static class Greeter {
            private final Clock clock;

            Greeter(Clock clock) {
                this.clock = clock;
            }

            // Step 2: The greeter only asks the clock. It never knows if the clock is real or fake.
            String greet() {
                int hour = clock.hour();
                if (hour >= 5 && hour <= 11) {
                    return "Good morning";
                }
                if (hour >= 12 && hour <= 16) {
                    return "Good afternoon";
                }
                return hour >= 17 && hour <= 20 ? "Good evening" : "Good night";
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            // Step 3: A fake clock is just a lambda that returns the hour from the input line.
            int hour = Integer.parseInt(lines[i]);
            output[i] = new Greeter(() -> hour).greet();
        }
        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)));
    }
}