Strategy Pattern: Customer Discounts

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : The amount to pay for each line.
 * Approach: Choose a rule object by key.
 * Time    : O(lines)
 * Space   : O(lines)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The strategy interface. Every discount rule has the same method.
        interface DiscountStrategy {
            long priceToPay(long amount);
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        // Step 2: A map from the customer type to its strategy. A new customer type only needs a new entry here.
        java.util.Map<String, DiscountStrategy> strategies = new java.util.HashMap<>();
        strategies.put("REGULAR", amount -> amount * 100 / 100);
        strategies.put("STUDENT", amount -> amount * 90 / 100);
        strategies.put("SENIOR", amount -> amount * 80 / 100);
        strategies.put("VIP", amount -> amount * 70 / 100);

        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] parts = lines[i].split(" ");

            // Step 3: Pick the strategy and use it. There is no if / else chain.
            DiscountStrategy strategy = strategies.get(parts[0]);
            if (strategy == null) {
                output[i] = "Error: unknown customer";
            } else {
                output[i] = String.valueOf(strategy.priceToPay(Long.parseLong(parts[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)));
    }
}