Vending Machine Simulator

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : One answer per command.
 * Approach: A machine class with state.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The machine keeps its stock (price and count for each item) and the money the customer inserted.
        static class VendingMachine {
            private final java.util.Map<String, long[]> items = new java.util.HashMap<>();
            private long balance = 0;

            String process(String[] parts) {
                String command = parts[0];

                if (command.equals("stock")) {
                    items.put(parts[1], new long[] {Long.parseLong(parts[2]), Long.parseLong(parts[3])});
                    return "ok";
                }
                if (command.equals("insert")) {
                    balance += Long.parseLong(parts[1]);
                    return "balance " + balance;
                }
                if (command.equals("refund")) {
                    long refund = balance;
                    balance = 0;
                    return "refunded " + refund;
                }

                // Step 2: Buy. Check the item, the stock and the money, in this order.
                long[] item = items.get(parts[1]);
                if (item == null) {
                    return "unknown item";
                }
                if (item[1] == 0) {
                    return "sold out";
                }
                if (balance < item[0]) {
                    return "need " + (item[0] - balance) + " more";
                }

                // Step 3: Sell the item. The rest of the money is returned as change.
                balance -= item[0];
                item[1]--;
                long change = balance;
                balance = 0;
                return "dispensed " + parts[1] + " change " + change;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        VendingMachine machine = new VendingMachine();
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            // Step 4: Send every command line to the machine and keep its answer.
            output[i] = machine.process(lines[i].split(" "));
        }
        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)));
    }
}