Bank Account Simulator

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : One answer line for each command.
 * Approach: A class with private data and checked methods.
 * Time    : O(commands)
 * Space   : O(commands)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The class keeps its data private. Only its methods can change the balance (encapsulation).
        static class BankAccount {
            private long balance = 0;

            // Step 2: Each method checks its input and throws an exception for a bad request.
            void deposit(long amount) {
                if (amount <= 0) {
                    throw new IllegalArgumentException("invalid amount");
                }
                balance += amount;
            }

            void withdraw(long amount) {
                if (amount <= 0) {
                    throw new IllegalArgumentException("invalid amount");
                }
                if (amount > balance) {
                    throw new IllegalStateException("insufficient funds");
                }
                balance -= amount;
            }

            long getBalance() {
                return balance;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        BankAccount account = new BankAccount();
        String[] output = new String[lines.length];

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

            // Step 3: Run the command. A failed request throws an exception, and we print its message.
            try {
                if (parts[0].equals("deposit")) {
                    account.deposit(Long.parseLong(parts[1]));
                    output[i] = "OK";
                } else if (parts[0].equals("withdraw")) {
                    account.withdraw(Long.parseLong(parts[1]));
                    output[i] = "OK";
                } else {
                    output[i] = "Balance: " + account.getBalance();
                }
            } catch (IllegalArgumentException | IllegalStateException error) {
                output[i] = "Error: " + error.getMessage();
            }
        }
        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)));
    }
}