Deadlock-Free Transfers (Lock Ordering)

/*
 * Problem : Final balances.
 * Input   : Transfer lines.
 * Output  : Four balance lines.
 * Approach: Ordered locks.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: One lock object per account.
        static final Object[] LOCKS = {new Object(), new Object(), new Object(), new Object()};
        static final long[] BALANCE = new long[4];

        // Step 2: Always lock the smaller account number first. Then two threads can never wait for each other in a circle.
        static void transfer(int from, int to, long amount) {
            if (from == to) {
                return;
            }
            Object first = LOCKS[Math.min(from, to)];
            Object second = LOCKS[Math.max(from, to)];
            synchronized (first) {
                synchronized (second) {
                    BALANCE[from] -= amount;
                    BALANCE[to] += amount;
                }
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        java.util.Arrays.fill(BALANCE, 1000);
        // Step 3: Start one thread for every transfer line.
        Thread[] threads = new Thread[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] part = lines[i].split(" ");
            int from = Integer.parseInt(part[0]);
            int to = Integer.parseInt(part[1]);
            long amount = Long.parseLong(part[2]);
            threads[i] = new Thread(() -> transfer(from, to, amount));
            threads[i].start();
        }
        // Step 4: Wait for all threads. join() throws a checked exception, so we handle it.
        try {
            for (Thread thread : threads) {
                thread.join();
            }
        } catch (InterruptedException error) {
            throw new IllegalStateException(error);
        }
        String[] output = new String[4];
        for (int i = 0; i < 4; i++) {
            output[i] = "account " + i + ": " + BALANCE[i];
        }
        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)));
    }
}