Implement all TODOs

src/main/java/dev/banking/model/BankAccount.java

@@ -1,18 +1,13 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
-
-    /*
-     * Students may introduce additional fields
-     * such as:
-     * - Lock / ReentrantLock
-     * - ReadWriteLock
-     * - Object monitor
-     * - etc.
-     */
+    private final Lock lock = new ReentrantLock();
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +18,48 @@ public class BankAccount {
         return accountId;
     }
 
-    /*
-     * TODO:
-     * Return the current balance in a thread-safe way.
-     *
-     * Requirements:
-     * - Must be safe under concurrent reads/writes
-     * - Should not block unnecessarily if using read/write locks
-     */
     public long getBalance() {
-        throw new UnsupportedOperationException("TODO: implement thread-safe balance read");
+        lock.lock();
+        try {
+            return balance;
+        } finally {
+            lock.unlock();
+        }
     }
 
-    /*
-     * TODO:
-     * Increase balance atomically.
-     *
-     * Requirements:
-     * - Must not lose updates under concurrency
-     */
     public void deposit(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+        lock.lock();
+        try {
+            balance += amount;
+        } finally {
+            lock.unlock();
+        }
     }
 
-    /*
-     * TODO:
-     * Decrease balance atomically.
-     *
-     * Requirements:
-     * - Must not cause race conditions
-     * - Negative balance handling is NOT required unless you decide
-     *   to extend the system (optional)
-     */
     public void withdraw(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
+        lock.lock();
+        try {
+            balance -= amount;
+        } finally {
+            lock.unlock();
+        }
     }
 
-    /*
-     * TODO:
-     * Transfer money between two accounts atomically.
-     *
-     * IMPORTANT REQUIREMENTS:
-     * - Must be atomic (no partial transfer)
-     * - Must be deadlock-free
-     * - Must protect both source and target accounts
-     *
-     * HINT:
-     * - Consider global lock ordering using accountId
-     * - Or tryLock with retry strategy
-     */
     public void transfer(BankAccount target, long amount) {
-        throw new UnsupportedOperationException("TODO: implement atomic deadlock-free transfer");
+        BankAccount first = this.accountId < target.accountId ? this : target;
+        BankAccount second = this.accountId < target.accountId ? target : this;
+
+        first.lock.lock();
+        try {
+            second.lock.lock();
+            try {
+                this.withdraw(amount);
+                target.deposit(amount);
+            } finally {
+                second.lock.unlock();
+            }
+        } finally {
+            first.lock.unlock();
+        }
     }
 }

src/main/java/dev/banking/model/DepositTransaction.java

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Represents a deposit operation.
- */
 public final class DepositTransaction extends Transaction {
 
     private final int accountId;

src/main/java/dev/banking/model/Transaction.java

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Base class for all transaction types.
- */
 public abstract class Transaction {
 
     private final int amount;

src/main/java/dev/banking/model/TransferTransaction.java

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Represents a transfer operation between two accounts.
- */
 public final class TransferTransaction
         extends Transaction {
 

src/main/java/dev/banking/model/WithdrawTransaction.java

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Represents a withdrawal operation.
- */
 public final class WithdrawTransaction extends Transaction {
 
     private final int accountId;

src/main/java/dev/banking/monitor/LiveMonitor.java

@@ -1,7 +1,6 @@
 package dev.banking.monitor;
 
 import dev.banking.model.BankAccount;
-
 import java.util.Collection;
 
 public class LiveMonitor {
@@ -9,15 +8,12 @@ public class LiveMonitor {
     public void update(
             Collection<BankAccount> accounts
     ) {
-
         for (BankAccount account : accounts) {
-
             System.out.printf(
                     "Account %d -> %d%n",
                     account.getAccountId(),
                     account.getBalance()
             );
-
         }
     }
 }

src/main/java/dev/banking/processor/TransactionProcessor.java

@@ -1,7 +1,6 @@
 package dev.banking.processor;
 
 import dev.banking.model.*;
-
 import java.util.Map;
 
 public class TransactionProcessor {
@@ -15,33 +14,19 @@ public class TransactionProcessor {
     }
 
     public void process(Transaction tx) {
-
         if (tx instanceof DepositTransaction deposit) {
-
             BankAccount account = accounts.get(deposit.getAccountId());
-
             account.deposit(deposit.getAmount());
         }
-
         else if (tx instanceof WithdrawTransaction withdraw) {
-
             BankAccount account = accounts.get(withdraw.getAccountId());
-
             account.withdraw(withdraw.getAmount());
         }
-
         else if (tx instanceof TransferTransaction transfer) {
-
             BankAccount source = accounts.get(transfer.getSourceAccountId());
-
             BankAccount target = accounts.get(transfer.getTargetAccountId());
-
-            source.transfer(
-                    target,
-                    transfer.getAmount()
-            );
+            source.transfer(target, transfer.getAmount());
         }
-
         else {
             throw new IllegalArgumentException(
                     "Unknown transaction type: " + tx.getClass()

src/main/java/dev/banking/service/BankingSystem.java

@@ -2,22 +2,9 @@ package dev.banking.service;
 
 import dev.banking.model.*;
 import dev.banking.processor.TransactionProcessor;
-
 import java.util.List;
 import java.util.concurrent.ExecutorService;
 
-/**
- * Dispatches a list of transactions to a shared ExecutorService
- * for concurrent (asynchronous) processing.
- *
- * Each transaction is submitted as an independent task and may
- * be executed in parallel depending on thread availability.
- *
- * No ordering guarantees are provided between transactions.
- *
- * Lifecycle management of the ExecutorService (creation,
- * shutdown, termination) is handled outside this class.
- */
 public class BankingSystem {
 
     private final ExecutorService executor;
@@ -34,13 +21,10 @@ public class BankingSystem {
     public void processTransactions(
             List<Transaction> transactions
     ) {
-
         for (Transaction tx : transactions) {
-
             executor.submit(() -> {
                 processor.process(tx);
             });
-
         }
     }
 }