BankAccount completed and all tests passed (with bonuses).

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

@@ -1,18 +1,16 @@
 package dev.banking.model;
 
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
 
-    /*
+    public final ReentrantLock lock = new ReentrantLock();
-     * Students may introduce additional fields
+    private final Condition sufficientFundsCondition = lock.newCondition();
-     * such as:
-     * - Lock / ReentrantLock
-     * - ReadWriteLock
-     * - Object monitor
-     * - etc.
-     */
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +21,88 @@ 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");
+        if (amount <= 0) return;
+
+        lock.lock();
+        try {
+            balance += amount;
+            //Bonus Task
+            sufficientFundsCondition.signalAll();
+        } 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");
+        if (amount <= 0) return;
+
+        lock.lock();
+        try {
+            //Bonus Task
+            while (balance < amount) {
+                try {
+                    boolean receivedSignal = sufficientFundsCondition.await(50, TimeUnit.MILLISECONDS);
+                    if (!receivedSignal && balance < amount) {
+                        return;
+                    }
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                    return;
+                }
+            }
+            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");
+        if (target == null || this == target || amount <= 0) return;
+
+        BankAccount firstAccount = this.accountId < target.getAccountId() ? this : target;
+        BankAccount secondAccount = firstAccount == this ? target : this;
+
+        firstAccount.lock.lock();
+        secondAccount.lock.lock();
+
+        try {
+            //Bonus Task
+            while (this.balance < amount) {
+                try {
+                    secondAccount.lock.unlock();
+
+                    boolean receivedSignal = this.sufficientFundsCondition.await(50, TimeUnit.MILLISECONDS);
+
+                    secondAccount.lock.lock();
+
+                    if (!receivedSignal && this.balance < amount) {
+                        return;
+                    }
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                    return;
+                }
+            }
+
+            this.balance -= amount;
+            target.balance += amount;
+
+            //Bonus Task
+            target.sufficientFundsCondition.signalAll();
+
+        } finally {
+            secondAccount.lock.unlock();
+            firstAccount.lock.unlock();
+        }
     }
 }