Implement thread-safe BankAccount operations

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

@@ -1,18 +1,12 @@
 package dev.banking.model;
 
+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 ReentrantLock lock = new ReentrantLock();
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +17,61 @@ 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;
+        BankAccount second;
+
+        if (this.accountId < target.accountId) {
+            first = this;
+            second = target;
+        } else {
+            first = target;
+            second = this;
+        }
+
+        first.lock.lock();
+        second.lock.lock();
+
+        try {
+            this.balance -= amount;
+            target.balance += amount;
+        } finally {
+            second.lock.unlock();
+            first.lock.unlock();
+        }
     }
 }