implement TODOs

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

@@ -1,18 +1,15 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
+    private final ReadWriteLock lock = new ReentrantReadWriteLock();
 
-    /*
-     * Students may introduce additional fields
-     * such as:
-     * - Lock / ReentrantLock
-     * - ReadWriteLock
-     * - Object monitor
-     * - etc.
-     */
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,40 +20,36 @@ 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.readLock().lock();
+        try {
+            return balance;
+        }
+        finally {
+            lock.readLock().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.writeLock().lock();
+        try {
+            balance += amount;
+        }
+        finally {
+            lock.writeLock().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.writeLock().lock();
+        try {
+            balance -= amount;
+        }
+        finally {
+            lock.writeLock().unlock();
+        }
     }
 
     /*
@@ -73,6 +66,21 @@ public class BankAccount {
      * - 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 ? this : target;
+
+        first.lock.writeLock().lock();
+        try {
+            second.lock.writeLock().lock();
+            try {
+                this.balance -= amount;
+                target.balance += amount;
+            } finally {
+                second.lock.writeLock().unlock();
+            }
+        }
+        finally{
+            first.lock.writeLock().unlock();
+        }
     }
 }