complete practical part

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

@@ -1,18 +1,15 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
 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 ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
+    private final Lock readLock = rwLock.readLock();
+    private final Lock writeLock = rwLock.writeLock();
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +20,71 @@ 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");
+        readLock.lock();
+        try
+        {
+            return balance;
+        } finally
+        {
+            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");
+        writeLock.lock();
+        try
+        {
+            balance += amount;
+        } finally
+        {
+            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");
+        writeLock.lock();
+        try
+        {
+            balance -= amount;
+        } finally
+        {
+            writeLock.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 == this)
+        {
+            return;
+        }
+        BankAccount first = this;
+        BankAccount second = target;
+
+        if (this.getAccountId() > target.getAccountId())
+        {
+            first = target;
+            second = this;
+        }
+
+
+        first.writeLock.lock();
+        try
+        {
+            second.writeLock.lock();
+            try
+            {
+                //"first" and "second":only for the order of locking to prevent deadlock (have nothing to do with the direction of the transfer)
+                this.balance -= amount;
+                target.balance += amount;
+            } finally
+            {
+                second.writeLock.unlock();
+            }
+        } finally
+        {
+            first.writeLock.unlock();
+        }
     }
+
 }