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

@@ -1,10 +1,14 @@
 package dev.banking.model;
 
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
-
+    private final Lock lock = new ReentrantLock();
     /*
      * Students may introduce additional fields
      * such as:
@@ -32,7 +36,13 @@ public class BankAccount {
      * - 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();
+        }
     }
 
     /*
@@ -43,7 +53,13 @@ public class BankAccount {
      * - 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();
+        }
     }
 
     /*
@@ -56,7 +72,13 @@ public class BankAccount {
      *   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();
+        }
     }
 
     /*
@@ -73,6 +95,19 @@ 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.getAccountId() > target.getAccountId() ? target : this;
+        BankAccount second = this.getAccountId() > target.getAccountId() ? this : target;
+
+        first.lock.lock();
+        second.lock.lock();
+        try {
+            this.balance -= amount;
+            target.balance += amount;
+        }
+        finally {
+            second.lock.unlock();
+            first.lock.unlock();
+        }
+
     }
 }