Implement thread-safe BankAccount operations

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

@@ -1,4 +1,5 @@
 package dev.banking.model;
+import java.util.concurrent.locks.ReentrantLock;
 
 public class BankAccount {
 
@@ -13,7 +14,7 @@ public class BankAccount {
      * - Object monitor
      * - etc.
      */
-
+    private final ReentrantLock lock = new ReentrantLock();
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
         this.balance = initialBalance;
@@ -32,7 +33,13 @@ 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 +50,13 @@ 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 +69,13 @@ 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 +92,28 @@ 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 lock1;
+        BankAccount lock2;
+
+        if (this.accountId < target.accountId){
+            lock1 = this;
+            lock2 = target;
+        }
+        else{
+            lock1 = target;
+            lock2 = this;
+        }
+
+        lock1.lock.lock();
+        lock2.lock.lock();
+
+        try {
+            this.balance -= amount;
+            target.balance += amount;
+        }
+        finally{
+            lock2.lock.unlock();
+            lock1.lock.unlock();
+        }
     }
 }