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

@@ -1,10 +1,16 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
 
+    private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
+
     /*
      * Students may introduce additional fields
      * such as:
@@ -32,7 +38,12 @@ 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.readLock().lock();
+        try {
+            return balance;
+        } finally {
+            lock.readLock().unlock();
+        }
     }
 
     /*
@@ -43,7 +54,15 @@ public class BankAccount {
      * - Must not lose updates under concurrency
      */
     public void deposit(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+        if(amount < 0)
+            throw new IllegalArgumentException("Amount can't be negative!");
+
+        lock.writeLock().lock();
+        try {
+            balance += amount;
+        } finally {
+            lock.writeLock().unlock();
+        }
     }
 
     /*
@@ -56,7 +75,15 @@ public class BankAccount {
      *   to extend the system (optional)
      */
     public void withdraw(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
+        if(amount < 0)
+            throw new IllegalArgumentException("Amount can't be negative!");
+
+        lock.writeLock().lock();
+        try {
+            balance -= amount;
+        } finally {
+            lock.writeLock().unlock();
+        }
     }
 
     /*
@@ -73,6 +100,28 @@ public class BankAccount {
      * - Or tryLock with retry strategy
      */
     public void transfer(BankAccount target, long amount) {
-        throw new UnsupportedOperationException("TODO: implement atomic deadlock-free transfer");
+        if (amount < 0) {
+            throw new IllegalArgumentException("Transfer amount cannot be negative");
+        }
+        if (this == target) {
+            throw new IllegalArgumentException("Cannot transfer to the same account");
+        }
+
+        // we order them by their id so that we always lock the first one to prevent deadLocks taking place.
+        BankAccount first = this.accountId < target.accountId ? this : target;
+        BankAccount second = this.accountId < target.accountId ? target : first;
+
+        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();
+        }
     }
 }