complete

src/main/java/dev/banking/AtomicDemo.java

@@ -0,0 +1,40 @@
+package dev.banking;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class AtomicDemo {
+    private static int normalCounter = 0;
+    private static final AtomicInteger atomicCounter = new AtomicInteger(0);
+
+    public static void main(String[] args) throws InterruptedException {
+        int threads = 10;
+        int increments = 1000;
+
+        Thread[] threadList = new Thread[threads * 2];
+
+        for (int i = 0; i < threads; i++) {
+            threadList[i * 2] = new Thread(() -> {
+                for (int j = 0; j < increments; j++) {
+                    normalCounter++;
+                }
+            });
+
+            threadList[i * 2 + 1] = new Thread(() -> {
+                for (int j = 0; j < increments; j++) {
+                    atomicCounter.incrementAndGet();
+                }
+            });
+
+            threadList[i * 2].start();
+            threadList[i * 2 + 1].start();
+        }
+
+        for (Thread t : threadList) {
+            t.join();
+        }
+        int expected = threads * increments;
+        System.out.println("Expected value: " + expected);
+        System.out.println("Normal int:      " + normalCounter);
+        System.out.println("AtomicInteger:   " + atomicCounter.get());
+    }
+}

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

@@ -5,15 +5,6 @@ public class BankAccount {
     private final int accountId;
     private long balance;
 
-    /*
-     * Students may introduce additional fields
-     * such as:
-     * - Lock / ReentrantLock
-     * - ReadWriteLock
-     * - Object monitor
-     * - etc.
-     */
-
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
         this.balance = initialBalance;
@@ -23,56 +14,46 @@ 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");
+    public synchronized long getBalance() {
+        return balance;
     }
 
-    /*
-     * TODO:
-     * Increase balance atomically.
-     *
-     * Requirements:
-     * - Must not lose updates under concurrency
-     */
-    public void deposit(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+    public synchronized void deposit(long amount) {
+        balance += amount;
+        notifyAll();
     }
 
-    /*
-     * 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");
+    public synchronized void withdraw(long amount) {
+        while (balance < amount) {
+            try {
+                wait();
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                return;
+            }
+        }
+        balance -= amount;
     }
 
-    /*
-     * 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 (this == target) {
+            return;
+        }
+
+        BankAccount first  = (this.accountId < target.accountId) ? this : target;
+        BankAccount second = (this.accountId < target.accountId) ? target : this;
+
+        synchronized (first) {
+            synchronized (second) {
+                if (this.balance >= amount) {
+                    this.balance -= amount;
+                    target.balance += amount;
+                    target.notifyAll();
+                    return;
+                }
+            }
+            this.withdraw(amount);
+            target.deposit(amount);
+        }
     }
 }