.idea/.gitignore

@@ -0,0 +1,10 @@
+# Default ignored files
+/shelf/
+/workspace.xml
+# Ignored default folder with query files
+/queries/
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml
+# Editor-based HTTP Client requests
+/httpRequests/

.idea/compiler.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="CompilerConfiguration">
+    <annotationProcessing>
+      <profile name="Maven default annotation processors profile" enabled="true">
+        <sourceOutputDir name="target/generated-sources/annotations" />
+        <sourceTestOutputDir name="target/generated-test-sources/test-annotations" />
+        <outputRelativeToContentRoot value="true" />
+        <module name="HW-09-Advanced-Multithreading" />
+      </profile>
+    </annotationProcessing>
+  </component>
+</project>

.idea/encodings.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Encoding">
+    <file url="file://$PROJECT_DIR$/src/main/java" charset="UTF-8" />
+    <file url="file://$PROJECT_DIR$/src/main/resources" charset="UTF-8" />
+  </component>
+</project>

.idea/jarRepositories.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="RemoteRepositoriesConfiguration">
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Central Repository" />
+      <option name="url" value="https://repo.maven.apache.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Maven Central repository" />
+      <option name="url" value="https://repo1.maven.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="jboss.community" />
+      <option name="name" value="JBoss Community repository" />
+      <option name="url" value="https://repository.jboss.org/nexus/content/repositories/public/" />
+    </remote-repository>
+  </component>
+</project>

.idea/misc.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ExternalStorageConfigurationManager" enabled="true" />
+  <component name="MavenProjectsManager">
+    <option name="originalFiles">
+      <list>
+        <option value="$PROJECT_DIR$/pom.xml" />
+      </list>
+    </option>
+  </component>
+  <component name="ProjectRootManager" version="2" project-jdk-name="25" project-jdk-type="JavaSDK" />
+</project>

.idea/vcs.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="" vcs="Git" />
+  </component>
+</project>

REPORT.md

@@ -0,0 +1,75 @@
+# REPORT
+### Hesam Ghazi
+### 403222015
+## 1. Atomic Variables
+Atomic variables provide thread-safe single-variable operations performed atomically using CPU-supported compare-and-swap (CAS) instructions. Unlike ordinary variables, they prevent race conditions without explicit synchronization for simple operations.
+
+## 2. Atomic Classes
+- `AtomicInteger`
+- `AtomicLong`
+- `AtomicBoolean`
+- `AtomicReference<T>`
+
+**Use case:** `AtomicInteger` is commonly used as a thread-safe counter shared among multiple threads.
+
+## 3. Locks vs Atomic Variables
+**Atomic variables**
+- Best for simple read-modify-write operations.
+- Non-blocking and usually faster under low contention.
+- Limited to simple operations.
+
+**Locks**
+- Suitable for protecting multiple variables or complex critical sections.
+- Easier to implement compound operations atomically.
+- Introduce blocking and context-switch overhead.
+
+## Bonus Task 
+
+```java
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class AtomicDemo {
+    static int normal = 0;
+    static AtomicInteger atomic = new AtomicInteger(0);
+
+    public static void main(String[] args) throws Exception {
+        Thread[] threads = new Thread[10];
+        for (int i = 0; i < threads.length; i++) {
+            threads[i] = new Thread(() -> {
+                for (int j = 0; j < 100000; j++) {
+                    normal++;
+                    atomic.incrementAndGet();
+                }
+            });
+        }
+        for (Thread t : threads) t.start();
+        for (Thread t : threads) t.join();
+
+        System.out.println("Normal: " + normal);
+        System.out.println("Atomic: " + atomic.get());
+    }
+}
+```
+
+Expected: `AtomicInteger` always prints 1000000, while `normal` is usually smaller because of race conditions.
+
+## 4. Correct but Poor Performance
+A program may be race-free but still scale poorly because:
+1. High lock contention forces threads to wait.
+2. Excessive synchronization increases overhead.
+3. False sharing and cache coherence traffic reduce CPU efficiency.
+4. Frequent blocking decreases parallelism.
+
+## 5. Why More Threads Can Hurt
+- **Context switching:** CPU spends time switching threads.
+- **Contention:** Threads compete for shared resources.
+- **Cache coherence:** Shared data invalidates CPU caches.
+- **Synchronization overhead:** Locks and coordination consume execution time.
+- Too many threads may exceed available CPU cores, reducing throughput.
+
+## 6. Why Deadlocks Often Appear Only in Production
+Deadlocks depend on thread scheduling, which is nondeterministic. Testing usually explores only a small subset of possible execution orders, whereas production workloads create many timing combinations.
+
+Two strategies to expose deadlocks:
+1. Perform stress tests with many threads and randomized execution timing.
+2. Insert artificial delays (sleep/yield) around lock acquisition to increase unfavorable interleavings.

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

@@ -1,18 +1,15 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
 
-    /*
+    // explicit ReentrantLock associated with each individual account
-     * Students may introduce additional fields
+    // to provide independent lock contention per account
-     * such as:
+    private final ReentrantLock lock = new ReentrantLock();
-     * - Lock / ReentrantLock
-     * - ReadWriteLock
-     * - Object monitor
-     * - etc.
-     */
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -24,55 +21,75 @@ public class BankAccount {
     }
 
     /*
-     * 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");
+        lock.lock();
+        try {
+            return this.balance;
+        } finally {
+            lock.unlock();
+        }
     }
 
     /*
-     * TODO:
+     * increase balance atomically
-     * Increase balance atomically.
-     *
-     * Requirements:
-     * - 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("Deposit amount must be positive.");
+        }
+        lock.lock();
+        try {
+            this.balance += amount;
+        } finally {
+            lock.unlock();
+        }
     }
 
     /*
-     * TODO:
+     * Decrease balance atomically
-     * 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");
+        if (amount <= 0) {
+            throw new IllegalArgumentException("Withdrawal amount must be positive.");
+        }
+        lock.lock();
+        try {
+            this.balance -= amount;
+        } finally {
+            lock.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 == null) {
+            throw new IllegalArgumentException("Target account cannot be null.");
+        }
+        if (this.accountId == target.getAccountId()) {
+            throw new IllegalArgumentException("Cannot transfer to the same account.");
+        }
+        if (amount <= 0) {
+            throw new IllegalArgumentException("Transfer amount must be positive.");
+        }
+
+        // Establish an absolute ordering to acquire locks
+        BankAccount firstLock = this.accountId < target.getAccountId() ? this : target;
+        BankAccount secondLock = this.accountId < target.getAccountId() ? target : this;
+
+        firstLock.lock.lock();
+        try {
+            secondLock.lock.lock();
+            try {
+                // Perform the atomic transfer operations
+                this.withdraw(amount);
+                target.deposit(amount);
+            } finally {
+                secondLock.lock.unlock();
+            }
+        } finally {
+            firstLock.lock.unlock();
+        }
     }
 }