.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="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>
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Central Repository" />
+      <option name="url" value="https://maven.myket.ir/" />
+    </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>

Answers.md

@@ -0,0 +1,47 @@
+## Question 1
+An atomic variable in Computer Science refers to a basic data or input variable that is used to build performance variables. These variables are not summaries or ratios, but rather fundamental building blocks in operational systems.
+
+Atomic variables allow multiple threads to safely read and update a shared value without using explicit locks, guaranteeing that operations like increment-and-update happen as a single, uninterruptible step. Ordinary variables don't provide this guarantee—if multiple threads modify them concurrently, updates can be lost due to race conditions.
+
+## Question 2
+`AtomicInteger`
+
+`AtomicLong`
+
+`AtomicBoolean`
+
+`AtomicReference<T>` for any type of object.
+
+## Question 3
+| | Locks (`synchronized`/`ReentrantLock`) | Atomic Variables |
+|---|---|---|
+| Mechanism | Blocking (mutual exclusion) | Lock-free |
+| Scope | Can protect multiple statements/variables | Single variable only |
+| Performance | Slower under contention | Generally faster |
+| Deadlock risk | Possible | None |
+| Best for | Complex critical sections | Simple counters, flags, single values |
+
+## Question 4
+A program can be completely free of race conditions yet still perform poorly, because the very mechanisms used to guarantee correctness — such as locks or CAS — introduce overhead. When many threads compete for the same shared resource, they end up **blocking or repeatedly retrying**, which sharply reduces throughput even though correctness is fully preserved.
+
+Some concurrency-related factors that may limit scalability even when correctness iss guaranteed:
+
+1. Lock contention
+2. Context switching overhead
+3. Cache coherence traffic
+
+## Question 5
+Despite the three factors given in the previous question there is a vital factor which is **limited CPU cores**.
+
+once threads exceed available cores, they compete for the same processing units, adding scheduling overhead instead of true parallelism.
+
+Context-switching overhead – the OS spends more time switching between threads than executing actual work.
+
+Cache coherence traffic – shared/false-shared memory locations cause costly cross-core cache invalidation.
+
+## Question 6
+A precise timing where two or more threads each acquire one lock and then attempt to acquire the other's lock simultaneously.
+
+1. Stress testing with high concurrency – run many more threads than in normal testing.
+2. Deliberate interleaving control / thread scheduling tools – use tools or techniques that artificially manipulate thread timing to force specific interleavings, such as:
+   Inserting `Thread.sleep()` or `yield()` calls strategically between lock acquisitions.

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();
+        }
     }
 }