Update Answer.md

.idea/.gitignore

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+# 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

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+<?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

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+<?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

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+<?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://maven.devneeds.ir/" />
+    </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

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+<?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

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

Answer.md

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+## Question 1
+
+
+1 - What are atomic variables?
+
+Atomic variables are variables that provide thread-safe operations without using explicit locks. Operations such as incrementing or updating a value are performed atomically, meaning they cannot be interrupted by other threads.
+
+Ordinary variables do not provide this guarantee. For example, the expression `counter++` consists of multiple steps, and different threads may interfere with each other, causing race conditions.
+
+Atomic variables are mainly used to safely share data between multiple threads.
+
+## Question 2
+
+2 - Name at least four classes from the java.util.concurrent.atomic package that provide atomic operations for different data types.
+
+Some classes from the `java.util.concurrent.atomic` package are:
+
+- AtomicInteger
+- AtomicLong
+- AtomicBoolean
+- AtomicReference
+
+A common use case for `AtomicInteger` is a shared counter. Multiple threads can safely increment the counter without using locks.
+
+## Question 3
+
+3 - Compare locks with atomic variables.
+
+Atomic variables are usually faster and simpler for operations on a single variable. They have lower overhead and do not require explicit locking.
+
+Locks are better when multiple operations or multiple shared variables must be protected together. They provide more flexibility but usually have higher overhead.
+
+Use atomic variables for simple updates such as counters. Use locks for complex critical sections that involve several operations.
+
+## Question 4
+
+4 - A program is completely free of race conditions but still performs poorly under high contention.
+
+A program can be free of race conditions and still perform poorly because correctness does not guarantee scalability.
+
+Some factors that limit performance are:
+
+1. Lock contention, where many threads wait for the same lock.
+2. Thread blocking, which reduces parallel execution.
+3. Synchronization overhead, which adds extra work for coordinating threads.
+
+As a result, the program remains correct but may not scale well under heavy load.
+
+## Question 5
+
+5 - Many concurrent systems experience performance degradation as the number of threads increases.
+
+Adding more threads does not always improve performance.
+
+- Context switching takes CPU time when the operating system switches between threads.
+- Contention happens when many threads compete for the same resources.
+- Cache coherence creates extra work for processors to keep shared data consistent.
+- Synchronization overhead increases because locks and coordination mechanisms require additional processing.
+
+Because of these costs, too many threads can actually reduce performance.
+
+## Question 6
+
+6 - Deadlocks often only appear in production, not during testing.
+
+Deadlocks often depend on specific thread schedules. During testing, the required scheduling order may never occur. In production, higher load and different timing make deadlocks more likely.
+
+Two ways to expose deadlocks during testing are:
+
+1. Stress testing with many threads and repeated executions.
+2. Adding random delays to create different thread interleavings.
+
+These techniques increase the chance of reproducing deadlock situations before deployment.

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

@@ -1,18 +1,12 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
-
-    /*
-     * Students may introduce additional fields
-     * such as:
-     * - Lock / ReentrantLock
-     * - ReadWriteLock
-     * - Object monitor
-     * - etc.
-     */
+    private final ReentrantLock lock = new ReentrantLock();
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +17,61 @@ 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");
+        lock.lock();
+
+        try {
+            return balance;
+        } finally {
+            lock.unlock();
+        }
     }
 
-    /*
-     * TODO:
-     * Increase balance atomically.
-     *
-     * Requirements:
-     * - 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();
+        }
     }
 
-    /*
-     * 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");
+        lock.lock();
+
+        try {
+            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");
+        BankAccount first;
+        BankAccount second;
+
+        if (this.accountId < target.accountId) {
+            first = this;
+            second = target;
+        } else {
+            first = target;
+            second = this;
+        }
+
+        first.lock.lock();
+        second.lock.lock();
+
+        try {
+            this.balance -= amount;
+            target.balance += amount;
+        } finally {
+            second.lock.unlock();
+            first.lock.unlock();
+        }
     }
 }