Implement all TODOs

.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://mirror-maven.runflare.com/maven2" />
+    </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="openjdk-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 +1,48 @@
+# Java Concurrency: Atomic Variables & Locks
+
+### 1. Atomic Variables
+Atomic variables are tools that let us update a value safely when many threads are running. They ensure that an action (like adding 1) happens **all at once** without being interrupted.
+
+*   **Difference:** Normal variables can lose data if two threads change them at the exact same time. Atomic variables use a "try-and-retry" method (CAS) to stay accurate without stopping other threads.
+
+### 2. Common Atomic Classes
+*   `AtomicInteger`
+*   `AtomicLong`
+*   `AtomicBoolean`
+*   `AtomicReference`
+
+**Use Case:** We use `AtomicInteger` for a **hit counter** on a website. It keeps the total count correct even if thousands of users click a button at the same second.
+
+### 3. Locks vs. Atomic Variables
+
+| Feature | Atomic Variables | Locks (`synchronized`) |
+| :--- | :--- | :--- |
+| **Speed** | Very Fast | Slower |
+| **Thread Behavior** | Keep running (no waiting) | Stop and wait their turn |
+| **Best For** | Single numbers or flags | Groups of variables or big tasks |
+
+*   **We use Atomic Variables when:** We only need to change one value quickly and want to avoid the slowdown of locking.
+*   **We use Locks when:** We need to update **multiple related values** together or when the work takes a long time (like saving a file).
+
+### 4. Poor Performance without Race Conditions
+Even if our code is "correct" (no data is lost), it can still be slow if many threads fight for the same resources. This is called **high contention**.
+
+**Factors that limit scaling:**
+1.  **Lock Contention:** Many threads wait in a long line for one lock, so only one thread actually works at a time.
+2.  **Context Switching:** The CPU spends more time "swapping" threads in and out than actually running our code.
+3.  **Memory Bottlenecks:** Threads might be waiting for data to move between the main memory and the CPU.
+
+### 5. Why More Threads Can Slow Us Down
+Adding more threads has a "cost" that eventually outweighs the benefits.
+
+-   **Context Switching:** Every time the CPU moves from one thread to another, it has to save and load data. This wastes time.
+-   **Contention:** If we have 100 threads but only 1 resource, 99 threads are sitting idle, which wastes memory.
+-   **Cache Coherence:** When one thread changes data, the CPU must tell all other cores to update their private "caches." This constant communication slows down the whole system.
+-   **Synchronization Overhead:** The tools we use to stay safe (like locks or atomic signals) require extra CPU work to manage.
+
+### 6. Deadlocks in Production vs. Testing
+Deadlocks depend on **timing**. In testing, threads might always run in a "safe" order because the computer is less busy. In production, unexpected delays or high traffic can cause threads to grab locks in the "wrong" order, causing a freeze.
+
+**Strategies to find deadlocks during testing:**
+1.  **Thread Fuzzing:** We can add random, tiny delays (like `Thread.sleep()`) in our code during testing. This forces different timings and helps expose hidden deadlocks.
+2.  **Stress Testing:** We can run the program with a much higher number of threads and data than we expect. This makes it more likely that the rare "wrong timing" will happen.

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

@@ -1,18 +1,13 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.Lock;
+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 Lock lock = new ReentrantLock();
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +18,48 @@ 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 = this.accountId < target.accountId ? this : target;
+        BankAccount second = this.accountId < target.accountId ? target : this;
+
+        first.lock.lock();
+        try {
+            second.lock.lock();
+            try {
+                this.withdraw(amount);
+                target.deposit(amount);
+            } finally {
+                second.lock.unlock();
+            }
+        } finally {
+            first.lock.unlock();
+        }
     }
 }

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

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Represents a deposit operation.
- */
 public final class DepositTransaction extends Transaction {
 
     private final int accountId;

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

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Base class for all transaction types.
- */
 public abstract class Transaction {
 
     private final int amount;

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

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Represents a transfer operation between two accounts.
- */
 public final class TransferTransaction
         extends Transaction {
 

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

@@ -1,8 +1,5 @@
 package dev.banking.model;
 
-/**
- * Represents a withdrawal operation.
- */
 public final class WithdrawTransaction extends Transaction {
 
     private final int accountId;

src/main/java/dev/banking/monitor/LiveMonitor.java

@@ -1,7 +1,6 @@
 package dev.banking.monitor;
 
 import dev.banking.model.BankAccount;
-
 import java.util.Collection;
 
 public class LiveMonitor {
@@ -9,15 +8,12 @@ public class LiveMonitor {
     public void update(
             Collection<BankAccount> accounts
     ) {
-
         for (BankAccount account : accounts) {
-
             System.out.printf(
                     "Account %d -> %d%n",
                     account.getAccountId(),
                     account.getBalance()
             );
-
         }
     }
 }

src/main/java/dev/banking/processor/TransactionProcessor.java

@@ -1,7 +1,6 @@
 package dev.banking.processor;
 
 import dev.banking.model.*;
-
 import java.util.Map;
 
 public class TransactionProcessor {
@@ -15,33 +14,19 @@ public class TransactionProcessor {
     }
 
     public void process(Transaction tx) {
-
         if (tx instanceof DepositTransaction deposit) {
-
             BankAccount account = accounts.get(deposit.getAccountId());
-
             account.deposit(deposit.getAmount());
         }
-
         else if (tx instanceof WithdrawTransaction withdraw) {
-
             BankAccount account = accounts.get(withdraw.getAccountId());
-
             account.withdraw(withdraw.getAmount());
         }
-
         else if (tx instanceof TransferTransaction transfer) {
-
             BankAccount source = accounts.get(transfer.getSourceAccountId());
-
             BankAccount target = accounts.get(transfer.getTargetAccountId());
-
-            source.transfer(
-                    target,
-                    transfer.getAmount()
-            );
+            source.transfer(target, transfer.getAmount());
         }
-
         else {
             throw new IllegalArgumentException(
                     "Unknown transaction type: " + tx.getClass()

src/main/java/dev/banking/service/BankingSystem.java

@@ -2,22 +2,9 @@ package dev.banking.service;
 
 import dev.banking.model.*;
 import dev.banking.processor.TransactionProcessor;
-
 import java.util.List;
 import java.util.concurrent.ExecutorService;
 
-/**
- * Dispatches a list of transactions to a shared ExecutorService
- * for concurrent (asynchronous) processing.
- *
- * Each transaction is submitted as an independent task and may
- * be executed in parallel depending on thread availability.
- *
- * No ordering guarantees are provided between transactions.
- *
- * Lifecycle management of the ExecutorService (creation,
- * shutdown, termination) is handled outside this class.
- */
 public class BankingSystem {
 
     private final ExecutorService executor;
@@ -34,13 +21,10 @@ public class BankingSystem {
     public void processTransactions(
             List<Transaction> transactions
     ) {
-
         for (Transaction tx : transactions) {
-
             executor.submit(() -> {
                 processor.process(tx);
             });
-
         }
     }
 }