.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>

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

@@ -1,18 +1,34 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * Thread-safe BankAccount implementation.
+ *
+ * Design choices:
+ * - Each account has its OWN ReentrantLock (per-account locking).
+ *   This means two threads working on DIFFERENT accounts never block each other.
+ *
+ * - deposit() and withdraw() simply acquire this account's lock,
+ *   modify the balance, then release the lock. Always inside try/finally
+ *   so the lock is GUARANTEED to be released even if an exception occurs.
+ *
+ * - getBalance() also acquires the lock so it never reads a half-written value.
+ *
+ * - transfer() is the most complex:
+ *   It must lock TWO accounts at the same time.
+ *   To prevent deadlock we ALWAYS lock the account with the LOWER accountId first.
+ *   Example: Thread 1 does A(id=1) -> B(id=2)  →  locks id=1 first, then id=2
+ *            Thread 2 does B(id=2) -> A(id=1)  →  ALSO locks id=1 first, then id=2
+ *   Because both threads always lock in the same order, they can never deadlock.
+ */
 public class BankAccount {
 
     private final int accountId;
     private long balance;
 
-    /*
+    // One lock per account instance. Never shared with other accounts.
-     * Students may introduce additional fields
+    private final ReentrantLock lock = new ReentrantLock();
-     * such as:
-     * - Lock / ReentrantLock
-     * - ReadWriteLock
-     * - Object monitor
-     * - etc.
-     */
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +39,93 @@ public class BankAccount {
         return accountId;
     }
 
-    /*
+    /**
-     * TODO:
+     * Returns the current balance in a thread-safe way.
-     * Return the current balance in a thread-safe way.
      *
-     * Requirements:
+     * We acquire the lock before reading so we never observe
-     * - Must be safe under concurrent reads/writes
+     * a partially-written value from another thread's deposit/withdraw.
-     * - 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:
+     * Adds the given amount to the balance atomically.
-     * Increase balance atomically.
      *
-     * Requirements:
+     * The lock ensures that if Thread A and Thread B both deposit
-     * - Must not lose updates under concurrency
+     * at the same time, one waits for the other. No update is ever lost.
      */
     public void deposit(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+        lock.lock();
+        try {
+            balance += amount;
+        } finally {
+            lock.unlock();
+        }
     }
 
-    /*
+    /**
-     * TODO:
+     * Subtracts the given amount from the balance atomically.
-     * Decrease balance atomically.
      *
-     * Requirements:
+     * Same guarantee as deposit — no lost updates under concurrency.
-     * - Must not cause race conditions
+     * Negative balances are permitted as per the assignment spec.
-     * - 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:
+     * Transfers the given amount from THIS account to the TARGET account atomically.
-     * Transfer money between two accounts atomically.
      *
-     * IMPORTANT REQUIREMENTS:
+     * --- HOW DEADLOCK IS PREVENTED ---
-     * - Must be atomic (no partial transfer)
+     * A deadlock would happen if:
-     * - Must be deadlock-free
+     *   Thread 1 locks Account A, then waits for Account B
-     * - Must protect both source and target accounts
+     *   Thread 2 locks Account B, then waits for Account A
+     *   → both wait forever.
      *
-     * HINT:
+     * The fix: ALWAYS lock the account with the lower accountId first,
-     * - Consider global lock ordering using accountId
+     * regardless of which direction the money is flowing.
-     * - Or tryLock with retry strategy
+     *
+     * Thread 1 (A→B): locks id=1 first, then id=2  ✅
+     * Thread 2 (B→A): locks id=1 first, then id=2  ✅  (same order!)
+     * → Thread 2 simply waits until Thread 1 finishes. No deadlock.
+     *
+     * --- HOW ATOMICITY IS GUARANTEED ---
+     * Both locks are held before any balance changes.
+     * No other thread can touch either account during the transfer.
+     * If an exception occurs, finally blocks release both locks safely.
+     * Money is never lost or created.
      */
     public void transfer(BankAccount target, long amount) {
-        throw new UnsupportedOperationException("TODO: implement atomic deadlock-free transfer");
+
+        // Determine which account to lock first (lower id = first)
+        BankAccount first  = this.accountId < target.accountId ? this : target;
+        BankAccount second = this.accountId < target.accountId ? target : this;
+
+        first.lock.lock();
+        try {
+            second.lock.lock();
+            try {
+                // Both accounts are now locked exclusively.
+                // No other thread can deposit, withdraw, or transfer
+                // on either account until we release both locks.
+                this.balance   -= amount;
+                target.balance += amount;
+            } finally {
+                second.lock.unlock();
+            }
+        } finally {
+            first.lock.unlock();
+        }
     }
 }

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

@@ -1,36 +1,11 @@
 package dev.banking.model;
 
-/**
- * Represents a deposit operation.
- */
 public final class DepositTransaction extends Transaction {
+    private final long accountId;
 
-    private final int accountId;
+    public DepositTransaction(long accountId, long amount) {
-
-    public DepositTransaction(
-            int accountId,
-            int amount
-    ) {
         super(amount);
-
-        if (accountId < 0) {
-            throw new IllegalArgumentException(
-                    "Invalid account id."
-            );
-        }
-
         this.accountId = accountId;
     }
-
+    public long getAccountId() { return accountId; }
-    public int getAccountId() {
-        return accountId;
-    }
-
-    @Override
-    public String toString() {
-        return "DepositTransaction{" +
-                "accountId=" + accountId +
-                ", amount=" + getAmount() +
-                '}';
-    }
 }

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

@@ -1,24 +1,11 @@
 package dev.banking.model;
 
-/**
- * Base class for all transaction types.
- */
 public abstract class Transaction {
+    private final long amount;
 
-    private final int amount;
+    protected Transaction(long amount) {
-
+        if (amount <= 0) throw new IllegalArgumentException("Transaction amount must be positive.");
-    protected Transaction(int amount) {
-
-        if (amount <= 0) {
-            throw new IllegalArgumentException(
-                    "Transaction amount must be positive."
-            );
-        }
-
         this.amount = amount;
     }
-
+    public long getAmount() { return amount; }
-    public int getAmount() {
-        return amount;
-    }
 }

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

@@ -1,57 +1,15 @@
 package dev.banking.model;
 
-/**
+public final class TransferTransaction extends Transaction {
- * Represents a transfer operation between two accounts.
+    private final long sourceAccountId;
- */
+    private final long targetAccountId;
-public final class TransferTransaction
-        extends Transaction {
 
-    private final int sourceAccountId;
+    public TransferTransaction(long source, long target, long amount) {
-    private final int targetAccountId;
-
-    public TransferTransaction(
-            int sourceAccountId,
-            int targetAccountId,
-            int amount
-    ) {
         super(amount);
-
+        if (source == target) throw new IllegalArgumentException("Accounts must be different.");
-        if (sourceAccountId < 0) {
+        this.sourceAccountId = source;
-            throw new IllegalArgumentException(
+        this.targetAccountId = target;
-                    "Invalid source account id."
-            );
-        }
-
-        if (targetAccountId < 0) {
-            throw new IllegalArgumentException(
-                    "Invalid target account id."
-            );
-        }
-
-        if (sourceAccountId == targetAccountId) {
-            throw new IllegalArgumentException(
-                    "Source and target accounts must be different."
-            );
-        }
-
-        this.sourceAccountId = sourceAccountId;
-        this.targetAccountId = targetAccountId;
-    }
-
-    public int getSourceAccountId() {
-        return sourceAccountId;
-    }
-
-    public int getTargetAccountId() {
-        return targetAccountId;
-    }
-
-    @Override
-    public String toString() {
-        return "TransferTransaction{" +
-                "sourceAccountId=" + sourceAccountId +
-                ", targetAccountId=" + targetAccountId +
-                ", amount=" + getAmount() +
-                '}';
     }
+    public long getSourceAccountId() { return sourceAccountId; }
+    public long getTargetAccountId() { return targetAccountId; }
 }

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

@@ -1,36 +1,11 @@
 package dev.banking.model;
 
-/**
- * Represents a withdrawal operation.
- */
 public final class WithdrawTransaction extends Transaction {
+    private final long accountId;
 
-    private final int accountId;
+    public WithdrawTransaction(long accountId, long amount) {
-
-    public WithdrawTransaction(
-            int accountId,
-            int amount
-    ) {
         super(amount);
-
-        if (accountId < 0) {
-            throw new IllegalArgumentException(
-                    "Invalid account id."
-            );
-        }
-
         this.accountId = accountId;
     }
-
+    public long getAccountId() { return accountId; }
-    public int getAccountId() {
-        return accountId;
-    }
-
-    @Override
-    public String toString() {
-        return "WithdrawTransaction{" +
-                "accountId=" + accountId +
-                ", amount=" + getAmount() +
-                '}';
-    }
 }