add Answers.md

Answers.md

@@ -0,0 +1,44 @@
+# Answers
+
+## Question 1
+
+Atomic variables are thread-safe variables provided by Java.
+
+They allow multiple threads to update a value safely without using synchronized blocks or locks.
+
+Regular variables do not provide this protection and may cause race conditions.
+
+## Question 2
+
+Four classes from the `java.util.concurrent.atomic` package are:
+
+- AtomicInteger
+- AtomicLong
+- AtomicBoolean
+- AtomicReference
+
+`AtomicInteger` is used for thread-safe integer operations such as incrementing or decrementing a counter.
+
+## Question 3
+
+Atomic variables are useful for simple operations on a single variable.
+
+For more complex operations involving multiple variables or multiple steps, locks are usually a better choice.
+
+## Question 4
+
+Yes. A program can be free of race conditions but still have poor performance.
+
+Too much synchronization, lock contention, and thread management overhead can slow down the program.
+
+## Question 5
+
+Adding more threads does not always improve performance because threads compete for CPU time and shared resources.
+
+In some cases, too many threads can actually reduce performance.
+
+## Question 6
+
+Deadlocks are difficult to detect because they depend on thread scheduling.
+
+A program may work correctly many times and then suddenly deadlock under different execution conditions.

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 +18,58 @@ 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");
+        if (target == this) {
+            return;
+        }
+
+        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();
+        }
     }
 }