diff --git a/.idea/.gitignore b/.idea/.gitignore
new file mode 100644
index 0000000..ab1f416
--- /dev/null
+++ b/.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/
diff --git a/.idea/compiler.xml b/.idea/compiler.xml
new file mode 100644
index 0000000..812c3f9
--- /dev/null
+++ b/.idea/compiler.xml
@@ -0,0 +1,13 @@
+
+
+
+
+
+
+
+
+
+
+
+
+
\ No newline at end of file
diff --git a/.idea/encodings.xml b/.idea/encodings.xml
new file mode 100644
index 0000000..aa00ffa
--- /dev/null
+++ b/.idea/encodings.xml
@@ -0,0 +1,7 @@
+
+
+
+
+
+
+
\ No newline at end of file
diff --git a/.idea/jarRepositories.xml b/.idea/jarRepositories.xml
new file mode 100644
index 0000000..4158879
--- /dev/null
+++ b/.idea/jarRepositories.xml
@@ -0,0 +1,20 @@
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
\ No newline at end of file
diff --git a/.idea/misc.xml b/.idea/misc.xml
new file mode 100644
index 0000000..eba6e1f
--- /dev/null
+++ b/.idea/misc.xml
@@ -0,0 +1,12 @@
+
+
+
+
+
+
+
+
\ No newline at end of file
diff --git a/.idea/vcs.xml b/.idea/vcs.xml
new file mode 100644
index 0000000..35eb1dd
--- /dev/null
+++ b/.idea/vcs.xml
@@ -0,0 +1,6 @@
+
+
+
+
+
+
\ No newline at end of file
diff --git a/Answers.md b/Answers.md
new file mode 100644
index 0000000..e84b64b
--- /dev/null
+++ b/Answers.md
@@ -0,0 +1,133 @@
+
+#### ****1 -** What are atomic variables?**
+
+#### Explain their purpose and how they differ from ordinary (non-atomic) variables.
+
+
+#### **Answer:**
+Atomic variables are variables on which reading and writing operations are atomic.
+It means that when a Thread is making changes to a variable, no other Thread can interfere with it.
+They Use hardware-level instructions to ensure that only one thread can modify the value at a time without locks.
+But for ordinary variables, operations like balance += amount are not atomic for example if another thread interrupts between these steps, it causes Race Conditions and data corruption.
+---
+
+#### **2 -** Name at least four classes from the `java.util.concurrent.atomic` package that provide atomic operations for different data types.
+
+#### For one of them, briefly describe a typical use case.
+
+
+#### **Answer:**
+
+- AtomicInteger
+- AtomicLong
+- AtomicBoolean
+- AtomicReference
+
+Example :
+```
+private AtomicLong balance = new AtomicLong(0);
+
+public void deposit(long amount)
+{
+ balance.addAndGet(amount);
+}
+```
+Atomically adds amount to current balance.
+It is fast & it does not need `synchronized`
+.
+But it is only good for simple operations on one variable.
+
+
+---
+#### **3 -** Compare locks with atomic variables.
+
+#### In which scenarios is using a lock a better choice than an atomic variable, and vice versa?
+
+
+#### **Answer:**
+
+
+| Feature | Lock (ReentrantLock / synchronized) | Atomic Variables |
+|:-------------------------|:----------------------------------------------------------------------------:|:------------------------------------------------------------------:|
+| Complexity of operations | Great for complex, multi-step operations | Great for simple operations on a variable |
+| Scope of Locking | You can lock blocks of code | It only locks the variable itself |
+| Conditional Logic | Supports conditional checks;You can check if (balance > 0) and then withdraw | Difficult; requires loops with compareAndSet which can be complex. |
+| Resource Management | Requires manual unlocking in finally | Automatic |
+| Deadlock | Dangerous | None |
+
+**When to Use Which?**
+
+**Use Locks When:**
+
+- You need to perform compound operations involving multiple state variables atomically (e.g., in our banking project: balance -= amount AND target.balance += amount must happen together).
+- You need to wait for a condition (using Condition.await()/signal()).
+- The critical section contains complex logic that cannot be reduced to a single atomic instruction.
+
+**Use Atomic Variables When:**
+
+- You are modifying a single variable (e.g., a global counter, a status flag).
+- Performance is critical and contention is expected to be low.
+- You want to avoid the complexity of managing lock lifecycles (acquire/release).
+---
+
+#### **4 -** A program is completely free of race conditions but still performs poorly under high contention.
+
+#### Explain how this situation can occur.
+
+#### Discuss at least three concurrency-related factors that may limit scalability even when correctness is guaranteed.
+
+
+#### **Answer:**
+
+
+Even without race conditions, high contention causes bottlenecks due to:
+
+- **Thread Contention:** Threads block waiting for locks, turning parallel execution into sequential processing. CPU time is wasted managing queues rather than computing.
+- **Context Switching Overhead:** Frequent blocking/unblocking forces the OS to save/restore thread states. High switch rates consume CPU cycles needed for actual work.
+- **False Sharing:** Unrelated variables in the same cache line cause unnecessary cache invalidations across cores, forcing slow main memory accesses despite logical independence.
+
+
+---
+####
+#### **5 -** Many concurrent systems experience performance degradation as the number of threads increases.
+
+#### Explain why adding more threads does not always improve performance.
+
+#### Your answer should discuss concepts such as:
+
+#### Context switching
+#### Contention
+#### coherence
+#### Synchronization overhead
+
+
+#### **Answer:**
+
+Performance degrades with excessive threads due to:
+
+- **Synchronization Overhead:** Lock acquisition/release costs exceed computation time for small tasks.
+- **Contention:** Increased probability of lock conflicts leads to long wait times, shifting from parallel to serialized execution.
+- **Context Switching:** Beyond core limits, CPUs spend more time switching contexts than executing instructions (“thread explosion”).
+- **Cache Coherence:** Frequent writes to shared data trigger inter-core synchronization (MESI protocol), saturating the communication bus and increasing latency
+---
+
+#### **6 -** Deadlocks often only appear in production, not during testing.
+
+#### Explain why this might happen from a thread-scheduling perspective.
+
+#### Describe two strategies a developer can use to increase the likelihood of exposing deadlocks during testing.
+
+
+#### **Answer:**
+
+**Why they hide in testing:**
+
+- Timing: Tests are fast and deterministic; production has I/O/network delays that alter thread interleaving.
+- Concurrency: Tests use few threads; deadlocks often require high concurrency to trigger naturally.
+
+
+**Strategies to Expose Deadlocks:**
+
+- Stress Testing: Spawn hundreds of threads performing random operations to exponentially increase the chance of hitting cyclic dependency windows.
+- Inject Delays: Add Thread.sleep() or use tools like JMH to disrupt natural flow, increasing the likelihood that a thread holds one lock while waiting for another.
+---
\ No newline at end of file
diff --git a/src/main/java/dev/banking/model/BankAccount.java b/src/main/java/dev/banking/model/BankAccount.java
index 745ede2..9cdf507 100644
--- a/src/main/java/dev/banking/model/BankAccount.java
+++ b/src/main/java/dev/banking/model/BankAccount.java
@@ -1,10 +1,16 @@
package dev.banking.model;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
public class BankAccount {
private final int accountId;
private long balance;
+ private final ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
+ private final ReentrantLock transferLock = new ReentrantLock();
+
/*
* 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");
+ rwLock.readLock().lock();
+ try {
+ return balance;
+ } finally {
+ rwLock.readLock().unlock();
+ }
}
/*
@@ -43,7 +54,12 @@ public class BankAccount {
* - Must not lose updates under concurrency
*/
public void deposit(long amount) {
- throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+ rwLock.writeLock().lock();
+ try {
+ balance += amount;
+ } finally {
+ rwLock.writeLock().unlock();
+ }
}
/*
@@ -56,7 +72,12 @@ public class BankAccount {
* to extend the system (optional)
*/
public void withdraw(long amount) {
- throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
+ rwLock.writeLock().lock();
+ try {
+ balance -= amount;
+ } finally {
+ rwLock.writeLock().unlock();
+ }
}
/*
@@ -73,6 +94,29 @@ public class BankAccount {
* - 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.getAccountId() < target.getAccountId()) {
+ first = this;
+ second = target;
+ } else {
+ first = target;
+ second = this;
+ }
+
+ first.transferLock.lock();
+
+ try {
+ second.transferLock.lock();
+ try {
+ first.balance -= amount;
+ target.balance += amount;
+ } finally {
+ second.transferLock.unlock();
+ }
+ } finally {
+ first.transferLock.unlock();
+ }
}
}
\ No newline at end of file