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..f4fe6ed
--- /dev/null
+++ b/.idea/jarRepositories.xml
@@ -0,0 +1,20 @@
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
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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 @@
+
+
+
+
+
+
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diff --git a/Answers.md b/Answers.md
new file mode 100644
index 0000000..1546cbb
--- /dev/null
+++ b/Answers.md
@@ -0,0 +1,47 @@
+## Question 1
+An atomic variable in Computer Science refers to a basic data or input variable that is used to build performance variables. These variables are not summaries or ratios, but rather fundamental building blocks in operational systems.
+
+Atomic variables allow multiple threads to safely read and update a shared value without using explicit locks, guaranteeing that operations like increment-and-update happen as a single, uninterruptible step. Ordinary variables don't provide this guarantee—if multiple threads modify them concurrently, updates can be lost due to race conditions.
+
+## Question 2
+`AtomicInteger`
+
+`AtomicLong`
+
+`AtomicBoolean`
+
+`AtomicReference` for any type of object.
+
+## Question 3
+| | Locks (`synchronized`/`ReentrantLock`) | Atomic Variables |
+|---|---|---|
+| Mechanism | Blocking (mutual exclusion) | Lock-free |
+| Scope | Can protect multiple statements/variables | Single variable only |
+| Performance | Slower under contention | Generally faster |
+| Deadlock risk | Possible | None |
+| Best for | Complex critical sections | Simple counters, flags, single values |
+
+## Question 4
+A program can be completely free of race conditions yet still perform poorly, because the very mechanisms used to guarantee correctness — such as locks or CAS — introduce overhead. When many threads compete for the same shared resource, they end up **blocking or repeatedly retrying**, which sharply reduces throughput even though correctness is fully preserved.
+
+Some concurrency-related factors that may limit scalability even when correctness iss guaranteed:
+
+1. Lock contention
+2. Context switching overhead
+3. Cache coherence traffic
+
+## Question 5
+Despite the three factors given in the previous question there is a vital factor which is **limited CPU cores**.
+
+once threads exceed available cores, they compete for the same processing units, adding scheduling overhead instead of true parallelism.
+
+Context-switching overhead – the OS spends more time switching between threads than executing actual work.
+
+Cache coherence traffic – shared/false-shared memory locations cause costly cross-core cache invalidation.
+
+## Question 6
+A precise timing where two or more threads each acquire one lock and then attempt to acquire the other's lock simultaneously.
+
+1. Stress testing with high concurrency – run many more threads than in normal testing.
+2. Deliberate interleaving control / thread scheduling tools – use tools or techniques that artificially manipulate thread timing to force specific interleavings, such as:
+ Inserting `Thread.sleep()` or `yield()` calls strategically between lock acquisitions.
\ 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..a179dc3 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.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
public class BankAccount {
private final int accountId;
private long balance;
+ private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
+
/*
* 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");
+ lock.readLock().lock();
+ try {
+ return balance;
+ } finally {
+ lock.readLock().unlock();
+ }
}
/*
@@ -43,7 +54,15 @@ public class BankAccount {
* - Must not lose updates under concurrency
*/
public void deposit(long amount) {
- throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+ if(amount < 0)
+ throw new IllegalArgumentException("Amount can't be negative!");
+
+ lock.writeLock().lock();
+ try {
+ balance += amount;
+ } finally {
+ lock.writeLock().unlock();
+ }
}
/*
@@ -56,7 +75,15 @@ public class BankAccount {
* to extend the system (optional)
*/
public void withdraw(long amount) {
- throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
+ if(amount < 0)
+ throw new IllegalArgumentException("Amount can't be negative!");
+
+ lock.writeLock().lock();
+ try {
+ balance -= amount;
+ } finally {
+ lock.writeLock().unlock();
+ }
}
/*
@@ -73,6 +100,28 @@ public class BankAccount {
* - Or tryLock with retry strategy
*/
public void transfer(BankAccount target, long amount) {
- throw new UnsupportedOperationException("TODO: implement atomic deadlock-free transfer");
+ if (amount < 0) {
+ throw new IllegalArgumentException("Transfer amount cannot be negative");
+ }
+ if (this == target) {
+ throw new IllegalArgumentException("Cannot transfer to the same account");
+ }
+
+ // we order them by their id so that we always lock the first one to prevent deadLocks taking place.
+ BankAccount first = this.accountId < target.accountId ? this : target;
+ BankAccount second = this.accountId < target.accountId ? target : first;
+
+ first.lock.writeLock().lock();
+ try {
+ second.lock.writeLock().lock();
+ try {
+ this.balance -= amount;
+ target.balance += amount;
+ } finally {
+ second.lock.writeLock().unlock();
+ }
+ } finally {
+ first.lock.writeLock().unlock();
+ }
}
}
\ No newline at end of file