ANSWERS.md

@@ -0,0 +1,54 @@
+# Answers
+
+## Q1
+Atomic variable is a type of variable that combines all three process of: read, write and update into a single operation.  
+as an example when we write `i++` it may seem like a single operation  
+but the compiler needs to read `i`(read), add 1 to it (update) and change `i` to new value (write)  
+since all these operations are combined into one operation race conditions doesn't occur  
+unlike normal variables.
+
+## Q2
+`AtomicInteeger`, `AtomicBoolean`.`AtomicLong` & `AtomicReference`
+
+#### `AtomicInteeger` Use case
+used as counters that are shared across threads
+#### `AtomicBoolean` Use case
+used as flags that are used across threads (like `running` flag)
+#### `AtomicLong` Use case
+in general anything that needs long operations for multiple threads such as a bank account balance or an id counter
+#### `AtomicReference` Use case
+it's used whenever you need atomic operations on any object 
+
+## Q3
+locks are used to lock a full block of code, this prevents race conditions as a thread locks the section it's working on when it reaches it
+
+therefore we should use locks when we wan't to prevent race conditions for a block of code  
+and use atomic variables on more basic things
+## Q4
+this situation may occur because if there are too many threads trying to access a shared resource performance drops because workers sleep more than they do work
+
+#### Usage of locks
+locks make a block of code completely unaccessible for every thread except one
+this would cause those other threads to sleep meaning even if you have 100 threads only one of them can work at a time.
+
+#### Usage of atomic variables
+when we use atomic variables and atomic operations, at a curtain point many threads my try to change one variable   
+only one can successfully access the variable, meaning the rest would fail  
+this failure is just waste of cpu power.
+
+## Q5
+majority of mainstream cpus has something around 8 to 16 cores right now  
+this means one thing  
+logically we can't run 100 threads at the same time in parallel  
+this causes something known as context switching: cpu cores need to switch between threads 
+and this continuous switching would make the program slow
+
+
+cpu cores have their own distinct cache, when a core changes a synced variable all the cpu cores change their cache value of that variable
+when we have too many threads working on a synced variable, cpu cores constantly follow the changes in order to keep their cache synced 
+this would cause cpu cores to spend more time synchronizing the cache rather than processing the threads 
+this situation is known cache coherence synchronization overhead.
+
+## Q6
+Deadlocks are rare because you can't determine threads execution order, OS determines this by itself
+to increase likelihood of deadlocks to happen a developer can start too many threads and delay the threads

src/main/java/bonus/AtomicVsNormal.java

@@ -0,0 +1,42 @@
+package bonus;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class AtomicVsNormal {
+    private static int counter = 0;
+    private static AtomicInteger atomicCounter = new AtomicInteger(0);
+
+    public static void main(String[] args){
+        int incrementCount = 1000;
+        List<Thread> workers = new ArrayList<>();
+
+        for (int i = 0; i < 10; i++){
+            workers.add(new Thread(() ->{
+                for (int j = 0; j < incrementCount; j++) {
+                    counter++;
+                    atomicCounter.incrementAndGet();
+                }
+            }
+        ));
+        }
+
+        for (Thread worker : workers){
+            worker.start();
+        }
+
+        for (Thread worker : workers){
+            try {
+                worker.join();
+            } catch (InterruptedException e) {
+                throw new RuntimeException(e);
+            }
+        }
+
+        System.out.println("Expected: 10000");
+        System.out.println("Normal increment: " + counter);
+        System.out.println("Atomic increment: " + atomicCounter.get());
+
+    }
+}

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

@@ -1,10 +1,14 @@
 package dev.banking.model;
 
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
-
+    private final Lock lock = new ReentrantLock();
     /*
      * Students may introduce additional fields
      * such as:
@@ -32,7 +36,13 @@ 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.lock();
+        try {
+            return balance;
+        }
+        finally {
+            lock.unlock();
+        }
     }
 
     /*
@@ -43,7 +53,13 @@ public class BankAccount {
      * - 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();
+        }
     }
 
     /*
@@ -56,7 +72,13 @@ public class BankAccount {
      *   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();
+        }
     }
 
     /*
@@ -73,6 +95,19 @@ 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 = this.getAccountId() > target.getAccountId() ? target : this;
+        BankAccount second = this.getAccountId() > target.getAccountId() ? this : target;
+
+        first.lock.lock();
+        second.lock.lock();
+        try {
+            this.balance -= amount;
+            target.balance += amount;
+        }
+        finally {
+            second.lock.unlock();
+            first.lock.unlock();
+        }
+
     }
 }