Solutions.md

@@ -0,0 +1,148 @@
+1 - What are atomic variables?
+
+Atomic variables are variables that support thread-safe operations without using explicit locks.
+
+They ensure that operations such as incrementing, decrementing, or updating a value are performed as a single indivisible action.
+
+Difference from ordinary variables:
+
+Ordinary variable: Multiple threads can modify it simultaneously, causing race conditions.
+Atomic variable: Operations are performed atomically, preventing race conditions for single-variable updates.
+
+2 - Name at least four classes from the java.util.concurrent.atomic package that provide atomic operations for different data types.
+
+AtomicInteger  
+AtomicLong  
+AtomicBoolean  
+AtomicReference<T>
+
+Typical use case for AtomicInteger:     
+AtomicInteger is commonly used as a thread-safe counter when multiple threads need to increment a shared value concurrently.
+
+3 - Compare locks with atomic variables.
+
+atomic variables:
+
+Advantages:
+
+Faster than locks for simple operations.  
+Non-blocking.  
+Lower overhead.  
+
+Best for:
+
+Counters.   
+Flags.  
+Single-variable updates.    
+Locks
+
+locks:
+
+Advantages:     
+Can protect multiple variables together.
+Support complex critical sections.
+
+Best for:
+
+Operations involving multiple shared objects.
+Complex business logic requiring mutual exclusion.
+
+Summary:
+
+Use atomic variables for simple thread-safe updates.
+Use locks when multiple operations must be performed as one atomic unit.
+
+Bonus Task
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class AtomicDemo
+{
+    static int normalCounter = 0;
+    static AtomicInteger atomicCounter = new AtomicInteger(0);
+
+    public static void main(String[] args) throws InterruptedException 
+    {
+
+        int numThreads = 10;
+        int incrementsPerThread = 100000;
+
+        Thread[] threads = new Thread[numThreads];
+
+        for (int i = 0; i < numThreads; i++) {
+            threads[i] = new Thread(() -> {
+                for (int j = 0; j < incrementsPerThread; j++) {
+                    normalCounter++;
+                    atomicCounter.incrementAndGet();
+                }
+            });
+
+            threads[i].start();
+        }
+
+        for (Thread thread : threads) {
+            thread.join();
+        }
+
+        System.out.println("Normal Counter: " + normalCounter);
+        System.out.println("Atomic Counter: " + atomicCounter.get());
+    }
+}
+
+Expected Result:
+
+Normal Counter: 823451   (varies)
+Atomic Counter: 1000000
+
+The normal counter may be incorrect because multiple threads overwrite each other's updates. The atomic counter always produces the correct result.
+
+4 - A program is completely free of race conditions but still performs poorly under high contention.
+Explain how this situation can occur.
+
+A program can be completely correct yet still perform poorly because threads spend too much time waiting for shared resources.
+
+Three factors that limit scalability:
+
+Lock Contention     
+Many threads compete for the same lock and must wait.
+
+Synchronization Overhead    
+Acquiring and releasing locks consumes CPU time.
+
+Thread Blocking     
+Threads frequently pause while waiting for locks or resources, reducing parallelism.
+
+Correctness is guaranteed, but throughput decreases as contention increases.
+
+5 - Many concurrent systems experience performance degradation as the number of threads increases.
+Explain why adding more threads does not always improve performance.
+
+The CPU must save and restore thread state when switching between threads. Too many threads increase this overhead.
+
+Contention      
+More threads compete for the same locks and resources, causing waiting.
+
+Cache Coherence     
+When multiple CPUs modify shared data, cache contents must be synchronized, creating extra communication overhead.
+
+Synchronization Overhead    
+Locks, atomic operations, and coordination mechanisms consume time and reduce the benefits of parallel execution.
+
+Conclusion: After a certain point, adding more threads increases overhead more than useful work, causing performance to stagnate or even decrease.
+
+6 - Deadlocks often only appear in production, not during testing.Explain why this might happen from a thread-scheduling perspective.
+
+Deadlocks depend on the exact timing and scheduling of threads.
+During testing, threads may execute in a favorable order and never enter the deadlock state. In production, different workloads, hardware, and timing conditions can cause the problematic scheduling sequence to occur.
+
+Two ways to expose deadlocks during testing
+
+1. Increase concurrency     
+Run with many threads.execute tests repeatedly under heavy load.
+
+
+2. Introduce timing variations   
+Add random delays (Thread.sleep()).
+Use stress-testing tools to force different thread interleavings.
+
+These techniques increase the chance that threads acquire locks in a problematic order, revealing deadlocks before deployment.

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

@@ -1,5 +1,8 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
 public class BankAccount {
 
     private final int accountId;
@@ -13,6 +16,7 @@ public class BankAccount {
      * - Object monitor
      * - etc.
      */
+    private final ReadWriteLock lock = new ReentrantReadWriteLock();
 
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
@@ -23,56 +27,96 @@ 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
+    /**
+     * Thread-safe balance read.
      */
     public long getBalance() {
-        throw new UnsupportedOperationException("TODO: implement thread-safe balance read");
+
+        lock.readLock().lock();
+
+        try
+        {
+            return balance;
+        }
+        finally
+        {
+            lock.readLock().unlock();
+        }
     }
 
-    /*
-     * TODO:
-     * Increase balance atomically.
-     *
-     * Requirements:
-     * - Must not lose updates under concurrency
+    /**
+     * Thread-safe deposit.
      */
     public void deposit(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+
+        lock.writeLock().lock();
+
+        try
+        {
+            balance += amount;
+        }
+        finally
+        {
+            lock.writeLock().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)
+    /**
+     * Thread-safe withdrawal.
      */
     public void withdraw(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
+
+        lock.writeLock().lock();
+
+        try
+        {
+            balance -= amount;
+        }
+        finally
+        {
+            lock.writeLock().unlock();
+        }
     }
 
-    /*
-     * TODO:
-     * Transfer money between two accounts atomically.
+    /**
+     * Atomic deadlock-free transfer.
      *
-     * 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
+     * Lock ordering rule:
+     * Always acquire the lock of the account with the
+     * smaller accountId first.
      */
     public void transfer(BankAccount target, long amount) {
-        throw new UnsupportedOperationException("TODO: implement atomic deadlock-free transfer");
+
+        if (target == null)
+            throw new IllegalArgumentException("Target account cannot be null.");
+
+        if (this == target)
+            return;
+
+        BankAccount first = this.accountId < target.accountId ? this : target;
+
+        BankAccount second = this.accountId < target.accountId ? target : this;
+
+        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();
+        }
     }
 }