BAEL-7430 first draft (#15728)

* BAEL-7430 first draft

* Revised the code

---------

Co-authored-by: Wynn Teo <wynnteo@Wynns-MacBook-Pro.local>

core-java-modules/core-java-concurrency-advanced-5/src/main/java/com/baeldung/countdownlatchvssemaphore/CountDownLatchDemo.java

@@ -0,0 +1,33 @@
+package com.baeldung.countdownlatchvssemaphore;
+
+import java.util.concurrent.CountDownLatch;
+
+public class CountDownLatchDemo {
+
+    public static void main(String[] args) throws InterruptedException {
+        // Create a CountDownLatch with an initial count equal to the number of tasks to be completed
+        int numberOfTasks = 3;
+        CountDownLatch latch = new CountDownLatch(numberOfTasks);
+
+        // Simulate completion of tasks by worker threads
+        for (int i = 1; i <= numberOfTasks; i++) {
+            new Thread(() -> {
+                System.out.println("Task completed by Thread " + Thread.currentThread()
+                    .getId());
+
+                // Decrement the latch count to signal completion of a task
+                latch.countDown();
+            }).start();
+        }
+
+        // Main thread waits until all tasks are completed
+        latch.await();
+        System.out.println("All tasks completed. Main thread proceeds.");
+
+        // Attempting to reset will have no effect
+        latch.countDown();
+        // Latch is already at zero, await() returns immediately
+        latch.await(); // This line won't block
+        System.out.println("Latch is already at zero and cannot be reset.");
+    }
+}

core-java-modules/core-java-concurrency-advanced-5/src/main/java/com/baeldung/countdownlatchvssemaphore/SemaphoreDemo.java

@@ -0,0 +1,42 @@
+package com.baeldung.countdownlatchvssemaphore;
+
+import java.util.concurrent.Semaphore;
+
+public class SemaphoreDemo {
+
+    public static void main(String[] args) {
+        // Create a Semaphore with a fixed number of permits
+        int NUM_PERMITS = 3;
+        Semaphore semaphore = new Semaphore(NUM_PERMITS);
+
+        // Simulate resource access by worker threads
+        for (int i = 1; i <= 5; i++) {
+            new Thread(() -> {
+                try {
+                    // Acquire a permit to access the resource
+                    semaphore.acquire();
+                    System.out.println("Thread " + Thread.currentThread().getId() + " accessing resource.");
+
+                    // Simulate resource usage
+                    Thread.sleep(2000);
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                } finally {
+                    // Release the permit after resource access is complete
+                    semaphore.release();
+                }
+            }).start();
+        }
+
+        // Simulate resetting the Semaphore by releasing additional permits after a delay
+        try {
+            Thread.sleep(5000);
+
+            // Resetting the semaphore permits to the initial count
+            semaphore.release(NUM_PERMITS);
+            System.out.println("Semaphore permits reset to initial count.");
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+    }
+}