docs: update singleton

singleton/src/main/java/com/iluwatar/singleton/App.java

@@ -89,7 +89,7 @@ public class App {
     LOGGER.info("enumIvoryTower1={}", enumIvoryTower1);
     LOGGER.info("enumIvoryTower2={}", enumIvoryTower2);
 
-    // double checked locking
+    // double-checked locking
     var dcl1 = ThreadSafeDoubleCheckLocking.getInstance();
     LOGGER.info(dcl1.toString());
     var dcl2 = ThreadSafeDoubleCheckLocking.getInstance();

singleton/src/main/java/com/iluwatar/singleton/BillPughImplementation.java

@@ -45,7 +45,7 @@ public final class BillPughImplementation {
   }
 
   /**
-   * The InstanceHolder is a static inner class and it holds the Singleton instance.
+   * The InstanceHolder is a static inner class, and it holds the Singleton instance.
    * It is not loaded into memory until the getInstance() method is called.
    */
   private static class InstanceHolder {

singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java

@@ -62,7 +62,7 @@ public final class ThreadSafeDoubleCheckLocking {
     // Check if singleton instance is initialized.
     // If it is initialized then we can return the instance.
     if (result == null) {
-      // It is not initialized but we cannot be sure because some other thread might have
+      // It is not initialized, but we cannot be sure because some other thread might have
       // initialized it in the meanwhile.
       // So to make sure we need to lock on an object to get mutual exclusion.
       synchronized (ThreadSafeDoubleCheckLocking.class) {
@@ -72,7 +72,7 @@ public final class ThreadSafeDoubleCheckLocking {
         // just like the previous null check.
         result = instance;
         if (result == null) {
-          // The instance is still not initialized so we can safely
+          // The instance is still not initialized, so we can safely
           // (no other thread can enter this zone)
           // create an instance and make it our singleton instance.
           result = new ThreadSafeDoubleCheckLocking();

singleton/src/test/java/com/iluwatar/singleton/SingletonTest.java

@@ -85,7 +85,7 @@ abstract class SingletonTest<S> {
    * Test singleton instance in a concurrent setting.
    */
   @Test
-  void testMultipleCallsReturnTheSameObjectInDifferentThreads() throws Exception {
+  void testMultipleCallsReturnTheSameObjectInDifferentThreads() {
     assertTimeout(ofMillis(10000), () -> {
       // Create 10000 tasks and inside each callable instantiate the singleton class
       final var tasks = IntStream.range(0, 10000)
@@ -96,7 +96,7 @@ abstract class SingletonTest<S> {
       final var executorService = Executors.newFixedThreadPool(8);
       final var results = executorService.invokeAll(tasks);
 
-      // wait for all of the threads to complete
+      // wait for all the threads to complete
       final var expectedInstance = this.singletonInstanceMethod.get();
       for (var res : results) {
         final var instance = res.get();