docs: add explanation for Double-Checked Locking (#2914)

double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/AppTest.java

@@ -35,7 +35,6 @@ class AppTest {
 
   /**
    * Issue: Add at least one assertion to this test case.
-   *
    * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
    * throws an exception.
    */

double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/InventoryTest.java

@@ -80,7 +80,7 @@ class InventoryTest {
    * item limit.
    */
   @Test
-  void testAddItem() throws Exception {
+  void testAddItem() {
     assertTimeout(ofMillis(10000), () -> {
       // Create a new inventory with a limit of 1000 items and put some load on the add method
       final var inventory = new Inventory(INVENTORY_SIZE);
@@ -109,7 +109,7 @@ class InventoryTest {
   }
 
 
-  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
+  private static class InMemoryAppender extends AppenderBase<ILoggingEvent> {
     private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender(Class clazz) {