📍Use lombok, reformat, and optimize the code (#1560)

* Use lombok, reformat, and optimize the code

* Fix merge conflicts and some sonar issues

Co-authored-by: va1m <va1m@email.com>

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

@@ -23,8 +23,7 @@
 
 package com.iluwatar.singleton;
 
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
+import lombok.extern.slf4j.Slf4j;
 
 /**
  * <p>Singleton pattern ensures that the class can have only one existing instance per Java
@@ -61,10 +60,9 @@ import org.slf4j.LoggerFactory;
  * {@link InitializingOnDemandHolderIdiom}. However, this implementation requires at least Java 8
  * API level to work.</p>
  */
+@Slf4j
 public class App {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
-
   /**
    * Program entry point.
    *

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

@@ -58,7 +58,7 @@ public final class ThreadSafeDoubleCheckLocking {
   public static ThreadSafeDoubleCheckLocking getInstance() {
     // local variable increases performance by 25 percent
     // Joshua Bloch "Effective Java, Second Edition", p. 283-284
-    
+
     var result = instance;
     // Check if singleton instance is initialized.
     // If it is initialized then we can return the instance.

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

@@ -49,7 +49,7 @@ import org.junit.jupiter.api.Test;
  * @author Jeroen Meulemeester
  * @author Richard Jones
  */
-public abstract class SingletonTest<S> {
+abstract class SingletonTest<S> {
 
   /**
    * The singleton's getInstance method.
@@ -69,7 +69,7 @@ public abstract class SingletonTest<S> {
    * Test the singleton in a non-concurrent setting.
    */
   @Test
-  public void testMultipleCallsReturnTheSameObjectInSameThread() {
+  void testMultipleCallsReturnTheSameObjectInSameThread() {
     // Create several instances in the same calling thread
     var instance1 = this.singletonInstanceMethod.get();
     var instance2 = this.singletonInstanceMethod.get();
@@ -84,7 +84,7 @@ public abstract class SingletonTest<S> {
    * Test singleton instance in a concurrent setting.
    */
   @Test
-  public void testMultipleCallsReturnTheSameObjectInDifferentThreads() throws Exception {
+  void testMultipleCallsReturnTheSameObjectInDifferentThreads() throws Exception {
     assertTimeout(ofMillis(10000), () -> {
       // Create 10000 tasks and inside each callable instantiate the singleton class
       final var tasks = IntStream.range(0, 10000)