Adjust checkstyle rules. Make checkstyle fail the build when violations are found. Correct all current checkstyle violations.

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

@@ -8,7 +8,7 @@ public final class IvoryTower {
   /**
    * Static to class instance of the class.
    */
-  private static final IvoryTower instance = new IvoryTower();
+  private static final IvoryTower INSTANCE = new IvoryTower();
 
   /**
    * Private constructor so nobody can instantiate the class.
@@ -21,6 +21,6 @@ public final class IvoryTower {
    * @return instance of the singleton.
    */
   public static IvoryTower getInstance() {
-    return instance;
+    return INSTANCE;
   }
 }

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

@@ -11,14 +11,14 @@ package com.iluwatar.singleton;
  */
 public class ThreadSafeDoubleCheckLocking {
 
-  private static volatile ThreadSafeDoubleCheckLocking INSTANCE;
+  private static volatile ThreadSafeDoubleCheckLocking instance;
 
   /**
    * private constructor to prevent client from instantiating.
    */
   private ThreadSafeDoubleCheckLocking() {
     // to prevent instantiating by Reflection call
-    if (INSTANCE != null) {
+    if (instance != null) {
       throw new IllegalStateException("Already initialized.");
     }
   }
@@ -31,12 +31,12 @@ public class ThreadSafeDoubleCheckLocking {
   public static ThreadSafeDoubleCheckLocking getInstance() {
     // local variable increases performance by 25 percent
     // Joshua Bloch "Effective Java, Second Edition", p. 283-284
-    ThreadSafeDoubleCheckLocking result = INSTANCE;
+    ThreadSafeDoubleCheckLocking result = instance;
     if (result == null) {
       synchronized (ThreadSafeDoubleCheckLocking.class) {
-        result = INSTANCE;
+        result = instance;
         if (result == null) {
-          INSTANCE = result = new ThreadSafeDoubleCheckLocking();
+          instance = result = new ThreadSafeDoubleCheckLocking();
         }
       }
     }

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

@@ -16,7 +16,7 @@ public class ThreadSafeLazyLoadedIvoryTower {
   /**
    * The instance gets created only when it is called for first time. Lazy-loading
    */
-  public synchronized static ThreadSafeLazyLoadedIvoryTower getInstance() {
+  public  static synchronized ThreadSafeLazyLoadedIvoryTower getInstance() {
 
     if (instance == null) {
       instance = new ThreadSafeLazyLoadedIvoryTower();