fix: Remove unnecessary and possibly not threadsafe flag (#1811)

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

@@ -36,16 +36,12 @@ public final class ThreadSafeDoubleCheckLocking {
 
   private static volatile ThreadSafeDoubleCheckLocking instance;
 
-  private static boolean flag = true;
-
   /**
    * private constructor to prevent client from instantiating.
    */
   private ThreadSafeDoubleCheckLocking() {
     // to prevent instantiating by Reflection call
-    if (flag) {
-      flag = false;
-    } else {
+    if (instance != null) {
       throw new IllegalStateException("Already initialized.");
     }
   }

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

@@ -34,9 +34,7 @@ public final class ThreadSafeLazyLoadedIvoryTower {
 
   private ThreadSafeLazyLoadedIvoryTower() {
     // Protect against instantiation via reflection
-    if (instance == null) {
-      instance = this;
-    } else {
+    if (instance != null) {
       throw new IllegalStateException("Already initialized.");
     }
   }