Add missing "volatile"s (#2307)

* Add volatile keywords

* Update SafeCounterWithoutLock.java

core-java/src/main/java/com/baeldung/concurrent/atomic/SafeCounterWithLock.java

@@ -1,7 +1,7 @@
 package com.baeldung.concurrent.atomic;
 
 public class SafeCounterWithLock {
-    int counter;
+    private volatile int counter;
     
     public int getValue() {
         return counter;

core-java/src/main/java/com/baeldung/concurrent/atomic/SafeCounterWithoutLock.java

@@ -3,7 +3,7 @@ package com.baeldung.concurrent.atomic;
 import java.util.concurrent.atomic.AtomicInteger;
 
 public class SafeCounterWithoutLock {
-    AtomicInteger counter = new AtomicInteger(0);
+    private final AtomicInteger counter = new AtomicInteger(0);
     
     public int getValue() {
         return counter.get();