Added changes based on review comments

core-java-modules/core-java-collections-maps-3/src/test/java/com/baeldung/map/concurrenthashmap/ConcurrentModificationErrorUnitTest.java

@@ -0,0 +1,41 @@
+package com.baeldung.map.concurrenthashmap;
+
+import java.util.Collections;
+import java.util.ConcurrentModificationException;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.concurrent.ConcurrentHashMap;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class ConcurrentModificationErrorUnitTest {
+
+    @Test(expected = ConcurrentModificationException.class)
+    public void whenRemoveAndAddOnHashMap_thenConcurrentModificationError() {
+        Map<Integer, String> map = new HashMap<>();
+        map.put(1, "baeldung");
+        map.put(2, "HashMap");
+        Map<Integer, String> synchronizedMap = Collections.synchronizedMap(map);
+        Iterator<Entry<Integer, String>> iterator = synchronizedMap.entrySet().iterator();
+        while (iterator.hasNext()) {
+            synchronizedMap.put(3, "Modification");
+            iterator.next();
+        }
+    }
+
+    public void whenRemoveAndAddOnConcurrentHashMap_thenNoError() {
+        Map<Integer, String> map = new ConcurrentHashMap<>();
+        map.put(1, "baeldung");
+        map.put(2, "HashMap");
+        Iterator<Entry<Integer, String>> iterator = map.entrySet().iterator();
+        while (iterator.hasNext()) {
+            map.put(3, "Modification");
+            iterator.next();
+        }
+
+        Assert.assertEquals(3, map.size());
+    }
+}

core-java-modules/core-java-collections-maps-3/src/test/java/com/baeldung/map/concurrenthashmap/NullAllowInMapUnitTest.java

@@ -0,0 +1,40 @@
+package com.baeldung.map.concurrenthashmap;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class NullAllowInMapUnitTest {
+
+    @Test
+    public void allowNullKey_In_SynchronizedMap() {
+        Map<String, Integer> map = Collections
+                .synchronizedMap(new HashMap<String, Integer>());
+        map.put(null, 1);
+        Assert.assertTrue(map.get(null).equals(1));
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void allowNullKey_In_ConcurrentHasMap() {
+        Map<String, Integer> map = new ConcurrentHashMap<>();
+        map.put(null, 1);
+    }
+    
+    @Test
+    public void allowNullValue_In_SynchronizedMap() {
+        Map<String, Integer> map = Collections.synchronizedMap(new HashMap<String, Integer>());
+        map.put("1", null);
+        Assert.assertNull(map.get("1"));
+    }
+    
+    @Test(expected = NullPointerException.class)
+    public void allowNullValue_In_ConcurrentHasMap() {
+        Map<String, Integer> map = new ConcurrentHashMap<>();
+        map.put("1", null);
+    }
+
+}