BAEL-2438 Difference between HashMap and HashTable (#5931)

* BAEL-2368 convert string array to string

* BAEL-2368 Convert array to string code

* BAEL-2368 Change package

* Fix for test

* BAEL-2438 Difference between HashMap and HashTable

core-java-collections/src/test/java/com/baeldung/hashmapvshashtable/HashmapVsHashtableDifferenceUnitTest.java

@@ -0,0 +1,98 @@
+package com.baeldung.hashmapvshashtable;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.Collections;
+import java.util.ConcurrentModificationException;
+import java.util.Enumeration;
+import java.util.HashMap;
+import java.util.Hashtable;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.junit.Test;
+
+import com.google.common.collect.Lists;
+
+public class HashmapVsHashtableDifferenceUnitTest {
+
+    // null values
+    @Test(expected = NullPointerException.class)
+    public void givenHashtable_whenAddNullKey_thenNullPointerExceptionThrown() {
+        Hashtable<String, String> table = new Hashtable<String, String>();        
+        table.put(null, "value");
+    }
+    
+    @Test(expected = NullPointerException.class)
+    public void givenHashtable_whenAddNullValue_thenNullPointerExceptionThrown() {
+        Hashtable<String, String> table = new Hashtable<String, String>();        
+        table.put("key", null);
+    }
+    
+    @Test
+    public void givenHashmap_whenAddNullKeyAndValue_thenObjectAdded() {
+        HashMap<String, String> map = new HashMap<String, String>();
+        map.put(null, "value");
+        map.put("key1", null);
+        map.put("key2", null);
+        
+        assertEquals(3, map.size());
+    }
+    
+    // fail-fast iterator
+    @Test(expected = ConcurrentModificationException.class)
+    public void givenHashmap_whenModifyUnderlyingCollection_thenConcurrentModificationExceptionThrown() {
+        HashMap<String, String> map = new HashMap<String, String>();
+        map.put("key1", "value1");
+        map.put("key2", "value2");
+        map.put("key3", "value3");
+        
+        Iterator<String> iterator = map.keySet().iterator();
+        while(iterator.hasNext()){ 
+            iterator.next();
+            map.put("key4", "value4");
+        }        
+    }
+
+    @Test
+    public void givenHashtable_whenModifyUnderlyingCollection_thenItHasNoEffectOnIteratedCollection() {
+        Hashtable<String, String> table = new Hashtable<String, String>();        
+        table.put("key1", "value1");
+        table.put("key2", "value2");
+        
+        List<String> keysSelected = Lists.newArrayList(); 
+        Enumeration<String> keys = table.keys();
+        while (keys.hasMoreElements()) {
+            String key = keys.nextElement();
+            keysSelected.add(key);
+            
+            if (key.equals("key1")) {
+                table.put("key3", "value3");
+            }
+        }
+        
+        assertEquals(2, keysSelected.size());
+    }
+    
+    // synchronized map
+    @Test
+    public void givenHashmap_thenCreateSynchronizedMap() {
+        HashMap<String, String> map = new HashMap<String, String>();
+        map.put("key1", "value1");
+        map.put("key2", "value2");
+        map.put("key3", "value3");
+        
+        Set<Entry<String, String>> set = map.entrySet();
+        synchronized (map) {
+            Iterator<Entry<String, String>> it = set.iterator();
+            while(it.hasNext()) {
+                Map.Entry<String, String> elem = (Map.Entry<String, String>)it.next();
+             }            
+        }
+        
+        Map<String, String> syncMap = Collections.synchronizedMap(map);
+    }
+}