add Demo suffix to avoid naming conflict

Author: fishercoder1534

src/main/java/java_collections/HashMap.java renamed to src/main/java/java_collections/HashMapDemo.java

@@ -2,13 +2,13 @@
 
 import java.util.Map;
 
-public class HashMap {
+public class HashMapDemo {
     private static final int NUMBER_OF_MAP_ENTRIES = 2;
 
     public static void main(String... args) {
         System.out.println("Program started.");
-        HashMap hashMap = new HashMap();
-        hashMap.understandHashMapInternalWorkings();
+        HashMapDemo hashMapDemo = new HashMapDemo();
+        hashMapDemo.understandHashMapInternalWorkings();
         System.out.println("Program finished.");
     }