how does hashmap work in java code example

Example 1: java hashmap example

//Hash map creation
Map< String,Integer> hm =  
                        new HashMap< String,Integer>(); 
//inserting elements into hashmap
       hm.put("a", new Integer(100)); 
       hm.put("b", new Integer(200)); 
       hm.put("c", new Integer(300)); 
       hm.put("d", new Integer(400));

Example 2: how to create a hashmap in java

HashMap<Integer,String> map=new HashMap<Integer,String>();//key is integer, value is String