why we use map.put(num[i] ,i) instead of map.put(i,num[i]) code example

Example: java map declaration

Map< String,Integer> hm =  
                        new HashMap< String,Integer>(); 
       hm.put("a", new Integer(100)); 
       hm.put("b", new Integer(200)); 
       hm.put("c", new Integer(300)); 
       hm.put("d", new Integer(400));