It is just Warring,do not have any affect on compilation or execution.
Accorind to Java Doc
-Xlint:unchecked  
Give more detail for unchecked conversion warnings that are mandated by the Java Language Specification.
Generally This comes up in Java 5 and later if you're using collections without type specifiers.
Example :
using Arraylist() instead of Arraylist<String>().  
You can avoid that warring by using @SuppressWarnings("unchecked"). 
class NoWarn {
   public static Map<String, String[]> getParameterMap(ServletRequest r) 
   {
      @SuppressWarnings("unchecked")
      Map<String, String[]> result = r.getParameterMap();
      return result;
    }
 }
For more information please look into this link