title: why we use MapSqlParameterSource
body: <p>I am new to spring world, in above code i understand query but i don't get why 
  "new MapSqlParameterSource("username" ,username)" is used?</p>
  
  <pre><code>  public boolean exists(String username) {      
         return jdbc.queryForObject("select count(*) from users where username=:username",
             new MapSqlParameterSource("username" ,username),Integer.class)&gt;0;                                        }
  </code></pre>
  
  <p>what is the purpose of using it? </p>
  
  <p>Thanks in advance</p>