How to get Map data using JDBCTemplate.queryForMap

How to load data from JDBCTemplate.queryForMap() as it returns the Map Interface? How is the query data maintained internally in the map? I tried to load it, but I got this exception: org.springframework.dao.IncorrectResultSizeDataAccessException: Incorrect result

Code:-

public List getUserInfoByAlll() {
    List profilelist=new ArrayList();
    Map m=new HashMap();
    m=this.jdbctemplate.queryForMap("SELECT userid,username  FROM USER");
    Set s=m.keySet();
    Iterator it=s.iterator();
    while(it.hasNext()){
        String its=(String)it.next();
        Object ob=(Object)m.get(its);
        log.info("UserDAOImpl::getUserListSize()"+ob);
    }
    return profilelist;
}