Tomcat and xmlrpc

first,tnks for your time..

im my client(the server is tomcat) i have a servlet which sends(ans receives the xmlrpc response-request).the data that i send is simply 2 strings and i want
to take back from my server HashMap<String, List<String>>.
i do that:

....
HashMap<String,List<String>> result= new HashMap<String,List<String>>;

....
....Object[] params = new Object[]{new String(x1), new String(x2)};

try {
result=
(HashMap<String,List<String>>)client.execute("Open.OpenEnvironment1",
params);

the problem is here...i can't cast to see my response since i use
jdk6,i have to use generics??collection??any advise??