HashMap and Array issue

I have this JSP where I have alot of fields with conditions.
I would like to make it more efficient and use a for loop.
Here is an example (showing 2 fields for example only):

<%@ page language="java" import="java.util.*" %>
<%
HashMap errors = new HashMap();
String firstname = "Joe";
String lastname = "Miller";

    if (!firstname.equals(""))
    {
        errors.put("firstname",firstname);
    }
    if (!lastname.equals(""))
    {
        errors.put("lastname",lastname);
    }

out.println(errors.get("firstname"));
out.println(errors.get("lastname"));
%>

It prints out Joe Miller

Now my attempt below to put this in a loop prints out null null:

<%@ page language="java" import="java.util.*" %>
<%
HashMap errors = new HashMap();
String firstname = "Joe";
String lastname = "Miller";
//String[] keys = {"firstname", "lastname"};
String[] keys = {firstname, lastname};
for(int i = 0;i < keys.length;i++)
{
     if(!keys[i].equals(""))
    {
        errors.put(keys[i],keys[i]);
    }
}

out.println(errors.get("firstname"));
out.println(errors.get("lastname"));

%>

Please advise.