Issues to note about the length () method in string

Today, when using struts, get a checkbox value from a JSP form (multiple records), if my first line of records has no choice, then use
String UserID = Request.getparameter ("Uselect" +i); When I get the value, I return null, and in the end I judge whether the value is empty, and I use the following method to judge
if (userid.length > 0 && userid!= null)
To judge, always report to Java. Nullpointexception exception, the original reason is not to use the length method, because UserID if there is no choice to return the null,null is not the length, so the correct way is
if (userid!= null)
That's the right thing to do.