Post methods have post parameters, but get methods do not.
Query parameters are embedded in the URL. The current version of HttpClient accepts a string in the constructor. If you wanted to add the key, value pair above, you could use:
String url = "http://www.example.com/page?key=value";
GetMethod method = new GetMethod(url);
A good starting tutorial can be found on the Apache Jakarta Commons page.
Update: As suggested in the comment, NameValuePair works.
GetMethod method = new GetMethod("example.com/page");
method.setQueryString(new NameValuePair[] {
new NameValuePair("key", "value")
});