Retrofit: multiple query parameters in @GET command?

If you have a bunch of GET params, another way to pass them into your url is a HashMap.

class YourActivity extends Activity {

    private static final String BASEPATH = "http://www.example.com";

    private interface API {
        @GET("/thing")
        void getMyThing(@QueryMap Map<String, String>, new Callback<String> callback);
    }

    public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.your_layout);

       RestAdapter rest = new RestAdapter.Builder().setEndpoint(BASEPATH).build();
       API service      = rest.create(API.class);

       Map<String, String> params = new HashMap<String, String>();
       params.put("foo", "bar");
       params.put("baz", "qux");
       // ... as much as you need.

       service.getMyThing(params, new Callback<String>() {
           // ... do some stuff here.
       });
    }
}

The URL called will be http://www.example.com/thing/?foo=bar&baz=qux

You should be using this syntax:

@GET("/my/API/call")
Response getMyThing(
    @Query("param1") String param1,
    @Query("param2") String param2);

Specifying query parameters in the URL is only for when you know both the key and value and they are fixed.