I would like to type some text in textfield, and then select a, if a is not available, then select b, finally submit the information.
I just write these code and get no idea what can I do next.
URL page_1 = new URL("https://google.com.hk");
HttpURLConnection urlconnection = (HttpURLConnection) page_1.openConnection();
DataOutputStream data_out = new DataOutputStream(urlconnection.getOutputStream());
DataInputStream data_in = new DataInputStream(urlconnection.getInputStream());
Java won't load the page and execute the JS code it contains like a browser would do. All it will do is load the data sent by the server in the response.
If you want a programmable browser in Java, then look at Selenium or HtmlUnit.