how java backend get value in a dropdown from frontend

In java backend which method can be used to get value in the frontend dropdown? eg: enter a country name in frontend dropdown, how can I pass this value to backend to search in database?

Answer

Your back end will expose a rest api that your front end will access via Rest Api call. Your dropdown list will be filled by a list of country coming from your back end or a predefined list in your front end.

Google about Rest Client, Rest API your will have a good understanding of the subject.