Please suggest a way to post to salesforce rest resource without using the username and password as the authentication

I’m trying to post to a Salesforce Rest Resource from a java console app. Currently I’m exposing it as a public resource via custom portals.
I’m looking for suggestions for a way to implement the authentication without passing username, password. I have checked OAuth flows but I believe all of them require a callback implementation to read the tokens. It will be helpful if you can suggest a way to authorize in an implicit manner.

Accessing Apex rest class with OAuth 2.0 with Named Credential authentication mechanism

I have been trying to implement Oauth 2.0 in my org and my requirement is to use Named credentials only when making a apex callout. I am using Named credentials, Oauth, Connected Apps and auth. provider to establish a connection with an external Salesforce org. The external org has exposed an apex class as REST service that I am trying to access using my org. I have followed the steps given in this link to setup everything :