Design patterns/ Structure to flow when creating chatbots through java without other frameworks like SLACK,MICROSOFT BOT FRAMEWORK

I want to know what should be my approach if I want to create a domain specific chatbot. More specifically what is the best approach to create the brain of chatbot i.e. the part which decides which method to call to retrieve information.

I know a message should parsed to get the interrogative pronouns which will decide the INTENT and other keywords should be compared with my domain specific words collection, which will be my ENTITIES. Now with these INTENT and ENTITIES I am deciding which method to call based on a long if-else ladder. I am worried that with increase in capabilities this if-else ladder will grow huge and is definitely not the approach, the smart people in the world are following.

Though the implementation of methods is not important, in my case the methods are fetching data from linux server and oracle database. I am using JAVA.

How should I move forward now, any algorithm or design pattern being followed in the Industry? Any link,article,book or guidance is appreciated. Thanks!!!