Hi Harsh,
Good question! Nice reading that you would like to extend your app. If you are already familiar with Java itself, I think the following docs should help you out. They guide you through the setup process of Eclipse.
https://docs.mendix.com/refguide/java-programming/
Additionally, it can be helpful to open an existing Marketplace module in Eclipse to see how the code has been developed. For example; the new AWS connectors are a good example of how Java can be used within Mendix.
There are a few how-to's and video's out there, but unfortunately, I don't have them on my clipboard right now. Hopefully, this helps.