I see that the only way to get api key for a sender is through eSL API.
on another note is there any information that you can share with me about the difference between eSL Java SDK features/functions and eSL API features/functions.
Essentially, the SDKs take away a lot of the work you’ll need to do compared to going through REST. Work such as building the json payload, httpclient, defining the headers, etc.. It is very powerful and easy way to get started with eSignLive. However, through REST, you have a lot more flexibility. Anything you can do with the SDKs you can do in REST. However, the reverse is not true (e.g. getting sender api key). You can have a look at our feature guides and documentationfor a list of all the calls available through the SDKs and REST.