The interview was average, but you have to be clear on those average things.

Be confident, but not with your wrong answers. Just say no if you don't know anything about the topic.

They ask very basic questions like:

Abstraction and the difference between an abstract class and an interface.

They will check your logical understanding by asking some algorithms, for example, sorting and searching algorithms. The difficulty will depend on your experience. In my case, an array was given, and the interviewer asked me to sort it using any algorithm.

I was asked about singleton, factory, and DAO design patterns, as they were on my CV.

JavaScript basic knowledge, like the difference between == and ===.

There were no questions on threading, but they might ask; it depends.

He asked me how a HashMap works internally. There were no other questions from collections, but one should be prepared with threading collections and generics. There were also questions from my CV, like MVC architecture and related topics. You can prepare your CV according to your knowledge and, instead of putting generic terms, go with the specific APIs or implementations you have done. They will directly jump on these topics and check if you know the things you have written on your CV.