What should I do if I am struggling at answering simple Java questions?

I am developing a web application using the Java programming language over the previous 7 years. I got experience in various java frameworks specially Spring, Hibernate, ... etc. I worked also with Python/Django stack and I worked with some big data projects including Apache Kafka and Apache Spark.

But, when it comes to that old java questions I found myself either not remembering at all or not sure what is the correct answer.

The main reason is that during my development careers I did not touch much of these basics.

For example I do not remember when was the last time I used nested classes :) or needed to write multi-threaded code.

Most of my work was web development, writing business, CRUD operations,.. etc.

And when I got a change to write code for a large system we used Apache Spark where it removed much of the hassle of managing concurrency and distribution :).

Recently I tried to apply to a software developer at cross over but unfortunately, I did not pass the basic test :(

What should I do in this case?