There are several topics to study:
- Java fundamentals
- Modular migration and services
- Concurrency
- I/O & NIO
- JDBC
- Annotations
- Generics & collections
- Functional interfaces/ Lambda/ Stream
- Secure coding
- Localization/ Exception/ Assertion/ Optional
General resources covering all the topics:
Book
Testing
Other programmers feedbacks
Javin Paul article:
Karsten Thoms article:
https://dev.karakun.com/2021/04/29/ocp-java11.html
Here the details of some resources I used for different each topics:
1. Java fundamentals
2. Modular migration and services
Book
Stack
Other
MOOC
Cheatsheet
3. Concurrency
Book
Javadoc
Trail
4. I/O & NIO
Trail
Javadoc
5. JDBC
6. Annotations
My articles
Trail
7. Generics & collections
Trail
Stack
Javadoc
8. Functional interfaces/ Lambda/ Stream
My articles
Book
MOOC (free)
Javdoc
https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html (part on reduction)
9. Secure coding
My articles
Official
10. Localization/ Exception/ Assertion/ Optional
My articles
Trail