Covered next set of areas:
- annotations
- cache
- collections
- core java
- iterators
- java 8
- jvm
- masking
- multithreading
- numeric
- proxies
- recursion
- refactoring
- search
- serialization
- sorting
- strings
- testing
- trees
- xml
With some uncategorized staff:
Additionally, don't miss tests, they contain enough code which describe implementation somehow.