Recommend Java basics links where you can try more with examples and dig into the code for each topic?
Where can I download the collection of JAVA problems?
Answer 1, authority 100%
Answer 2, authority 67%
For examples, there is a great site http://java2s.com/Code/Java/CatalogJava.htmwith a bunch of examples both for the language itself and for various libraries and frameworks.
Answer 3, authority 33%
javarush.com is definitely a good collection of problems, more than 1000 pieces, and even with automatic checking