Question: I find reading coding style guidelines to be the best way to get to know a language and its peculiarities. I've been trying to find a good document for Java, and get some questions answered immediately. 

Firstly, what is the convention for very long lines? Writing in both Java and C/C++ leads me to have a great deal of confusion on this point. 

Secondly, what is the guideline for creating custom exception classes? I typically end up throwing an existing exception rather than creating my own. Is custom exception creation typically enforced?

Answer: 
"Effective Java" by Joshua Bloch is vital reading. It goes beyond the syntactic guidelines offered by Oracle.

Item 60 is "Favor the use of standard exceptions," but item 61 is "Throw exceptions appropriate to the abstraction." Sometimes custom exceptions are called for, and sometimes not.
Possible duplicate question. You can find the answer here: <https://stackoverflow.com/questions/1334204/official-java-code-guidelines-conventions>

However, the documentation on Sun/Oracle's website is older than 12 years. Although the core language hasn't changed, technology changes rapidly, which means the way we work with that technology changes. 

I would suggest using what works best for you and your team. As long as there is some agreed-upon standard within your organization that is considered mainstream, you should be okay.