Everything you can use is documented. At least in libraries that are worth using. All basic class usage information is there. Obviously their methods, too.
The Android has a site dedicated to this. You have to know how to use this site very well. You have to have a good idea of your organization, what you have there in general, know how to find what you need there. You have to learn what you use most and know what might be useful.
Java documentation. It is not exactly the same as Android and this version is not yet available in it.
You have to learn to read the documentation carefully, to understand the minutiae. And you need to know that not everything you need to know about using that class is written in the documentation. Using well depends on experience. Each day you wear it well, you’ll wear it better the next day.
You can find other documentation. Don’t know English? You will have difficulty developing software at a high level.
Look for other sources, confront what you read somewhere. Don’t accept the first information you receive as an absolute truth. Take a critical attitude and make sure you understand all the fundamentals of computing to be able to do it with quality.
Some people may give other alternatives, but the canonical way to learn something is documentation.
When in doubt ask who is more experienced.
Question being debated on https://pt.meta.stackoverflow.com/q/5965/101.
– Maniero