Everything you can use is documented. At least in libraries that are worth using. All basic information about using classes is there. Obviously their methods too.
Android has a website 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 in general, how to find what you need there. You have to memorize what you use the most and know what can be useful.
Java documentation . It is not exactly the same as Android and this version is not yet available on 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 that you use well, you will have better use the next day.
Other documentation can be found. The Stack Overflow itself has a multi-documentation site . Do not know English? You will have difficulty developing high-level software.
Look for other sources, compare what you read somewhere. Do not accept the first information you receive as an absolute truth. Have a critical attitude and make sure you understand all the basics of computing to be able to do it with quality.
Some people may come up with other alternatives, but the canonical way of learning something is documentation.
In doubt, ask the person who is most experienced.
