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Question

The Java API is a collection of classes and interfaces that have been written for you to use.
The Java API Documentation with all of the available APIs can be located on the Oracle website at
http://docs.oracle.com/javase/7/docs/api/

Answer 1

For a beginner, it's a good idea to read the documentation of the most important class (Sting, Integer, ArrayList...). just to be familiar with the presentation : signature of the class, description, constructor and methods.