Further reading
We have learned enough Java to proceed with this book. It is always beneficial, however, to see more examples of Java in action and to go beyond the minimum necessary to proceed. If you want a good source to learn Java in greater depth, then the official Oracle website is good. Note that you do not need to study this website to continue with this book. Also note that the tutorials on the Oracle website are not set in an Android context, but the site is a useful resource to bookmark and browse all the same:
The official Java tutorials can be found at https://docs.oracle.com/javase/tutorial/.