Enter any valid Java expression into the terminal and press enter to immediately get the result: http://www.javarepl.com/term.html. If you want to test something out that is too complicated for the REPL, you can write and run Java code in a simple in-browser editor at ideone: http://ideone.com/
Official Java tutorials: https://docs.oracle.com/javase/tutorial/java/index.html
Offline API doc manager Dash (highly recommended, the Java API docs are available to download within Dash and are then easily searchable even without internet access): https://kapeli.com/dash
Online API reference (if you don’t have a Mac or just prefer not to use Dash): https://docs.oracle.com/javase/8/docs/api/index.html
Sorting algorithms speed comparison video: https://www.youtube.com/watch?v=ZZuD6iUe3Pc
If you are having trouble with Java syntax, the Codecademy Java course is a good way to review the basics: https://www.codecademy.com/learn/learn-java
Practice Java by making visual art with Processing: https://processing.org/
Extra Java practice problems at exercism: http://exercism.io/languages/java/exercises