The Java Tutorials at Oracle's website:
https://docs.oracle.com/javase/tutorial/
Lessons on:
Getting Started
Learning the Java Language
Essential Java Classes
Collections
Date-Time APIs
Deployment
Preparation for Java Programming Language Certification
Creating a GUI with Swing
Creating a JavaFX GUI
Custom Networking
The Extension Mechanism
Full-Screen Exclusive Mode API
Generics
Internationalization
JavaBeans
JDBC Database Access
JMX
JNDI
JAXP
JAXB
RMI
Reflection
Security
Sound
2D Graphics
Sockets Direct Protocol