Resources

API specifications

Java 8.0

General Java resources

The Java Tutorials
Java in a Nutshell: A Desktop Quick Reference
Introduction to Programming Using Java
Think Java
Java for C++ programmers
From Python to Java: a collection of short videos

Code visualization

Java Tutor

Extra-practice exercises

CodingBat

Data structures and algorithms

Software Design and Data Structures
Interactive visualizations of algorithms
VisuAlgo.net (more visualizations)
Visualizations of sorting algorithms