Java

JDK documentation

• Java SE 6
• Java SE 7
• Java SE 8 - Java FX 8

Java core API

• Java SE 6
• Java SE 7
• Java SE 8

Networking in Java

• Sun tutorial
• NIO overview by Sun
• some NIO examples, by Sun
• the ROX Java NIO tutorial
• an article on DeveloperWorks on NIO
• five ways to maximize Java NIO
• a technical article providing an overview of the Generic Connection Framework, a J2ME API.

Concurrency

• Sun tutorial

Logging

• log4j page

Database

• Sun JDBC tutorial
• database development in Eclipse with the Data Tools Platform (DTP)
• Apache Derby
• Java DB, Sun's supported distribution of Apache Derby
• a WikiBook: Java JDBC using SQLite
• SQLite wrappers for Java
• SQLite documentation

Graphical User Interface

• the Standard Widget Toolkit (SWT)
• SWT widgets
• SWT Javadoc (available from inside Eclipse Help as well)

General documentation

• Thinking in Java - 3rd edition, by Bruce Eckel
• Introduction to programming with Java, by David J. Eck