The Oracle tutorials will help you fully understand the Java language used to write plugins.
- Learning the Java Language (Oracle)
- Trail Collections (Oracle)
- Lesson: Exception (Oracle)
- Lesson: Concurrency (Oracle)
Java video tutorials
Video Java tutorials
- iTechRemix Basic instructions from iTechRemix
- Thenewboston - Plenty of amazing videos
- JavaVideoTutes.com - Many important video tutorials Java