Here are some basic fundamentals and when to use them/

Abstract classes vs. interfaces

Abstract classes and interfaces practicum

Best Practices to improve Performance in J2SE

Here are some of the specification versions and the app server supported versions

J2EE Version 1.4 – Servlet specification 2.4, JSP specification – 2.0


J2EE Version 1.3 – Servlet specification – 2.3

BEA Support

Weblogic 8.1 SP2

J2EE specification -1.3
Servlet specification – 2.3
JSP specification – 1.2

WebSphere 5.1.2
J2EE specification -1.3
Servlet Specification Level: 2.3
JSP Specification Level: 1.2
EJB 2.0
JDBC 2.0
JTA 1.0
JavaMail 1.2
JAXP 1.1
JMS 1.0.2
http://java.sun.com/j2ee/sdk_1.3/techdocs/release/ReleaseNotes.html

Sometime the basics such as heap, stack causes many confusion. Here is a article that puts together.
The lean, mean, virtual machine