Here I am providing you links to some useful pages available on the net divided in different categories:

1)Inner classes:

• www.go4java.20m.com/inner_classes.html ( This page has a chart showing the relation between different modifiers and inner classes i.e. which modifier can be used with inner classes which methods and variables are accessible in inner classes, which type of variables is allowed to be declared in inner classes, etc).
• www.go4java.20m.com/modifiers.htm ( This page also has a chart showing the relation of different modifiers with classes, variables, constructors, methods, initializer blocks i.e. which modifiers are allowed with classes, variables, constructors, methods, initializer blocks, interfaces, etc).

2) Bitshift Operators:

• www.jchq.net/tutorial/BitShift.htm (An excellent document explaining the whole process of Bitshift operators)
• www.javaranch.com/campfire/StoryBits.jsp ( This document have good explanation of Bitshift operator using images)
• www.jchq.net/applets/BitShift/BitShiftAr.html ( This document have a Calculator performing Bitshifting calculations)

3) Garbage Collection:

• http://www.jchq.net/tutorial/03_01Tut.htm (A good one also have small questions on the topic)
• www.geocities.com/SiliconValley/Network/3693/obj_sec3.html#obj8 ( Another good one explaining the process of Garbage Collection)

4) IEEE754 Standard:

• www.research.microsoft.com/~hollasch/cgindex/coding/ieeefloat.html ( This article gives a brief overview of IEEE floating point and its representation.)

5) Java Language Specification:

• http://java.sun.com/docs/books/jls/ (You can Download the Java Language Specification book from here.)
• http://java.sun.com/docs/books/jls/second_edition/html/j.title.doc.html (OR you can read it online here.)

6) SCJP Exam Introduction:

• http://suned.sun.com/USA/certification/progexam_intro.html (Introduction to SCJP Exam)
• http://suned.sun.com/USA/certification/java/certification_details.html (Java Certification Details)

7) Study notes for SCJP2 Exam:

• http://www.geocities.com/velmurugan_p/notes.html ( Excellent study notes for SCJP2 Exam)

• http://2test.com/index.jsp (To find out authorized centers and register online for any kind of computer related exams taken by any company)

9) Some useful e-mail IDs:

• jls@java.sun.com (To send your comments and correction to the authors of Java Language Specification)
• who2contact@central.sun.com (For any type of enquiry on Sun Certification)
• khalid@ii.uib.no ( To send your comments and correction to the authors of "A Programmer's Guide to Java Certification" book)