Wow! You got lot of questions LOL! Well thank you for taking time out to post on JavaRanch.
I will try to answer them one by one.
1. By going through the content summary, I notice that the book is more focus on introducing Flex 3 to Java developers, and perhaps how to make use of Flex 3 front-end to work with Java back-end support? Correct me if I misunderstand the book's intentions.
You are correct. In fact the one of the primary goal of this book is to help beginners to get hands-on experience in Flex 3 programming with Java integration in mind.
2. How easy could it be to implement Flex 3 with Java? Would doing so require someone not to be a novice programmer, but someone possesses at least intermediate or preferably advanced Java knowledge and experiences?
Flex 3 is very easy to learn and adapt and so is the Java integration framework provided by Flex. Flex 3 provides APIs to integrate with Java backend and to invoke Java objects from server side. You do not need to be experienced programmer to implement Java integration with Flex 3. However basic understanding of Java such as writing Classes, Servlets
and deployment on web servers is assumed in this book.
3. What are the concerns or issues arise by using Flex 3 with Java in terms of security, robustness, and scalability of system being developed?
Flex 3 and Java are best married together and evidently this marriage is supported by Adobe by releasing their two major product lines to integrate Flex 3 with Java and those are “BlazeDS” an open source server side framework that facilitates Flex and Java integration and “LiveCycle Data Service” also known as “LCDS” it’s licensed enterprise grade data management framework that works seamlessly with Java. Having said that I personally do not see any issues since you can easily use best of both technologies together without any issues.
4. Is there any existing issue while implementing Flex 3 with Java? Any knowledge gap between Flex 3 and Java? If so, how to bridge them? How one programmer or developer can better make use of the advantages provided by both Flex 3 and Java for development?
Today Flex supports all major Java frameworks such as Spring, Hibernate and many others, so you can make use of these frameworks with Flex front-end and its data rich components to develop Rich Internet Applications.
I don’t anticipate any issues in integration but conceptually when a Java programmer comes to the Flex and wants to use its capabilities by replacing existing JSP/HTML based front-end then he or she may tend to mistake the Flex way of representing data with JSP/HTML page based representation. I had many questions from Java programmers such as how do I import Java APIs into Flex applications directly or how do I develop pages into Flex (similar to html pages) etc. These questions are commonly asked and I would suggest understanding Flex framework basics before actually starting coding using some sample programs.
5. For Flex Development, normally people develop using the in-house IDE developed by Adobe - Flex Builder (now known as Flash Builder), either standalone or as a plugin to Eclipse. Do you have any special preference towards it? Or you actually recommend using open source approach?
Flex Builder or Flash Builder are specially built IDEs for Flex development by Adobe. These IDEs are very powerful and provide you critical functionalities such as Visual Designing, Debugging, Profiling, Code hinting, Code generation, design time error reporting etc. and many others. However there are many open source and third party tools available for developing Flex applications but they lack some of these critical functionalities. Also Flex Builder/Flash Builder is built on top of Eclipse platform it makes Java and Flex development easy. I recommend using Flex Builder/Flash Builder as IDE for Flex development.
6. What do you think is the best approach to develop Flex 3 with Java? What's the best tools and resources you would recommend to someone going to begin development in Flex 3 with Java?
I would suggest reading Flex 3 with Java book
and understanding Flex 3 basics and integration concepts and few of the online references as given below.
Adobe Devnet http://www.adobe.com/devnet/flex/quickstart.html
Flex 3 online help http://livedocs.adobe.com/flex/3/html/help.html
Adobe TV video tutorials http://tv.adobe.com/product/flex/
Other documentation and resources http://www.adobe.com/support/documentation/en/flex/
Choose Flex Builder/Flash Builder as your development IDE and any server side framework such as BlazeDS or LCDS for integrating Flex with Java or any other open source third party remoting gateways.
7. Assuming this is not a book for Java programmer, would you be interested in writing a book using Flex 3 with Java which is catering for the needs of Java greenhorns?
This book does help you to get hands-on with Flex 3 with Java. I would plan for advanced integration guide for Java programmers in future.