I had some knowledge at the university and now I wanted to return to Java and to became an expert in that language.
I'm reading First Head Java which is a great great book, i'm loving it.
For my next step to became an expert I want to start practising and I would like if you can tell me a book or a site where I can have like "mini projects" to do for example in one week where I have to read the exercise design what I need to do and implement it. I would like something with good and different important exercises to practise....
Another question, what do you think that is more interesting and with more market. programing in J2SE more closer to the database, or doing GUI's and all the code behind it to connect with the core code?
What about the other question... GUI's or j2SE what's the best to learn and to become an expert?
Looks like you are confused among the two. Actually you need J2SE that is Java 2 standard edition to compile and run all your code including the one that does the GUI stuff.
As far as GUI is concerned you need to use either AWT or Swing but both require knowledge of Java programming.
But to be an expert you need to think beyond J2SE because Java is not as popular for desktop and intranet applications as it is for distributed and web applications.But there is some time for you before you get your hands on J2EE.
Why do you want to become a 'Java expert' - to get a job ? If so, the Java skills to learn are listed in the job advertisements on your local web site. Remember LOCAL - you do not say where you live but try Dice.com / NYTimes.com (USA) or jobserve.com (EU). Different localities require different job skills. A hint : Do not waste your time on Swing. Swing is a large topic and requires Threads (also a large topic). Most jobs are in companies (selling every thing from pizza to credit derivatives) so J2EE is the one.
Head First Java is a child's comic book. Try Sierra & Bates SCJP Guide or (the best) Mughal & Rasmussen 'Java Certification' with extra web material (it's only Java 1.4).