I've read the book or two about the Java. Have a become a programmer? I don't think so. What have I do next? I think, I must get some practice. Where can I get it? For example, involving in an open source project. But most of them are too difficult for me and any other beginner. So, this is my question. Can you recommend any source of programming experience for the beginner?
You could also invent your own little or big projects. Try programming a simple game, for example. One of the first larger Java programs that I wrote (together with a colleague, in 1998, using Java 1.1...) was a chess playing applet. We implemented our own chess playing algorithms and let them play against each other.
I've watched the Olympics for years, but that does not make me an Olympic athlete. Heck, it doesn't even make me an athlete.
The only way to be a programmer is to program. a lot. Project Euler has some interesting problems. the JavaRanch has the set of CattleDrive problems. There are probably thousands of open source projects you could get involved with.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors
there are quite a few similar sites with more problems.
Apart from that you can check out some of the freelance coding websites and see if someone will let you build something simple for way too little money. Just to get some real life experience with an actual customer under your belt.
And then there is the open-source stuff mentioned before.
Hope this gives you some ideas!
If it wasn't for physics and law enforcement, I'd be unstoppable!