well.. I've been looking for problems like that and what I've found.. is IMO.. not very essential for me in developing my skills.. and looking for a quiz like problem isn't really helpful for me..
well In my opinion (IMO) .. if you know the basics.. like arrays, loops, datatypes, classes, methods, etc.. then you don't need school like problem solving....
heres what you need to better develop your skill.. IMO..
1. Plan a big project. Think of what you want to learn and focus on it.. JAVA api is really big.. i'll bet you can't learn them all..
2. Develop or researching ability and your skill in using google.. by what I call.. what-do-I-need and how-do-I-get-it method..
example: .. If your project needs a JFrame, or needs an interface.. then search google for API documentation on how to use em..
3. If you got bored.. like me.. hang here at DIC and try to solve all the problem posted here.. this is very essential simply because most problems here are ACTUAL PROBLEMS not theoretical problems..
4. If you master GOOGLE.. and you develop your skill in understanding documentation.. well that's the time you go search for theoretical problems.. school like quiz problems.. that will consume your free time.. lmao.. but I don't think its essential..
..

.. LEARN how to understand the API documentation.. that is the most useful and fruitful thing you could do that you wont regret..