java based programming challenges [closed]

Is there any website which provides core java based programming challenges that tests programming logic?

What is the biggest problem with Java?

Carmack concluded in 2005 about Java on cell-phones: "The biggest problem is that Java is really slow. On a pure cpu / memory / display / communications level, most modern cell phones should be considerably better gaming platforms than a Game Boy Advance.

Which are the biggest challenges you have faced while working on Java project?

The biggest challenge in Java world, in my opinion, is the bunch of tools/libraries, that have different versions, bugs etc. And struggling to bring them all together takes a lot of time. And there is no guarantee that specific set of libraries/tools will work together.

Project Euler

It's not Java specific, but a good Programming Challenge site for any language.