Unfortunately, the porblem with new things is that they are not tested thoroughly. It's especially important in math libraries, that they were tested by many people. I deally, I'd love to gave GPL's CERNLib or BESMLib in Java.
GSL - GNU Scientific Library is in C/C++. It may be easier to convert to Java.
Also, I found out that Numerical Recipes is available on-line. It's a surprise.