Pure Ruby
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In Java, calling executables programs is hard (check out the amount of code it takes to process the input and output). Wrapping a C library is also difficult. You've got to do lots of futzing with the Java Native Interface.
Then Sun came up with the 100% Pure Java marketing campaign to promote writing applications without resorting to native code or calling external programs. This ensured maximum portability for Java applications. There is a lot of dissension and a number of spectacular failures (like Netscape's "Javagator" browser) but being 100% Pure Java is still highly regarded in the Java community, as a quick search will show. Look at the number of open source projects that come up, touting their 100% Pure Java status.
Between the difficulty of calling non-Java programs and the cultural bias against doing it, there is not a lot of this going on. That means there are things that either were re-implemented in Java, don't exist, or aren't considered.
Meanwhile, in Ruby (and Perl and Python -- the "glue" languages) the use of C extensions and calling off to external programs is widespread. Sure, a "Pure Ruby" API may be preferred if it exists, but it's not uncommon for performance bottlenecks to be re-written in C, or for Ruby libraries to wrap C libraries with a Ruby-esque interface.
This does have some problems. Not all C libraries are available on all platforms. Some scripts and libraries only work on Linux and other Unix-like operating systems. A solution that only works on most platforms would be unacceptable to many Java programmers. But it does get Ruby most of the way there, and it brings a huge advantage: Ruby can use much of the existing C libraries (or even Java, with GCJ).