100% Pure Java code is code that conforms to the Java ideal of universal portability. Writing an application that doesn't exploit operating system/platform specific features is a great start, but you also have to write the code in a platform neutral way. This includes things such as not hardwiring the '/' symbol as a file separator on Unix systems, as this will fail on a Wintel system that use '\'. If you don't rely on the core Java API's, and instead use native methods, this can also disqualify an application from being 100%.
Developers who wish to use the 100% Pure Java logo must have their application independently verified, using the Java Purity Checker software. Full details are available from Sun, but developers who wish to write portable code can read the 'cookbook' for writing pure applications for free.
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