Java is platform independent because you can take byte code compiled on one platform and run it in a compatible JVM on some other platform. Given a new platform, you can't run Java at all until somebody writes a JVM so I wouldn't count on running on a TRS80 real soon.
It's easy to write code that breaks out of the JVM with JNI or exec() or even line-end schemes. And some JVMs have subtle differences in behavior. These things mean there is code that won't run on every JVM & platform combination.
Does that line up with your question?
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