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The single most important feature of Java is reverse compatibility. It is a temporal prerequisite of "write once, run everywhere".

Do you mean forwards compatibility ?

I can't take Java 1.8 code and run it on a Java 1.7 JVM.

The 1.8 JVM implementation is compatible with the interface provided by earlier ones. That's what's meant by backwards-compatible.

"write once, debug everywhere".

I've had very little of that personally. Every project I've worked on has had no compatibility issues on different platforms.