If it's "Write once run anywhere", why do so many Java programs tell me to pick the version for my OS?

Question

I am fairly new to Java or programming in general. On my journeys through the internet to master this language I have come up the saying "write once run anywhere" multiple times.

But I have found many software that requires you to pick the right version for your OS. Sometimes there is only one version available.

Could you explain to me why that is so?

Answer 1

_{[expanded per the comments]}

Java runs on a Virtual Machine, the JVM. In an ideal world this means that the Operating System is abstracted away behind this and you only have to make sure your code works with the JVM which will make it work with the underlying OS. This can already be undone by using the wrong path separators or line endings; it is not an absolute truth.

An application can use many Operating System-specific approaches/libraries/functions/etc that might make it not feasible to restrict yourself to one general codebase. Instead they might want to leverage some advantages provided by a platform and create a separate application with it.

The statement should probably be somewhere along the lines of "Write once in a general fashion, run anywhere" but that's not as snappy.

This statement is often linked to Java but there are also other languages that incorporate this: weblanguages like Javascript and HTML will run on any browser because the browser itself forms the abstraction between the language and the underlying OS.

Other languages don't have this (entirely?) since they work differently: C# will use the underlying .NET framework which as it is only exists for Windows. There exists a cross platform variant (Mono) but it would be an overstatement to consider C# truly cross platform.