If JAVA language realy cross platformed, why i can't start it on linux deepin?

Answer 1

The JVM that runs the Java code is not platform independent. You can see that on the download page. There are different downloads for Windows, Linux, MacOS and Solaris for the JRE/JDK. But you don't have to change your code to run it in another JVM.
So Java code is somehow platform independent (although gui might be displayed different on another platform) whereas the runtime is definitely not.

Answer 2

It's as close to cross-platform as you can get, but sometimes needs a little work.