Please briefly describe why the java process running the program, as well as cross-platform java

First run is to use java javac tool to compile the file suffix .class files. Then put .class files on a virtual machine is running.

java has been able to cross-platform because it has a virtual machine (JVM), run java is run on the JVM is not on the computer, each platform has its own virtual machine, so you can pass the JVM is what platform so you compile and run the appropriate class file.