What do you mean by write once rin everywhere?

Question

It's very popular about java that you write it once and Run it everywhere
What does this exactly means?
Does it mean if i wrote my program or app that i can run it on Android, Windows, Linux, IOS ?

Answer 1

It means java runs on dark magic also known as JVM (java virtual machine) - as long as your machine, be it a phone, clock, vending machine, pc or whatever supports it, java classes get compiled into bytecode that can be read by JVM on any machine

Answer 2

Yes the same file can run the same on different os with no difference.