Platform independence in Java's Byte Code

Question

I sometimes wonder why Java is referred as a Platform Independent Language?

I couldn't find a proper explanation of the below points :

1. Is the JVM same for Windows/Linux/Mac OS?
2. Are the bytecode generated same for a same Class in the above environments?

If the answer to the above questions are NO then how the platform independence is achieved.

Please help me out in learning this basic concept.

Answer 1

Is the JVM same for Windows/Linux/Mac OS?

Not at all. Compiler is same across the platforms. But, since it is an executable file, the file itself will be different i.e. on Windows, it would be .exe, on Linux, it would be Linux executable etc.

Are the bytecode generated same for a same Class in the above environments?

Yes. That is why Java is COMPILE ONCE. RUN ANYWHERE.

Before starting please read this doc by oracle