what is the .class file

Answer 1

When you compile your program, it will automatically create a .class file for every class you have there.

Class files contain bytecode, which is instruction for JVM, which then translates the bytecode into platform-specific (Windows, Linux etc.) machine level instruction.

Thanks to this, Java is platform independent. :-)

Answer 2

is a file of java class, compiled :)

Answer 3

heores is correct during compilation of source codes it generates .class files for each class. .class now contains java byte code which jvm understands and executes it's human unreadable.