.class file in Java is the core of achieving platform independence which is a key feature of Java. class files are compiled form of any Java program written in byte codes. Since the Java program executes inside JVM, these instructions are read by JVM and then translated into machine code for the underlying platform. effectively you can run the class file on any platform if you have a corresponding JVM supported for that platform. Java compilers are responsible for generating .class files and they follow the class file format specified by Sun and it's different based on different Java versions.