Java Programming

When compiling java source programmes, will the java compiler (javac) add different extra information each time to the class files?

I encounter a problem when using default unix comparison tool - cmp. In the cygwin environment, if I wrote a hello world programe, e.g. hello.c and compiled it (`gcc -o hello hello.c` and gcc -o hello1 hello.c) into different binary name, say `hello' and `hello1'. Then compare these two binary files with command `cmp hello.exe hello1.exe`. The result show these binary files are not equal (hello.exe and hello1.exe differ: char 137, line 2).

I did this with java version (also in cygwin version) by compiling Hello.java into Hello1.class and Hello.class. Though result shows they are the same, I wonder if the java with different version or in different environment (e.g. un*x and window) will add different extra information each time during compilation or is there any command I can use to prevent javac to add any extra information to the class files?

Or anyone is able to point me to where containing related information/ document?

Thank you very much.