How do I extract code from an arduino?
The short answer: You don't.
With enough know-how, you could probably extract the executable binary from the Arduino, but the source code is not installed on the device. You would need to run a decompiler on the binary. (Or read the machine code directly.) The output of a decompiler is usually pretty ugly however, and will look quite different from the original source-code. It won't have meaningful variable names, class names, or function names, and the code structure will likely be a little different than the original source code.
EDIT:
As mckenzm pointed out in his answer, you could download the binary image from your Arduino, and then copy it onto another Arduino, but all that would let you do is to make an exact copy of the program. You wouldn't be able to modify it at all.