Key bindings:

Key : Command
C : Clear Screen
BACKSPACE : Backspace
DELETE Delete
! : Store
@ : Recall
H : ~(1/2)
S : Square
T : Cube
~ : ~
P : PI
( : (
) : )
/ : /
* : *
- : -
+ : +
= | ENTER : =
- : ( - )
0-9 : 0-9

If you don't want to be prompted to save your data each time you exit, then uncheck "Options->Save Prompt on Exit" from the menu bar.

To use Jac from the command-line interface use:
> java -jar jac.jar c

To process an input file on the command-line use:
> java -jar jac.jar c < input_file

To enable logging use:
> java -Djava.util.logging.config.file=config.properties -jar jac.jar c