I doubt many will download a rar to look at some code, but getMission and setMission presumably deal with a mission, so you'd expect setMission to take a Mission argument and getMission to return a Mission. Usually, in setMission, the Mission would be stored in a class variable, and getMission would return that variable. Similarly for isComplete and setComplete. The Java convention for methods returning a boolean value is to call them isXxxxx rather than getXxxxx because it makes the code read better. But they're usually handled similarly - setComplete sets a boolean class variable and isComplete returns that variable. Of course, the setXxxx methods might do some other stuff related to setting the relevant variable, but more often than not, they just set the class variable.
As for SpaceMerchant, hoping for a little direction is a bit vague. You need to explain exactly what the bit you're having trouble with is supposed to do, and why you're having trouble with it. Posting some relevant
code wouldn't hurt either.