To answer your question you can find out what exactly is in your garage you do the following:
Vehicle v = myGarage[0];
if (v instanceof Car) {
// This vehicle is a car
((Car)v).doSomeCarStuff();
} else if(v instanceof Boat){
// This vehicle is a boat
((Boat)v).doSomeBoatStuff();
}
UPDATE: As you can read from the comments below, this method is okay for simple solutions but it is not good practice, particularly if you have a huge number of vehicles in your garage. So use it only if you know the garage will stay small. If that's not the case, search for "Avoiding instanceof" on stack overflow, there are multiple ways to do it.
