You could reassign one to an object of some other subclass of car:
one = new Ford(...);
But you can't reassign two like that, since it's restricted to being an opel.
If m is a method that's defined in the opel class but not the car class, then the compiler will give you an error if you do this:
one.m();
But this is OK:
two.m();
since it knows two is restricted to being an opel, so it knows that method m will exist.
Generally, you want to declare your variables to be the broadest type possible. That is, if you're only going to use methods in car, then declare it with type car (like you did with one), because you're telling the reader that the algorithm only needs to know that one is a car, it doesn't need to know what kind of car it is.
More: It's necessary to understand that a variable has both a compile-time type and a runtime type. The compiler sees one as a car, because it doesn't know what kind of car the variable will be at any given time. But the runtime type of both will be opel. If you have a method mm that is defined for car, and then overridden for opel, one.mm() and two.mm() will both call the same method. Once the compiler looks at the compile-time type and decides the call is legal, then which one gets called when the program is run depends on the runtime type.
