To take further design discussion,
As you have specified non setter/getter methods. I am interpreting it as interface methods which requires access to data/behavior of class There are two types of interface, push and pull interface. if pull-style interface for example web browser data, i.e. you desire information, you type URL and in response web page data shown. otherwise in push-style interface just transfer data to you, say Observer pattern.
You can use either style or both, depending on your design.
for e.g. in case of Collection object, both-style can work as follows,
pull-style: a for iterate collection object of cars for its each element as, car.startEngine();
push-style: for each element in 'otherObject', the startEngine() method calls property of car, i.e through otherObject.startEngine(car);
I would vote for the first approach, if you have multiple entities which you expect to have the startEngine() method, you can have an interface or abstract class (as per your need) like Vehicle or MachineWithEngine which has startEngine() method & then have your Car class extending/implementing it.