Inheritance and Polymorphism Java
- Polymorphism is an important topic of object-oriented programming. It is also very powerfull.
- Let's say you had a class called Controller that uses the classes discussed in the previous slide. It instantiates an objects of class Cavalier and Hybrid. Then, it passes them to another method as type Vehicle. The other method executes the go() method on both objects. Which go method gets executed?
- The answer may suprize you.
- For the Cavalier object, car1, there is only one go() method to run - the method defined in class Vehicle. Therefore, that is the method run.
- However, the Hybrid object, car2, overrides the go() method. But wait, the run() method of class Controller accepts a Vehicle type, not a Hybrid type, so wouldn't it run the method in Vehicle? No, it would not. Polymorphism states that the go() method in Hybrid should be run. Java knows how to do this via dynamic binding.
- What if we wanted to run one of the methods specific to class Hybrid in this method? We can't, unless we cast it to a Hybrid.
- And while we're at it...
- Yes, we did pass an object of type Car and an object of type Hybrid into a method that accepts a parameter of type Vehicle. This is perfectly legal. We can pass an object as a parameter if:
- That object type is the same as the object type in the parameter list of the method, or
- That object type is a subclass of the object type in the parameter list of the method.
// Class controller uses the Vehicle classes.
public class Controller {
// Default constructor.
public Controller() {
}
// Begin method instantiates two vehicles and
passes them to run.
public void begin() {
Cavalier car1 = new
Cavalier(6, 2, 4);
Hybrid car2 = new Hybrid(5,
8, 4, 5, 68);
run(car1);
run(car2);
}
// run executes the go() method on the vehicle
passed in.
public void run(Vehicle v) {
v.go(10);
}
// Instantiate and start a Controller object.
public static void main(String[] args) {
Controller controller
= new Controller();
controller.begin();
}
}