Explain Overriding Methods: The Solution
The object oriented solution to this problem is to describe a new class, call it SlowCar, that inherits from Car and imposes the additional constraint in which a car may not go faster than 70 mph (112.65 kph).
To do this you'll required to adjust the two places that speed can be changed, the constructor and the accelerate() method. The constructor has a different name since all constructors are named after their classes other than the accelerate() method must be overridden. This means the subclass has techniques with the similar signature as the method in the superclass.
public class SlowCar extends Car {
private static final double speedLimit = 112.65408; // kph == 70 mph
public SlowCar(String licensePlate, double speed, double maxSpeed,
String make, String model, int year, int numberOfPassengers, int numDoors) {
super(licensePlate, 0, maxSpeed, make, model, year,
numberOfPassengers, numDoors);
this.accelerate(speed);
}
public void accelerate(double deltaV) {
double speed = this.getSpeed() + deltaV;
if (speed > speedLimit) {
super.accelerate(speedLimit - this.getSpeed());
}
else {
super.accelerate(deltaV);
}
}
}
The first thing to remember about this class is what it doesn't have, getSpeed(), getLicensePlate(), getMaximumSpeed(), setLicensePlate() methods or speed, maxSpeed and numDoors fields. All of these are given through the superclass Car. Nothing about them has modified so they don't required to be repeated here.
Further look at the accelerate() method. This is various than the accelerate() method in Car. It imposes the additional constraint.
The constructor is a little more complicated. First remember that if you're going to use a non-default constructor, which is a constructor with arguments, you do required to write a constructor for the subclass, even if it's just going to do the exact similar thing as the matching constructor in the superclass. You cannot easily inherit Car's constructor because in which constructor is named Car() and this one must be named SlowCar().
The constructor requires to set the value of name, url, and description. Therefore they're not accessible from the subclass. In the other words they are set through calling the superclass's constructor using the keyword super. While super is used as a method in the first non-blank line of a constructor, it stands for the constructor of this class's superclass.
The immediate superclass's constructor will be known as in the first non-blank line of the subclass's constructor. If you don't call it explicitly, then Java will call it for you along with no arguments. It's a compile time error if the immediate superclass doesn't have a constructor with no arguments and you don't call a various constructor in the first line of the subclass's constructor.
The use of the ternary operator in the constructor call is unusual. Therefore, it's essential to meet the compiler's requirement that the invocation of super be the first line in the subclass constructor. Or else this could be written more clearly using only if-else.