We will cover all of the topics below during class. For review and further reading, check out these sections from the Java Tutorials handbook.
The example below illustrates subclasses and inheritance. The superclass (parent class) Bicycle contains fields and methods that are inherited by the subclass (child class) MountainBike.
public class Bicycle {
// the Bicycle class has three fields
public int cadence;
public int gear;
public int speed;
// the Bicycle class has one constructor
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
// the Bicycle class has five methods
public void setCadence(int newValue) {
cadence = newValue;
}
public void setGear(int newValue) {
gear = newValue;
}
public void applyBrake(int decrement) {
speed -= decrement;
}
public void speedUp(int increment) {
speed += increment;
}
public void printDescription(){
System.out.println("\nBike is in gear " + this.gear
+ " with a cadence of " + this.cadence +
" and travelling at a speed of " + this.speed + ". ");
}
}
A class declaration for a MountainBike class that is a subclass of Bicycle might look like the code below.
MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and methods to get and set it (mountain bikes have seats that can be moved up and down as the terrain demands).
The printDescription() method demonstrates the concept of polymorphism: Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class.
public class MountainBike extends Bicycle {
// the MountainBike subclass has one field
public int seatHeight;
// the MountainBike subclass has one constructor
public MountainBike(int startHeight, int startCadence,
int startSpeed, int startGear) {
super(startCadence, startSpeed, startGear); // call the superclass constructor
seatHeight = startHeight;
}
// the MountainBike subclass has two methods
public String getSeatHeight(){
return this.seatHeight;
}
public void setSeatHeight(int newValue) {
seatHeight = newValue;
}
public void printDescription() {
super.printDescription(); // call superclass method
System.out.println("The MountainBike has a seat height of " +
getSeatHeight() + ".");
}
}