Inheritance

Now suppose you also need a class for a motorcycle. A motorcycle also has a make, a model, a year, a speed, a maximum speed, a weight, a price, a number of passengers it can carry, two wheels and many other properties you can represent with fields. A class that represents a motorcycle might look like this:

public class Motorcycle {

  private String licensePlate;      // e.g. "New York A456 324"
  private double speed;             // kilometers per hour
  private double maxSpeed;          // kilometers per hour
  private String make;              // e.g. "Harley-Davidson"
  private String model;             // e.g. "panhead"
  private int    year;              // e.g. 1997, 1998, 1999, 2000, 2001, etc.
  private int    numberPassengers;  // e.g. 4
  private int    numberWheels = 2;  // all motorcycles have two wheels
  
  
  // constructors
  public Motorcycle(String licensePlate, double maxSpeed,
   String make, String model, int year, int numberOfPassengers) {

    this(licensePlate, maxSpeed, make, model, year, numberOfPassengers);
    
  }

  public Motorcycle(String licensePlate, double speed, double maxSpeed,
   String make, String model, int year, int numberOfPassengers) {

    this(licensePlate, speed, maxSpeed, make, model, year, 
     numberOfPassengers);
    
  }

  public Motorcycle(String licensePlate, double speed, double maxSpeed,
   String make, String model, int year, int numberOfPassengers) {

    // I could add some more constraints like the
    // number of doors being positive but I won't
    // so that this example doesn't get too big.
    this.licensePlate = licensePlate; 
    this.make = make; 
    this.model = model; 
    this.year = year; 
    this.numberPassengers = numberOfPassengers; 

    if (maxSpeed >= 0.0) {
      this.maxSpeed = maxSpeed;
    }
    else {
      maxSpeed = 0.0;
    }
    
    if (speed < 0.0) {
      speed = 0.0;
    }
    
    if (speed <= maxSpeed) {
      this.speed = speed;
    }
    else {
      this.speed = maxSpeed;
    }
    
  }
  
  
  // getter (accessor) methods
  public String getLicensePlate() {
    return this.licensePlate;
  }

  public String getMake() {
    return this.make;
  }

  public String getModel() {
    return this.model;
  }

  public int getYear() {
    return this.year;
  }
  
  public int getNumberOfPassengers() {
    return this.numberPassengers;
  }
  
  public int getNumberOfPassengers() {
    return this.numberWheels;
  }
  
  public double getMaxSpeed() {
    return this.maxSpeed;
  }

  public double getSpeed() {
    return this.speed;
  }

  // setter method for the license plate property
  public void setLicensePlate(String licensePlate) {
    this.licensePlate = licensePlate;
  }

  // accelerate to maximum speed
  // put the pedal to the metal
  public void floorIt() {
    this.speed = this.maxSpeed;  
  }
  
  public void accelerate(double deltaV) {

     this.speed = this.speed + deltaV;
     if (this.speed > this.maxSpeed) {
       this.speed = this.maxSpeed; 
     }
     if (this.speed < 0.0) {
       this.speed = 0.0; 
     }     
     
  }
  
}

There's a lot of overlap between the class definitions for Car and Motorcycle. In fact the only things that are different are the constructors and a few of the fields. Inheritance takes advantage of the overlap.