Example: Add Four Wheel Drive to Car

• Let's make an interface to add four wheel drive to our cars.
• We'll make a new car, too, called Talon.
• To make an interface, choose New-Interface.
• Then, you can add methods to this interface.  Remember, you cannot put any implementation (source code) in these methods, you can only define them.  We leave the logic up to the implementing class.
• Make two methods, fourWheelOn and fourWheelOff.
• Our new class Talon will be like class Cavalier, except the first line will state that it implements an interface.
• In implementing an interface, it must also define the methods we defined in the interface.
• So, let's make a boolean variable for fourWheelDriveOn
• Let's also decrease miles per gallon slightly when fourWheelDrive is on, and increase it again when fourWheelDrive is off.

First, our interface.

/*
 * FourWheelDrive.java
 *
 * Created on March 5, 2003, 12:54 AM
 */

/**
 *
 * @author  default
 */
public interface FourWheelDrive {

    public void fourWheelDriveOn();
 
    public void fourWheelDriveOff();
 
}

Now, our class Talon.

/*
 * Talon.java
 *
 * Created on March 5, 2003, 1:00 AM
 */

/**
 *
 * @author  jonesbr@email.uc.edu
 */
public class Talon extends Car implements FourWheelDrive {

    // true when on, false when off.
    boolean fourWheelDrive = false;
    static int talonMpg = 20;
 
    /** Creates a new instance of Talon */
    public Talon(int gallonsOfGas, int numberOfDoors, int numberOfGears)
    {
        super (gallonsOfGas, numberOfDoors, numberOfGears, talonMpg);
    }
 
 

    public void fourWheelDriveOff() {
        if (fourWheelDrive) {
            //increase the mpg back to normal, but only if four wheel drive was previously off.
            mpg += 5;
        }
        // change fourWheelDrive to off again.
        fourWheelDrive = false;
    }
 
    public void fourWheelDriveOn() {
        if (!fourWheelDrive) {
            System.out.println("Turning 4wd on, previously off.");
            // decrease mpg by 5 when turned on.
            mpg = mpg -5;
        }
        // turn it on.
        fourWheelDrive = true;
    }
 
}
 
 
 

Changes to Controller

• added three new cars.
• added method mixedTerrain, passed car4 to it to turn on four wheel drive indirectly.
• added a call to car5 to turn four wheel drive on directly.
• increased distance to 100 to demonstrate difference in miles per gallon.
• other minor changes.

/**
 *
 * @author  default
 */

// must import this class to use it.
import java.util.ArrayList;

// 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() {
        // instantiate our two vehicles.
        Cavalier car1 = new Cavalier(6, 2, 4);
        Hybrid car2 = new Hybrid(500, 8, 4, 5, 68);
 
        // Add a few Talons.
        Talon car3 = new Talon(10, 2, 4);
        Talon car4 = new Talon(10, 2, 4);
        Talon car5 = new Talon(10, 2, 4);
 
        // Tell car 4 it is going to be in mixed terrain.
        // Note the use of polymorphism with an interface.
        mixedTerrain(car4);
 
 
        // Turn four wheel drive on for car 5 as well.
        // This does the same thing as the method above, but
        // 1) it does not use polymorphism, and
        // 2) it demonstrates that we can call the method directly instead of using polymorphism.
        car5.fourWheelDriveOn();
 
        // Make an ArrayList to hold our vehicles.  Put them in it.
        ArrayList cars = new ArrayList();
        cars.add(car1);
        cars.add(car2);
 
        // Add the talons to our ArrayList.
        cars.add(car3);
        cars.add(car4);
        cars.add(car5);
 
        run(cars);

    }

    // run executes the go() method on the vehicle passed in through the ArrayList.
    public void run(ArrayList vehs) {
        // for loop iterates through the ArrayList.
        for (int i = 0; i < vehs.size(); i++) {
            // For informational purposes only.
            System.out.println("Car: " + (i + 1));
            // get the Vehicle out of the ArrayList and cast it from an object to a vehicle.
            Vehicle v = (Vehicle) vehs.get(i);
            // tell it to go 10 units.
            v.go(100);
        }
    }

    public void mixedTerrain(FourWheelDrive fwd) {
        fwd.fourWheelDriveOn();
    }
 
    // Instantiate and start a Controller object.
    public static void main(String[] args) {
        Controller controller = new Controller();
        controller.begin();
    }

}
 

All other classes remain the same.

Results:
Notice that, even though car3, car4, and car5 are all Talons, car4 and car5 have 1 less gallon of gas, because we turned four wheel drive on.

Turning 4wd on, previously off.
Turning 4wd on, previously off.
Car: 1
Travelled 100
Gallons of Gas remaining: 3
Car: 2
Travelling via electric mode
KWH remaining: 300
Car: 3
Travelled 100
Gallons of Gas remaining: 5
Car: 4
Travelled 100
Gallons of Gas remaining: 4
Car: 5
Travelled 100
Gallons of Gas remaining: 4