Singleton Example

• Singletons are often used with either a controller class, that controls the flow of the program, or a resource pool, such as an FTP pool or JDBC (database) pool.
• Let's say we have a class, Road, and we want to put Cars on the Road.  But we only have one Road, so we'll only have one road object.  And that road object has a maximum car capacity.
• Then, we can instantiate as many Cars as we want, but we can only have one road.  The road is our singleton.
• The Source:
• Road - the singleton


/*
 * Road.java
 *
 * Created on April 2, 2002, 8:38 PM
 */

package singleton;

/**
 *
 * @author  default
 */
public class Road {

    // Max capacity for this road.
    int capacity = 5;

    // The Array will hold our car objects.
    Car[] cars = new Car[capacity];

    // number of cars currently on the road.
    int carCount = 0;
 
    // This is an instance of the singleton Road object.
    static Road road = null;
 
    /** Creates a new instance of Road.  This is private, and can only be called by the Road class itself. */
    private Road() {
    }
 
    /***
     * get an instance of the Road singleton.
     *
     */
 
    public static Road roadInstance() {
        // Pardon my use of an if test.
        if (road == null) {
            // Call the constructor internally.
            road = new Road();
        }
        // Return the road singleton.
        return road;
    }
 
    /**
     * Add a car to the road.
     *
     * @param car the car to add.
     */
 
    public void addCar(Car car) throws ArrayIndexOutOfBoundsException {
        // Note that, if we exceed capacity, the method will automatically throw an exception.
        cars[carCount] = car;
        carCount++;
        System.out.println("Car # " + carCount + " added.");
    }

}
 

• Car


/*
 * Car.java
 *
 * Created on April 2, 2002, 8:48 PM
 */

package singleton;

/**
 *
 * @author  default
 */
public class Car {

    /**
     * Creates a new instance of Car
     *
     * This is nothing more than a holder class for our example.
     * We could add more detail if we really wanted to.
     */
    public Car() {
    }

}
 

• AddCarsToRoad (with the public static void main(String args[]) method) :


/*
 * AddCarsToRoad.java
 *
 * Created on April 2, 2002, 8:49 PM
 */

package singleton;

/**
 *
 * @author  default
 */
public class AddCarsToRoad {

    /** Creates a new instance of AddCarsToRoad */
    public AddCarsToRoad() {
    }
 
    /**
     * Start from the command line.  Add as many cars as we wish.
     *
     *
     */
 
    public static void main(String args[]) {
        // Get the number of cars to add from the command line.
        int numberOfCars = Integer.parseInt(args[0]);
        // Add the cars, one at a time.
        for (int i = 0; i < numberOfCars; i++) {
 
            try {
                Car car = new Car();
                Road road = Road.roadInstance();
                road.addCar(car);
            } catch (ArrayIndexOutOfBoundsException e) {
                System.out.println("You've added too many cars!");
            }
        }
    }

}
 
 

• The results, when attempting to add 7 cars:


Car # 1 added.
Car # 2 added.
Car # 3 added.
Car # 4 added.
Car # 5 added.
You've added too many cars!
You've added too many cars!
 

Created by:  Brandan Jones December 17, 2001