Java Using The Comparable Interface

Question

I am working on a homework assignment on implementing interfaces, and am a little lost. I need to implement the comparable interface and use the compareTo() method. Here is the code of my super class, it has three subclasses that are all different forms of vehicles. In this case I am trying to campare the number of doors that they have.

Below is the code for the "Vehicle" superclass

package vehicle;

abstract public class Vehicle implements Comparable {
    private String color;
    private int numberOfDoors;

    // Constructor
    /**
     * Creates a vehicle with a color and number of doors
     * @param aColor The color of the vehicle
     * @param aNumberOfDoors The number of doors
     */
    public Vehicle(String aColor, int aNumberOfDoors) {
        this.color = aColor;
        this.numberOfDoors = aNumberOfDoors;
    }

    // Getters
    /**
     * Gets the color of the vehicle
     * @return The color of the vehicle
     */
    public String getColor() {return(this.color);}
    /**
     * Gets the number of doors the vehicle has
     * @return The number of doors the vehicle has
     */
    public int getNumberOfDoors() {return(this.numberOfDoors);}

    // Setters
    /**
     * Sets the color of the vehicle
     * @param colorSet The color of the vehicle
     */
    public void setColor(String colorSet) {this.color = colorSet;}
    /**
     * Sets the number of doors for the vehicle
     * @param numberOfDoorsSet The number of doors to be set to the vehicle
     */
    public void setNumberOfDoors(int numberOfDoorsSet) {this.numberOfDoors = numberOfDoorsSet;}

    public int compareTo(Object o) {
        if (o instanceof Vehicle) {
            Vehicle v = (Vehicle)o;
        }
        else {
            return 0;
        }
    }

    /**
     * Returns a short string describing the vehicle
     * @return a description of the vehicle
     */
    @Override
    public String toString() {
        String answer = "The car's color is "+this.color
                +". The number of doors is"+this.numberOfDoors;
        return answer;
    }
}

Currently it is a work in progress, and I'm not to sure where to go from here on the compareTo method. Any help is much appreciated.

Thanks!

Edit Once I get the compareTo() method working in the superclass, is there anything that I need to add to the subclasses to make this function?

Thanks!

Louis Wasserman · Accepted Answer · 2012-04-12 19:17:42Z

4

You should have Vehicle implement Comparable<Vehicle>, so your compareTo method can take a Vehicle argument rather than having to cast.

But if you're asking how to implement the compareTo method, then if this vehicle is supposed to be less than the other vehicle, return a negative number; if it's supposed to be greater, return a positive number, and if they're supposed to be equal, return 0. You'll probably be using color.compareTo(otherVehicle.color) to compare the colors, since they are Strings.

That should be enough hints!

answered Apr 12, 2012 at 19:17

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

salxander Over a year ago

Thank you, this is helpful. Though I'm still unsure of how to properly use the compareTo() method from here. I'm not sure how to go about comparing the numberOfDoors int with these objects

Louis Wasserman Over a year ago

You can just compare those the old fashioned way. if (this.numberOfDoors < otherVehicle.numberOfDoors) { ... } else {...}.

Alexey Berezkin · Accepted Answer · 2012-04-13 07:11:32Z

2

If the comparison should be based only on doors number, try the following:

public int compareTo(Object o) {
    if (o instanceof Vehicle) {
        Vehicle v = (Vehicle) o;
        return getNumberofDoors() - v.getNumberOfDoors();
    } else {
        return 0;
    }
}

edited Apr 13, 2012 at 7:11

answered Apr 12, 2012 at 19:21

Alexey Berezkin

1,5531 gold badge11 silver badges19 bronze badges

Comments

tskuzzy · Accepted Answer · 2012-04-12 19:19:53Z

1

compareTo() is supposed to return a int representing how the current object compares to the given object (the parameter). In other words, if your current object is "less than" the passed object, then it should return a negative number.

answered Apr 12, 2012 at 19:19

tskuzzy

36.6k15 gold badges78 silver badges145 bronze badges

Comments

jzworkman · Accepted Answer · 2012-04-12 19:23:25Z

1

First have your Vehicle implement Comparable<Vehicle> as Louis said. Then in your compareTo method you want to return a value comparing the aspect of the Vehicle you want them to be compared by.

public int compareTo(Vehicle v) {
        return this.getNumberOfDoors() - v.getNumberOfDoors();
    }

This will return a zero if the number of doors is the same. A negative value if v has more doors, and a positive value if v has less doors.

You could also compare something else(like make of the car if you add that)

public int compareTo(Vehicle v) {
            return this.make.compareTo(v.make);
        }

answered Apr 12, 2012 at 19:23

jzworkman

2,71316 silver badges20 bronze badges

Collectives™ on Stack Overflow

Java Using The Comparable Interface

4 Answers 4

2 Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related