Sort ArrayList using Comparable

Question

I'm working on a project where I need to be able to sort an ArrayList of Car objects by price. In my Car class, I have

public class Car implements Comparable

and in the body of the code is the compareTo method:

    public int compareTo(Object o)
    {
        Car rhs = (Car)o;

        if (price > rhs.price)
            return 1;
        else if (price < rhs.price)
            return -1;
        else
            return 0;
    }

I just don't understand how to implement this method to sort by price- what does carList need to be compared to? I know this isn't correct but so far this is the sorting method.

public void sortByPrice()
{
    Collections.sort(carList.compareTo(o));
}

David Conrad · Accepted Answer · 2015-02-12 19:29:26Z

5

Two problems: one syntatical and one conceptual.

The first issue is that while your compareTo is technically correct, you want to type-bind it to Car instead of Object.

public class Car implements Comparable<Car>

Inside of your compareTo method you'd then substitute Object for Car. You would also want to check for null.

The second is that sortByPrice sounds specific, but since compareTo is comparing based on price, that's somewhat okay.

All you'd need to do is call Collections.sort on the actual collection:

Collections.sort(carList);

edited Feb 12, 2015 at 19:29

David Conrad

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answered Feb 12, 2015 at 19:27

Makoto

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1 Comment

Laura Over a year ago

Thank you! this makes a lot more sense to me now

Smutje · Accepted Answer · 2015-02-12 19:28:08Z

1

Normally, one sorts a collection using

Collections.sort(collection)

while collection has to implement Comparable and sort uses the compareTo method to sort collection.

answered Feb 12, 2015 at 19:28

Smutje

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Comments

Giovanni Botta · Accepted Answer · 2015-02-12 19:29:11Z

0

Your Car class must implement Comparable<Car>. Then your compareTo method will have signature:

public int compareTo(Car other) {}

As per the documentation, this method should:

Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

Then given a List<Car>, say list, you can call Collections.sort(list).

answered Feb 12, 2015 at 19:29

Giovanni Botta

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Comments

buddy123 · Accepted Answer · 2015-02-12 19:29:45Z

0

You're almost done! Only call to Collections.sort(carList); and that will by itself, use the overridden compareTo. Actually, when you're not implementing compareTo, you'll have the very basic implementation, and calling Collections.sort(..) will use the basic implementation, which is comparing pointers in that case.

answered Feb 12, 2015 at 19:29

buddy123

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