Chapter 27 Ex 2

caa5042 · August 12th, 2011, 03:05 PM

In the solution for Ex 2, i dont see a call to the 'Compare' method in the CarComparer class...the method that has two car objects as parameters am i missing something?

Rod Stephens · August 14th, 2011, 04:59 PM

I can see it in my version it looks like this:

Code:

// Compare two cars.
public int Compare(Car car1, Car car2)
{
    // Compare differently depending on the sort type.
    switch (Sort)
    {
        case SortType.ByName:
            return car1.Name.CompareTo(car2.Name);
        case SortType.ByMaxSpeed:
            return -car1.MaximumSpeed.CompareTo(car2.MaximumSpeed);
        case SortType.ByHorsepower:
            return -car1.Horsepower.CompareTo(car2.Horsepower);
        case SortType.ByPrice:
            return -car1.Price.CompareTo(car2.Price);
    }

    // This should never happen (unless we add a
    // new SortStyle value and forget to update the cases.)
    throw new Exception("Unknown SortType value");
}

Does your version of the program run? It shouldn't work if that method isn't there. Perhaps there was something wrong with the download?

If you can't get your version to work, email me at [email protected] and I'll send you a new version.

caa5042 · August 14th, 2011, 06:01 PM

Hi Rod,
Yes I do see the method. But i dont see a call to the method. The program runs fine but I'm just trying to figure out what I am missing...

Rod Stephens · August 14th, 2011, 10:24 PM

Ah. Gotcha.

The ListBox control itself calls the Compare method when it needs to sort the items. If you set a breakpoint in the Compare method, you'll see the code stop several times when the control needs to sort the items.

(The ListBox knows that the object has a Compare method because it implements IComparer.)

caa5042 · August 14th, 2011, 11:45 PM

oh...so we dont have to invoke the method for it to actually execute its code?...i see the Compare method has two car objects as parameters....How does the program know to call that method and pass two car objects to it if there is no code in there that does that?...

Rod Stephens · August 15th, 2011, 08:56 AM

Sometimes you don't need to invoke the method yourself, particularly when you implement an interface. Often you are required to provide an object that implements an interface just so the thing using it knows that it provides a certain service, in this case comparing two objects.

An array's Sort method can take as a parameter an object that implements IComparer. that lets the Sort method know that this object can sort.

It's a generic interface so the Sort method doesn't know what type of things it can compare (in this case it's Cars), but it knows that the array contains whatever those things are. So it blindly uses the IComparer to compare the objects in the array without knowing what they are. It doesn't need to know, it just needs to know how to compare them.

You could even do that yourself. If I gave you a list of objects and a way to tell which was bigger for any two of them, you could run through the list comparing them to find the "biggest."

To summarize, usually you write code for your code to invoke, sometimes your code calls code provided by the system, and sometimes you write code that the system can call for you.