Which one is better?

[ankiit]

Hello Mentors,

I was going through a code, the code is as follows:-

/*
			BoxDemo1 application
		demonstrates the usage of a simple function with a return type, function name = getVol
*/
class Box
{
    double width;
    double height;
    double depth;

	//Method declaration

	double getVol(double w, double h, double d)

	 {

	return w*h*d;

	 }

}

class BoxDemo2
{

	public static void main(String[] args)
	{
	 Box mbox1 = new Box();
	 Box mbox2= new Box();
	 double vol;

/* Assigning  values to the instance variable*/

/*	mbox1.width=10;
	mbox1.height=20;
	mbox1.depth=15;

	mbox2.width=3;
	mbox2.height=6;
	mbox2.depth=10;
*/

/* Calculating volumes*/


	vol=mbox1.getVol(10,20,15);
	System.out.println("the volume of box1 is:=" + vol);
	vol=mbox2.getVol(3,6,10);
	System.out.println("the volume of box1 is:=" + vol);

	//System.out.println("Sum of the two volumes is:=" + vol1+vol2);


	}

}

The book from which I am studying Java, specifies a different way of performing the same work as the above code, and the code from the book is as follows:-

class Box {
double width;
double height;
double depth;
// compute and return volume
double volume() {
return width * height * depth;
}
// sets dimensions of box
void setDim(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
}
class BoxDemo5 {
public static void main(String args[]) {
Box mybox1 = new Box();
Box mybox2 = new Box();
double vol;
// initialize each box
mybox1.setDim(10, 20, 15);
mybox2.setDim(3, 6, 9);
// get volume of first box
vol = mybox1.volume();
System.out.println("Volume is " + vol);
// get volume of second box
vol = mybox2.volume();
System.out.println("Volume is " + vol);
}
}

I want to know, why is there a need to create an extra function, "setDimensions(<parameter>)" here, if the work can be done with one function only.

Though I feel that it is done only for better understanding of the example, in case there are other reasons request you to please share them.

Thanks in Advance
Ankit Gupta

[pbrockway2]

It is very often better to use a method like setDimensions() rather than allow the instance variables to be assigned to directly. That helps "protect" your Box class.

Suppose, for example, that someone using your Box class said

Box myBox = new Box();
myBox.width = -1.5;

That would be a crazy value, but you couldn't stop them. It might not matter in this case, but you often write classes where silly values for the instance variables would make the whole class fail to behave correctly.

If you use a so-called setter method you can stop this:

    /**
     * Sets the dimensions of the box to given values.  If the values are not greater
     * than zero that dimension will be set to one.
     */
void setDim(double w, double h, double d) {
    if(w > 0) {
        width = w;
    } else {
        width = 1.0;
    }
    if(h > 0) {
        height = h;
    } else {
        height = 1.0;
    }
    if(d > 0) {
        depth = d;
    } else {
        depth = 1.0;
    }
}

The code is longer - but safer! (When you get to them you will see that you can also use exceptions.)

-----

Have you read about access modifiers? (public/private) Often you will see only a few methods marked "public" and the instance variables and other methods marked "private" to stop anyone outside the class from being able to use them directly. Don't worry if you haven't come across them yet, but they are getting at the same thing: keeping most of the class secret and letting the outside world have access only to a few well defined methods.

[pbrockway2]

Also when you post code use the "code" tags. You put [code] at the start of the code and [/code] at the end. That way the code will be formatted when it appears here. As you can see if you post code that has been cut and pasted from somewhere else you may have to correct the formatting.

[ankiit]

Hi pbrockway2,

Got your point, and I have made the changes suggested by you, the code looks good now. :)

Yes, I have read about the access modifiers in an introductory chapter.

I'll keep the formatting in mind from next time.

Thanks
Ankit Gupta

[pbrockway2]

You're welcome.

[Tolls]

Um, that first code for getVol in the OP doesn't use anything inside the Box objects...so what's the point of the attributes?

[pbrockway2]

Oh yes, I didn't spot that!

@OP getVol() should take no arguments - like in your other thread.

[ankiit]

Hi Tolls,

I didn't got your point?

If I am correct, than I suppose you meant that the instance variables were not utilized(not initialized), so I must use width, depth and height variables instead of w,d & h.

Request you to kindly correct me, if I am wrong.

Thanks
Ankit

[Tolls]

Your Box has attributes.
You have a getVol method that presumably should be returning the volume of the Box object, so therefore should use those attributes and not have any parameters.

[ankiit]

Hi Toll,

Thanks a lot for the information.

I got your point, will be taking care of such details for further programming. :)

Thanks
Ankit