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Re: The Accessor Heresy

by gjb (Vicar)
 on Nov 28, 2005 at 16:40 UTC ( #512262=note: print w/replies, xml ) Need Help??

in reply to The Accessor Heresy

This may be an academic example, but it's very weird to see you set the area of a circle. Personally I'd have a constructor that takes two arguments or three arguments: either a point or two coordinates (this is open for debate) and the radius of the circle. I'd have a getter for the point and the radius, as well as getters for the area (which would be computed from the radius) and such other properties. Point may be an object if necessary, but it could also be coded as an x- and y-coordinate.

I'd (if necessary) have methods such as move to change the coordinates of the midpoint and scale to change the radius.

This seems to me the way to manipulate a circle without fussing with its implementation and that is after all one of the goals of OO.

Just my 2 cents, -gjb-

Update: Apparantly I should clarify a bit. The points I'm trying to make with my casual description of a Circle class are (at least ;) the following:

• Getters/setters don't necessarily come in pairs and
• you can manipulate an object other than by setting some of its properties directly
The former point is illustrated by the area example, the latter by the move method which could have been done just be modifying the coordinates of the midpoint.

As a last point: I don't care whether Radius is ann object or not since I'm not going to manipulate it directly in my example anyway.

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Re^2: The Accessor Heresy
by sauoq (Abbot) on Nov 28, 2005 at 17:07 UTC
This may be an academic example, but it's very weird to see you set the area of a circle.

I don't see that as weird at all. It's just the other side of the equation. If it seems weird to you, I'd guess that's because your math classes only taught you to think in one direction. If you'd seen R = sqrt(A/pi) printed inside the back cover of high school algebra textbook, this might not seem so strange.

```-sauoq
"My two cents aren't worth a dime.";
```

The point I'm trying to make is that certain properties are derived from others, and while you can go in both directions, there's often a good point not to do so. I agree that there are without any doubt some circumstances in which you'd determine the radius from the area, but this is irrelevant to the discussion.

My point is that getters and setters very often don't come in pairs.

I hope this clarifies things a bit, -gjb-

My point is that getters and setters very often don't come in pairs.

Okay, you have clarified your point successfully. I don't really agree with this either, though. It all depends on what you have to do with your objects. Having two or more inter-related properties, any of which can be set and thereby affect the others, is not particularly rare.

Running with the original example, if you sold custom made hand-woven circular rugs and charged by the square foot, you might be able to imagine situations in which setting either the area or the radius might be important.

```-sauoq
"My two cents aren't worth a dime.";
```
getters and setters very often don't come in pairs
To help in applying what you've read here to situations that aren't specifically covered, think about whether something is conceptually a property. If you've got a bunch of operate_on_Doohickey type methods, regardless of whether any of them happen to be get_ or set_, you probably have in your model the concept of a Doohickey property.

If you've got only one operate_on_Doohickey method, there's still probably a Doohickey object in your model, but it's not interesting enough to implement as an object. Of course, you might expand your functionality in the future, in which case you might end up wishing you had coded it up as an object.

I definitely wouldn't code up an object for a read-only property. If Area wasn't settable, then my Circle object might have an Area method that simply returned the area of the Circle. Note that it wouldn't be getArea. But if it had numerous possible output formats, I'd have it create a Measurement object (not a sub-object, but one that exists outside of the Circle class hierarchy, because it's not specific to the domain of Circles) so I could get Area->inSqFeet or whatever.

The design decisions are yours, of course.

Caution: Contents may have been coded under pressure.

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