Posts Tagged ‘variables’

This is another post about object oriented programming.

Ok, cool, you’re one of the 3% still reading. I just wanted to do a quick post about access modifiers on objects. It applies pretty much regardless of programming language, presuming you’re working with one that supports OO and has the standard public, private and protected for access by sub-classes only.

I think, what we need here is an attitude shift away from using private variables. Often, I see code that uses private variables and I have no idea why.

What I mean by this, is that almost every time I see a private variable, what it actually should be is a protected variable. Indeed, I think the default assumption when creating a variable, should be that you define it as protected.

Lets ignore public variables for the moment. I’ve previously argued you could simply do away with them altogether (that is what getters and setters are for), but in any case, I’m not concerned with them for this post. Lets just focus on private or protected.

The traditional teaching has always been that you should define a variable as private if you don’t want it to be publicly accessible, and giving child classes access to it later on is often an afterthought.

I don’t think this should be the case.

If we’re to adopt a true OO mindset (and it’s been around for sixty years, so given it was invented before most of us were born, you would hope we would have adopted it by now), surely you would work from a perspective that your class will be extended.

Protecting variables from external bodies makes sense, hence not making them public, but to by default place restrictions on what you can do with child classes in an OO world, doesn’t make sense to me. Why have the functionality at all? Why not make everything final if you need such protection?

That isn’t to say there aren’t plenty of instances where there is a reason to do this – but these aren’t the 90% most common use cases, so I think there is a good argument for making protected the default at least.

Worfolk Desktop News was an application which downloaded the latest news from Worfolk and displayed it in the application. The original application was built in flash and loaded the variables in from a text file stored on the Worfolk servers.

The text file was simple:

news=the news story...

Create a text file like this and upload it somethere, or even just leave it on your hard drive as long as you know the file path. Now to load it into the flash movie you use LoadVariables. Go to the first frame in your movie and go into actionscript. Now add a LoadVariables command and add in the url path or file path to your text file.

Once this is done you have the variables stored in your movie. Next create a text area and set it to dynamic text. Where it says “var” and has a space to enter a name such as “news,” or whatever name you gave your variable in the text file.

Now run your movie. If it worked right the text area should now display the value you have the variable. If you copied my text into your text file you should come out with “the news story…” If its blank you have probably given the wrong url or you have got different names for the variable in your text file then you do in your movie.

The request function, suprising requests a value. This could be a query string, form variable, or some other value. For example if you were at the following URL.

http://www.somepage.com/home.asp?ID=23

And you had a script as such.

Request.QueryString("ID")

The call would return the value 23.

An example of where this can be used is on the M World News channel. The news is stored dynamically in a databasde so when you click on a link to the full story from the news homepage you are taken to a dynamic page which selects the story based on the ID of the story you want passed to the page as a url variable. For example the link could be:

http://www.mworld.us/news/story.asp?ID=52

Examples of use:

Request.QueryString("SomeURLVariable")
Request.Form("SomeFormElement")
Request.Cookies("SomeCookie")

Problem: You are requesting QueryString’s in ASP but nothing happens and the value seems to be empty.

Cause: You cannot use default documents when requesting QueryString’s. For example:

http://www.somefile.com/search/index.asp?q=hello

Would pick up that the query string “q” is set to “hello.” However:

http://www.somefile.com/search/?q=hello

Would not pick up anything despite index.asp being loaded as the default document.

Fix: Enter the full url instead of skipping out the file name.

The request function, surprisingly, requests a value. This could be a URL variable, form variable, or some other value. For example if you were at the url.

http://www.somepage.com/home.asp?ID=23

You would use the following code in your script.

Request.QueryString("ID")

The line would then return the value 23.

An example of where this can be used is on the M World News channel. The news is stored dynamically in a database so when you click on a link to the full story from the news homepage you are taken to a dynamic page which selects the story based on the ID of the story you want passed to the page as a url variable. For example the link could be as follows.

http://www.mworld.us/news/story.asp?ID=52

Examples of use

Request.QueryString("SomeURLVariable")
Request.Form("SomeFormElement")
Request.Cookies("SomeCookie")