FLEX{er}

variables – public/protected/private/internal/static

Virgil Cristea | 07.01.08 | 10 Comments

One of the first questions any beginner in OOP and Flex will ask is: What do those public/protected stuff really mean?

Well here is a short table to describe them.

As you should know by now a flex application can be structured in packages. Imagine an application as a cupboard, and it is composed of shelves (packages). In each shelf you have some boxes (classes). Now inside this boxes (classes) you have some small post-its (variables).

Now if you want your post-it data to be available from the box (class) it is in then you need to declare that post-it (variable) as private.

Imagine now that you have a post-it (variable) that contains something that is required in 2 or more boxes (classes) from the same shelf. In that case you need to declare your post-it (variable) as internal.

Another common case is when you want your post-it to be available to all other boxes no matter where they reside in the cupboard. You need to declare it public.

In your programs you will want to extend the boxes at some point (class inheritance). If you want a post-it (variable) to be available to the box it resides in (declared in) and also all the other boxes that are based on this one (the base box is for example: Documents and it can be extended to Music Documents and Pictures Documents) you need to declare it as internal.

So if we list the variables from their restrictive access level to the most general the list would be:

Code placement	Private	Internal	Protected	Public
Same class	Allow	Allow	Allow	Allow
Descendants of class	Deny	Allow	Allow	Allow
Classes in same package	Deny	Allow	Deny	Allow
Other package	Deny	Deny	Deny	Allow

A different keyword is const. It can be applied to any post-it (variable). If you apply it that will mean that you cannot change the value that is kept in that post-it (variable). In fact the value there is set in code at the develop time (like the name of the program/cupboard) and cannot be changed later.

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Tags: ActionScript, flex, internal, private, protected, public, scope, variables

This post was written by Virgil Cristea

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10 Comments

On 03.21.08 Significance of package names - Ultrashock Forums wrote these pithy words:

[...] in regards to exposing classes and interfaces to the public. For example, you can make classes package protected so that users outside that package cannot use that class. An example is that you do have a public [...]

On 03.21.08 House of Code wrote these pithy words:

The Science of Packages

Everyone knows about packages, but do you know of the science of packages? There are quite a few principles that deal with maintainability and reusability. Read on to learn about them, as explained by Robert C. Martin.
…

On 04.19.08 Links for the Weekend, 4-19-2008 wrote these pithy words:

[...] Variables – Public/Protected/Private/Internal/Static – ActionScript, Flex resources… [...]

On 11.14.08 Erno wrote these pithy words:

Hi! This was good and simple. But what is static ? You didn’t tell that. Well this is old post and I google it right away, but maybe next one need to know that.

Thanks!

On 02.15.09 Daniel Rijkhof wrote these pithy words:

Data is not correct :/

Code placement Private Internal Protected Public
Classes in same package Deny _Allow_ _Deny_ Allow

On 02.15.09 Daniel Rijkhof wrote these pithy words:

Let me add… that comment was about flex (as) 3

On 02.16.09 Virgil Cristea wrote these pithy words:

You are right. I will modify it.

On 05.04.09 Mario Ruggier wrote these pithy words:

Thanks, useful little summary table (helps to remember the frequently badly explained difference between internal and protected).

But, would it not be more logical to switch the Internal and Protected columns? Like that the natural restriction-level progression is more evident… i.e. the table becomes:

Code placement			Private		Protected 	Internal	Public

Same class				Allow		Allow		Allow		Allow
Descendants of class	Deny		Allow		Allow		Allow
Classes in same package	Deny		Deny		Allow		Allow
Other package			Deny 		Deny 		Deny		Allow

On 05.06.09 smedini wrote these pithy words:

The column internal is wrong.
The descendant of Class should be Deny.

You can say it’s package internal and hierarchy protected.

On 02.15.10 pookka wrote these pithy words:

All types and type members have an accessibility level, which controls whether they can be used from other code in your assembly or other assemblies. You specify the accessibility of a type or member when you declare it by using one of these access modifiers:

public

The type or member can be accessed by any other code in the same assembly or another assembly that references it.

private

The type or member can only be accessed by code in the same class or struct.

protected

The type or member can only be accessed by code in the same class or struct, or in a derived class.

internal

The type or member can be accessed by any code in the same assembly, but not from another assembly.

protected internal

The type or member can be accessed by any code in the same assembly, or by any derived class in another assembly.

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