Tutorial 8 the constants

[Flep]

After having seen the properties (variables inside an Actionscript 3.0 class), we can now introduce the constants.
The constants are a new characteristic implemented in the version 3.0. In fact, with the 2.0 they were not present.

For convention, they are declared using a name in capital letters so to understand at first sight that we are dealing with constants and not variables (properties).

We can define the value of a constant but we cannot modify it.

If we use more words as the constant name, it is of good rule to add an underscore in between the words.

Examples:
- Event.COMPLETE, COMPLETE is a constant of the Event class.
- MouseEvent.CLICK, CLICK is a constant of the MouseEvent class.


Let us look at it with even more examples'

I create a FLA and save it as 'ottava.fla'.
I create the Document Class associated to the FLA and save it as 'Ottava.as', implemented the following way:

package
{
	import flash.display.MovieClip;
	
	public class Ottava extends MovieClip
	{
		public function Ottava()
		{
			
		}
	}
}

I will now declare the constant:

package
{
	import flash.display.MovieClip;
	
	public class Ottava extends MovieClip
	{
		private const NOME:String='Filippo';
		
		public function Ottava()
		{
			
		}
	}
}

private const NOME:String='Filippo';

- private :we do know by now the meaning of it
- const :declares to Flash that it is a constant and not a variable
- NOME :the constant's name in capital letter
- :String :is the type of value accepted by the constant (exactly as a variable)
- ='Filippo'; :the constant's value

We can access to the content of the constant in the same way as for the content of a variable:

package
{
	import flash.display.MovieClip;
	
	public class Ottava extends MovieClip
	{
		private const NOME:String='Filippo';
		
		public function Ottava()
		{
			trace(NOME);
		}
	}
}

with trace(NOME);

I obtain the following output:
Filippo

It is not possible to assign a value to a constant after its declaration.
As an example, I declare another constant:

package
{
	import flash.display.MovieClip;
	
	public class Ottava extends MovieClip
	{
		private const NOME:String='Filippo';
		private const ETA:int;
		
		public function Ottava()
		{
			trace(NOME);
		}
	}
}

if I publish the SWF, Flash would return the following error:
Warning: 1110: The constant was not initialized.
It simply tells us that we did not assign a value to the constant.

In fact, if we try to assign a value from the building function:

package
{
	import flash.display.MovieClip;
	
	public class Ottava extends MovieClip
	{
		private const NOME:String='Filippo';
		private const ETA:int;
		
		public function Ottava()
		{
			trace(NOME);
			ETA=35;
		}
	}
}

Flash would return the following error:
1049: Illegal assignment to a variable specified as constant.
It tells us that we are trying to assign a value to a constant in a forbidden way, meaning after the constant's declaration.

About the constant's attributes'
They are the same applied to the variables:
private, internal, protected, public and static
I recommend to look at tutorial 4 if you have any doubt about the variable's attributes.

Source files: