AS3 Casting as SuperClass returns null

Question

THE SOLUTION! Ensure that you're not mistakenly importing the wrong class.

In my case, I was explicitly importing TabButton, but there was another TabButton class that was being imported because it existed in the same directory as TabButtonSubclass. This happened because TabButtonSubclass exists in a folder specific to one application. This folder also contains a horribly generically named class, TabButton, which is completely unrelated and incompatible to the other TabButton class. The application-specific TabButton class was renamed so it's obvious it belongs to that application.

Had I written 'public class MBTabButtonSprited extends [specific location].TabButton', it would have worked fine. But it seems that since 'extends TabButton' was not prefaced by a specific path, the compiler defaulted to the local path.

I've been banging my head on my desk for a while now, trying to figure this one out.

My issue is that I've extended Sprite to create a TabButton class. I then extended the TabButton class (we'll call it TabButtonSubclass) for some additional functionality. When I try to cast an instance of TabButtonSubclass as TabButton, I get null.

The bizarre part though, is that I can cast an instance of TabButton or TabButtonSubclass as a Sprite, and it works fine.

Here is what my TabButton looks like:

import flash.display.Sprite;

public class TabButton extends Sprite {
    public function TabButton():void {
        super();
        // code
    }
}

And here is what my TabButtonSubclass looks like:

import ...TabButton;

public class TabButtonSubclass extends TabButton {

    public function TabButtonSubclass( bitmap:BitmapData ):void {
        // code
        super();
    }

}

To clarify the TabButtonSubclass class: The purpose of extending the TabButton was so that you could instantiate a TabButtonSubclass with a supplied Bitmap. This Bitmap would be used for various purposes. I have extended classes in the past and added parameters to the constructor, and it worked fine. Is this not a good idea?

Some sample code that illustrates my issue:

import flash.display.Sprite;
import ...TabButton;
import ...TabButtonSubclass;

var btn1:TabButton = new TabButton();
trace( "btn1:", btn1 );     // btn1: [object TabButton]
trace( "btn1 as Sprite:", (btn1 as Sprite) );     // btn1 as Sprite: [object TabButton]

var btn2:TabButtonSubclass = new TabButtonSubclass( new GangTabMail() );
trace( "btn2:", (btn2) );     // btn2: [object TabButtonSubclass]
trace( "btn2 as TabButton:", (btn2 as TabButton) );     // btn2 as TabButton: null
trace( "btn2 as Sprite:", (btn2 as Sprite) );     // btn2 as Sprite: [object TabButtonSubclass]

As you can see, I'm able to cast btn1 as Sprite with no issues. I can also cast btn2 as a Sprite, and it returns the expected results. However, if I try casting an instance of TabButtonSubclass as TabButton, it returns null.

I'm interested in casting an instance of TabButtonSubclass as TabButton because the application utilizes a couple of classes with that require objects of type TabButton to be passed.

Here's an example of some code that allows you to type a class as a super class:

import ...TabButton;
import flash.display.Sprite;

var myBtn:TabButton = new TabButton();
doStuff( myBtn );     // btn: [object TabButton]

function doStuff( btn:Sprite ):void {
    trace( "btn:", btn );
}

Here's an example of the code failing if you go one level deeper:

import ...TabButton;
import ...TabButtonSubclass;
import flash.display.Sprite;

var myBtn:TabButtonSubclass = new TabButtonSubclass( new BitmapData( ... ) );
doStuff( myBtn );     // Returns error: Implicit coercion of a value of type ...:TabButtonSubclass to an unrelated type ...:TabButton
doStuff( myBtn as TabButton );     // btn: null

function doStuff( btn:TabButton ):void {
    trace( "btn:", btn );
}

Can anybody offer any help as to why I get a null response when I attempt to trace it out? Or why I get that error? Am I missing something in regards to subclasses and type casting?

I might be totally off the track here, but is it possible that you have multiple swf being loaded and those swf have TabButton used in them? — catholicon

cmann cmann · Accepted Answer · 2013-07-10T18:57:05

I'm not sure why it would return null, but the way I would handle it would be to rather define the variable using the base class and cast up when needed, eg.

import ...TabButton;
import ...TabButtonSubclass;
import flash.display.Sprite;

var myBtn:TabButton = new TabButtonSubclass( new BitmapData( ... ) );
doStuff( myBtn ); // No need to case here since myBtn is already defined as TabButton

function doStuff( btn:TabButton ):void {
    trace( "btn:", btn );
}

Then if you need TabButtonSubclass functionality you cast it up:

var myBtnSub:TabButtonSubclass = myBtn as TabButtonSubclass;

AS3 Casting as SuperClass returns null

4 Answers