AS3 - Access object from another class

Question

I have three classes: a Main, a Player and a Car.

The main creates a Car and a Player. The Car needs to trace the player's x position. But the code below is not working. I have researched and found that declaring it as static should fix the problem although it is not the best way to do it?

Main:

package { 

import flash.display.MovieClip;
import flash.events.Event;
import flash.events.MouseEvent;

public class Main extends MovieClip
{       
    public static var _player:Player;
    public static var _car:Car;

    public function Main() 
    {
        createPlayer();
    }

    public function createPlayer():void
    {

        _player= new Player();
        _car = new Car();

        _player.x = stage.stageWidth / 2;
        _player.y = stage.stageHeight / 2;

        stage.addChild(_player);
        stage.addChild(_car);
    }

}

}

Car:

package { 

import flash.display.MovieClip;
import Main;

public class Player extends MovieClip
{
    public function Player()
    {
        trace(Main._car.x);

    }
}

}

But this causes an error: TypeError: Error #1009: Cannot access a property or method of a null object reference. at Player() at Main/createPlayer() at Main()

I want to know why this is giving me an error and what are the other good ways to do this?

//EDIT: Sorry _hero was my mistake, it is actually _car. Thank You Very Much...........

Gio · Accepted Answer · 2012-08-15 08:57:12Z

First of all: THIS IS NOT A RIGHT WAY TO PROGRAM! You shouldn't be using static properties everywhere. Secondly: Your _player instance is created before you assign new Car() to your _car instance, that's why it's still null. Try changing their lines, so that you have:

_car = new Car();
_player = new Player();

Again, this will solve your problem, but your coding is not the right way. If you want to have a reference to your _car object, pass it in Player class' constructor.

P.S. Don't forget to mark this post as an answer if it helped you.

EDIT:

Here is some code to get you started:

In your Player class add a private property for storing a reference on your Car object. Then add your Player's constructor a parameter, which you will pass when creating a new Player. Note that use this only if you want to store a reference to your Car object in your Main class too.

package { 

import flash.display.MovieClip;
import Main;

public class Player extends MovieClip
{
    private var _car:Car; // add a property to store a reference to the Car object

    public function Player(car:Car) //add a parameter to your Player constructor to pass in a Car object
    {
        _car = car; //assign passed Car object to your property _car
        trace(_car.x) //now you can use _car as you wish
    }
}
}

When defining a Player object, you'll now need to pass in your Car object to it's constructor. So do it like this:

_player = new Player(_car);

IF you don't need a reference to your Car object in your Main class, or better to say if a Car is assigned to a Player, so that each Player would have it's own Car, you'd better define a property in your Player Class, like this:

 package { 

import flash.display.MovieClip;
import Main;

public class Player extends MovieClip
{
    public var car:Car; // add a property to store a reference to the Car object, note that now we made the car a public property, so that it can be accessed from outside.

    public function Player() //add a parameter to your Player constructor to pass in a Car object
    {
        car = new Car(); // initialize your variable
        trace(car.x) //now you can use car as you wish from this class
    }
}
}

Also note that this Car object we initialized in Player class can be accessed from you Main class like this:

_player = new Player();
trace(_player.car.x);

I'd suggest you read basic concept of OOP (Object Oriented Programming), such as Encapsulation, Inheritance and Polymorphism. Search google for some good books which will help you get on your feet.

Thank You very much , it is working. As for the static identifier, I read it in a post and was trying to make this work before trying anything else. If you can elaborate on using the constructor thing, that would be helpful

Kolyunya · Accepted Answer · 2012-08-15 07:29:56Z

0

I can not see Main object in Player class, also I can not see a hero object in Main. So, Main._hero.x is NULL.

answered Aug 15, 2012 at 7:29

Kolyunya

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