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………..

Answer 1

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.