获取NullPointerException，不确定原因

Question

First I instantiate a gamestate

class GameState extends state{
ArrayList<Level> levels; 
int currentLevelID;
public GameState() {
    stateID = 2;
    levels = new ArrayList<Level>();
    createLevels();
    currentLevelID = 0;
}

which creates levels

    public void createLevels(){
    try {
        this.levels.add(new NormalLevel(0, 10, 10, null, null, new EmptyTile(1, 1, 1, 1, null) ));
    } catch (IOException e) {
        e.printStackTrace();
    }
}

using this bit of code (forgot the technical term lol)

    //level is the superclass of normallevel or whatever i named the default level
    public Level(int id, int height, int width, ArrayList<TileEntity> tiles, ArrayList<MobileEntity> mobiles, TileEntity fillBlock){
    this.height = height;
    this.levelID = id;
    this.width = width;
    if(tiles != null){
        this.tiles = tiles;
    } else {
        tiles = new ArrayList<TileEntity>();
        try {
            tiles.add(new EmptyTile(-50,-50,1,1,null));
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    if(mobiles != null){
        this.mobiles = mobiles;
    } else {
        mobiles = new ArrayList<MobileEntity>();
        try {
            mobiles.add(new DefaultEntity(-50,-50,1,1,null));
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    if(fillBlock != null){
        this.tiles = new ArrayList<TileEntity>();
        this.fillWith(fillBlock);
    }
}
public void fillWith(TileEntity tile){
    for(int i = 0; i < this.height; i++){
        for(int j = 0; j < this.width; j++){
            for(int k = 0; k < this.tiles.size(); k++){
                if (this.tiles.get(k).y == i && this.tiles.get(k).x == j){
                    break;
                }
                if(k == this.tiles.size()){
                    tile.x = j;
                    tile.y = i;
                    this.tiles.add(tile);
                }
            }
        }
    }
}

Then I try to update the level

    public void update(){
    this.draw();
    for(int i = 0; i < this.tiles.size(); i++){
        this.tiles.get(i).update();
    }
    for(int i = 0; i < this.mobiles.size(); i++){ //nullpointerexception here
        this.mobiles.get(i).update();
    }

    this.specialUpdate();
    }

But I get an error on the commented line. I'm confused as hell, help would be appreciated :)

Answer 1

Your mobiles object is null because in the following code

if(mobiles != null){
        this.mobiles = mobiles;
} else {
    mobiles = new ArrayList<MobileEntity>();

in the else case your assigning the local variable mobiles rather than this.mobiles

Answer 2

if(mobiles != null){
    this.mobiles = mobiles;
} else {
    mobiles = new ArrayList<MobileEntity>();
    try {
        mobiles.add(new DefaultEntity(-50,-50,1,1,null));
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

If mobiles != null you assign the instance variable ( this.mobiles ), yet if they are, you create a new List , but assign it to the local variable ( mobiles ), which is why the list never reaches the instance variable.