Which is called first the constructor or onDraw()

Question

I referred to this question but it does not offer what I am asking about.

I have some variables which are declared globally and, initially, they are assigned to NULL. Inside the constructor I call a function called "newGame()", this function initialise the variables. And inside onDraw() I am trying to draw a text contains the size of one of the variables that I have initialised inside the newGame() function, and when I run the app, it crashes, and logCat says: NPE.

So, i think, if the constructor is called first, my variables should have been initialsed, so that there should be no NPE. But, since there is NPE, it seems that, onDraw() is called before the constructor, is that true?

Update_1

I have also placed the newGame() function inside onSizeChanged() but, I receive the same NPE

Update_2

I am checking if an object of a hashmapis null or not like this in the following: if (obj == null) is it correct to check whether or not an object is null?

Update_3

here is how I initialise the "hand"

if (hand == null) {
        Log.i(TAG, "@dealCards: Hand hashMap was NULL, it will be initialised");
        hand = new HashMap<Integer, Card>();
    }

Code

private HashMap<Integer, Card> deck = null;
private HashMap<Integer, Card> tableHand = null;
private HashMap<Integer, Card> myHand = null;
private HashMap<Integer, Card> compHand = null;
....
....
//CONSTRUCTOR
    public GameView(Context context) {
    super(context);
    // TODO Auto-generated constructor stub
    mContext = context;
    scale = mContext.getResources().getDisplayMetrics().density;

    textPaint = new Paint();
    textBounds = new Rect();
    deckSize_String = "Deck_Size: ";
    cardArraySize_String = "cardsArraySize: ";

    textPaint.setAntiAlias(true);
    textPaint.setColor(Color.RED);
    textPaint.setStyle(Paint.Style.STROKE);
    textPaint.setTextAlign(Paint.Align.LEFT);
    textPaint.setTextSize(scale*15);

    newGame();
}
....
....
//OnDraw()
    protected void onDraw(Canvas canvas) {
    // TODO Auto-generated method stub
    super.onDraw(canvas);
    canvas.drawText("tableHand: "+tableHand.size(), 10, 200, textPaint);

}

private void newGame() {
    // TODO Auto-generated method stub
    Log.i(TAG, "@newGame()");

    initDeck();
    dealCards(deck, tableHand);
    myTurn = whosTurn();

    if (myTurn) {
        dealCards(deck, myHand);
        dealCards(deck, compHand);
    }else {
        dealCards(deck, myHand);
        dealCards(deck, compHand);
    }
}
...
...
...
 private void dealCards(HashMap<Integer, Card> deck, HashMap<Integer, Card> hand) {
    // TODO Auto-generated method stub
    if (hand == null) {
        Log.i(TAG, "@dealCards: Hand hashMap was NULL, it will be initialised");
        hand = new HashMap<Integer, Card>();
    }

    for (int i=0; i<4; i++) {
        hand.put( (hand.size()+1), deck.get( ((DECK_MAX_SIZE - deck.size())+1) ) );
        copyDealtCards( dealtCardsPile, deck.get( ((DECK_MAX_SIZE - deck.size())+1) ) );
        deck.remove( ((DECK_MAX_SIZE - deck.size())+1) );
    }
}

If the constructor wasn't invoked, what would `onDraw()` be invoked on? — Sotirios Delimanolis, Aug 02 '14 at 16:39
The constructor has to be called on an object before any non-static functions are called on it. — Gabe Sechan, Aug 02 '14 at 16:41
@SotiriosDelimanolis if there was no constructor, however, onDraw() will try to draw a text which has not been initialised. is it your question, did i got you right? — Amrmsmb, Aug 02 '14 at 16:49
`onDraw` is an instance method. You need an instance to invoke it on. To create an instance, you need to invoke a constructor. You don't seem to be initializing `tableHand` correctly. — Sotirios Delimanolis, Aug 02 '14 at 16:51
@SotiriosDelimanolis can i check if an object is null or not as the following: `if (obj == null)` or it is not correct — Amrmsmb, Aug 02 '14 at 17:49
@SotiriosDelimanolis please see the updated Code. `dealCards` methods, in which i initialise the hands — Amrmsmb, Aug 02 '14 at 19:26
Is `dealCards` called before `onDraw`? We keep going back and forth on this. If you get a NPE in `onDraw`, that's because something is `null`. Figure out what that is and initialize before `onDraw` is called. — Sotirios Delimanolis, Aug 02 '14 at 19:37
@SotiriosDelimanolis `dealcards` is a function and is called from inside `newGame()` which is called from the constructor.in other words, the constructor has `newGame()` and `newGame` has dealCards() — Amrmsmb, Aug 02 '14 at 20:41
@SotiriosDelimanolis sorry for any inconvenience caused. please see the code updated — Amrmsmb, Aug 03 '14 at 16:25

Peter Pei Guo · Answer 1 · 2014-08-02T16:58:49.200

1

Update

Saw your added code, where did you initialize tableHand?

By the way, it is better to declare tableHand as Map instead of HashMap, and initialize it with new HashMap<>().

No doubt, constructor. If the object does not exist first, no instance method can be invoked on it.

edited Aug 02 '14 at 16:58

answered Aug 02 '14 at 16:42

Peter Pei Guo

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For non-static methods – vsoftco Aug 02 '14 at 16:43
You can invoke static methods without creating an instance of the class. – Christian Tapia Aug 02 '14 at 16:44

score 0 · Answer 2 · answered Aug 02 '14 at 17:06

Why not try and follow this design to be sure you are initiating the variables properly

class Foo {
    private static String GLOBAL_VAR = null ;
    private        String instanceVariable = null ;

    static {
        // here you can do complicated stuff 
        //in order to correctly initiate your variables.
        GLOBAL_VAR = "FOO" ; 
    }// this block will run when the class is loaded

    {
        // here you can do complicated stuff 
        //in order to correctly initiate your variables.
        instanceVariable = "foo" ;  
    }// this block will run before constructor.

    Foo()
    {

    }//Constructor

    static String getGlobalVar()
    {
         return GLOBAL_VAL ;
    }
}

Which is called first the constructor or onDraw()

2 Answers2