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Thread: NullPointerException on JButton.setIcon.

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    Default NullPointerException on JButton.setIcon.

    Wasn't sure which subforum to put this in. It feels like it crosses a couple different aspects of Java.

    So I have these two classes that I wrote a few months back. The Card class represents a single playing card and the Deck class is just an array of Card objects. I'm trying to build 4 JPanels, each with 13 cards. I have a function that builds an individual panel. It in turn calls a function to deal a single card. Each panel has 13 JButtons with a graphical representation of the card on it.

    Here is the function that builds the panel.

    private JPanel buildCardPanel()
    	{
    		//Panel to add elements to.
    		JPanel returnPanel = new JPanel();
     
    		//JButton on which an individual card will be shown.
    		JButton[] handButton = new JButton[13];
     
    		//A hand of 13 cards.
    		Card[] hand = new Card[13];
     
    		returnPanel.setLayout(new GridLayout(1,13));
    		for(int i = 0; i < 13; i++)
    		{
    			//Deal a card.
    			hand[i] = dealCard();
    			//Set that card's image icon on the button.
    			System.out.println(hand[i].getImageIcon().toString());
    			handButton[i].setIcon(hand[i].getImageIcon());
    			returnPanel.add(handButton[i]);
    		}
    		return returnPanel;
    	}

    Here is the function that deals cards. It accesses a Deck object named starterDeck (I think baseball cards were on my mind when I came up with that name... it's kind of a stupid variable name) defined earlier. I realize that it doesn't prevent the same card from being drawn multiple times. I'll implement that later.

    	private Card dealCard()
    	{
    		Random rand = new Random();
     
    		//Draw a card
    		return starterDeck.cards[rand.nextInt(DECKSIZE)];		
    	}

    The Card.getImageIcon() method is a simple getter method:

    public ImageIcon getImageIcon()
    	{
    		return this.cardIcon;
    	}

    But I keep getting a NullPointerException on the line that says handButton[i].setIcon(hand[i].getImageIcon()); which I don't understand.

    If you notice the println statement before that, you probably realize that's just temporary for debugging purposes. It's returning a value. I get something like "javax.swing.ImageIcon@4ba2fc31", which matches what I get if I put a System.out.println(this.cardIcon); inside the Card.getImageIcon function.

    So I'm really confused. hand[i].getImageIcon() seems to be returning an ImageIcon object. That object is being passed to handButton[i].setIcon(); So where the heck is the Null Pointer Exception coming from?


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    Super Moderator Norm's Avatar
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    Default Re: NullPointerException on JButton.setIcon.

    What is the value of handButton[i]?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: NullPointerException on JButton.setIcon.

    But I keep getting a NullPointerException on the line that says handButton[i].setIcon(hand[i].getImageIcon()); which I don't understand
    Do you ever instantiate the objects in the handButton array? Creating an array is one thing, but if its an array of objects the default value will be null until instantiated.

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    Default Re: NullPointerException on JButton.setIcon.

    Quote Originally Posted by copeg View Post
    Do you ever instantiate the objects in the handButton array? Creating an array is one thing, but if its an array of objects the default value will be null until instantiated.
    I thought the statement JButton[] handButton = new JButton[13]; instantiated them. I guess I was wrong. I always screw up with object array. Blech. So I oughtta throw something like handButton[i] = new JButton(); at the start of that for loop?

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    Default Re: NullPointerException on JButton.setIcon.

    Yes, you also might want to set the icon for the buttons too. They won't have an ImageIcon by default unless you either set it in the constructor or call a setter method.

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    Default Re: NullPointerException on JButton.setIcon.

    Ok got it working. Thanks Norm and Copeg.

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