How can I create an object and then add that object to an ArrayList?

Question

For my Java project I am creating buttons from strings stored in an array as such:

public class UserInterface implements ActionListener {
ArrayList<CardButton> currButtonList;   // Current list of card buttons used

public void createAndShowGUI(String[] allCards){

//unrelated to question code

//for each String in allCards[]
for(int i=0; i<allCards.length; i++){
    //Assign the button names relative to which card has is being created
    String name = allCards[i];
    CardButton button = new CardButton(name);
    //add the button to the CardPanel
    button.setFaceDown();
    button.setBorderPainted(false);
    int width = button.getWidth();
    int height = button.getHeight();
    button.setSize( new Dimension( width, height ) );
    //button.setPreferredSize(new Dimension(150, 150));
    CardPanel.add(button);
    currButtonList.add(button);
    }
  }
 //rest of creating the Panels, more unrelated to question code

Code complies but: Exception in thread "main" java.lang.NullPointerException at memory_game.GameFunction.(GameFunction.java:47) Which is where I try to assign listeners to each object in the array list. Am I correct in assuming that the ArrayList is not having the buttons added to it correctly? If so, why? How can I make this work?

Elliott Frisch · Accepted Answer · 2013-12-07 01:50:47Z

3

You need to instantiate your ArrayList<CardButton>. This is wrong (because it leaves currButtonList undefined, that is null) -

ArrayList<CardButton> currButtonList;   // Current list of card buttons used

Try this instead

// Current list of card buttons used
List<CardButton> currButtonList = new ArrayList<CardButton>();

Or you could add this to your UserInterface constructor(s) -

currButtonList = new ArrayList<CardButton>(); // Instantiate the currButtonList.

Finally, as for your subject line question - you seem to be doing that correctly with these lines -

CardButton button = new CardButton(name); // Create a new CardButton instance.
currButtonList.add(button); // Add it to the list (currButtonList was null, not button)

answered Dec 7, 2013 at 1:50

Elliott Frisch

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1 Comment

Marcus Aurelius Over a year ago

Ah thank you, I had tried new ArrayList<CardButton> but must have forgotten the full new ArrayList<CardButton>();

Coder101101010 · Accepted Answer · 2013-12-07 01:51:46Z

1

Are you ever instantiating your ArrayList like

ArrayList<CardButton> currButtonList = new ArrayList<CardButton>();

I don't believe ArrayList is an intrinsic type that will work without instantiating it.

answered Dec 7, 2013 at 1:51

Coder101101010

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Comments

jayalalk · Accepted Answer · 2013-12-07 01:58:57Z

1

Your arrayList is not initialized, default is NULL. That is why you get null pointer exception when you try to add button to currentButtonList variable.

Initialize your array with empty list as below.

ArrayList<CardButton> currButtonList = new ArrayList<CardButton>();    // Current list of card buttons used

answered Dec 7, 2013 at 1:58

jayalalk

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