一个JLabel的setIcon与一个JLabel数组之间的区别

Question

I'm trying to design a very simple graphic interface where I will be adding some images to using a JLabel loaded with an Icon:

JFrame Table = new JFrame("Transporter Room");
Table.setSize(600, 600);
Table.setLocationRelativeTo(null);
Table.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Table.setLayout(new GridLayout(2, 2, 20, 20));

My problem is loading the icons for an array of JLabels... Something like this works:

JLabel playingCard = new JLabel(CardGUI.getIcon(myCard));
JLabel playingCard2 = new JLabel(CardGUI.getIcon(myCard2));
Table.add(playingCard);
Table.add(playingCard2);

Just a note, the CardGUI.getIcon(ob) is just a static method that sets up the directory correctly and loads up an Icon with a given card object.

Something like this also works (although the icon is not centered in the layout.. why?) :

JLabel playingCard = new JLabel();
JLabel playingCard2 = new JLabel();
playingCard.setIcon(CardGUI.getIcon(myCard));
playingCard2.setIcon(CardGUI.getIcon(myCard));
Table.add(playingCard);
Table.add(playingCard2);

Using arrays of JLabel also works as long as I don't use setIcon() but dup my code to declare twice:

JLabel[] myLabel = new JLabel[2];
myLabel[0] = new JLabel(CardGUI.getIcon(myCard));
myLabel[1] = new JLabel(CardGUI.getIcon(myCard2));
Table.add(myLabel[0]);
Table.add(myLabel[1]);

However, the problem arises when I try to do this (and I need to do this per spec):

JLabel[] myLabel = new JLabel[2];
myLabel[0].setIcon(CardGUI.getIcon(myCard));
myLabel[1].setIcon(CardGUI.getIcon(myCard2));
Table.add(myLabel[0]);
Table.add(myLabel[1]);

I get a Execption in thread "main" java.lang.NullPointerException . I have no idea why. I'm pretty sure the directory to the image is perfectly fine (that's taken care of by another class). Why does this only come up if a JLabel within an array is attempted to be set up?

This is an assignment I'm working on, and I prefer not using any new code that might be potentially be longer as a work around. It should be rather straightforward, if I can get pass this point, I literally use a for loop to load up Icons to JLabels, then another for loop to load up all the JLabels onto a table top (that's another Class I have already set up). Any help would be greatly appreciated!

Answer 1

When you create a JLabel[] , you're creating the array of labels. NOT the labels inside the array. In other words, the array elements are still null after you create the array. That's why your second to last bit of code works, but your last didn't -- in the last bit of code, you didn't initialize the elements in the array, so when you try to call a method on them, you get a NullPointerException .