Cannot find the source of a nullPointerException

Question

I am trying to make a simple GUI, but am running into some errors. I want to create 3 simple buttons, and have them display side by side, the length of the screen. How would I go about doing this ?

My code so far is :

  public static void main(String[] args) {
    JFrame frame = new JFrame ("JFrame");
    JPanel panel = new JPanel( );
    frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE);
    JButton buttons[] = new JButton[2];
    for(int i = 0;i<=buttons.length ; i++){
     panel.add(buttons[i]);
    }
    frame.getContentPane( ).add( panel );
    frame.setSize( 500, 500);
    frame.setVisible( true );
  }

Answer 1

Hm, if I recall, arrays of objects in Java are initialized with null references. So, in your for, you're adding null to the panel.

You would have to do this:

JButton buttons[] = new JButton[2];
for(int i = 0;i < buttons.length ; i++){
    buttons[i] = new JButton(/* whatever */);
    panel.add(buttons[i]);
}

Then again, this raises a question: why use an array of buttons if you can just add them as you create them?

Answer 2

You need to initialize the buttons in the array first. You can do that like in the following example.

JButton buttons[] = new JButton[2];
for(int i = 0; i < buttons.length; i++){
    buttons[i] = new JButton("" + i);
}
for(int i = 0; i < buttons.length; i++){
    panel.add(buttons[i]);
}

The problem is that all of your buttons get initialized to null initially. And components don't let you add null to them.

Answer 3

You need to initialize your buttons before adding them to the panel.

for(int i = 0;i<buttons.length ; i++){
    buttons[i] = new JButton();
    panel.add(buttons[i]);
}

Also, take note of the modification I made to the for loop i < buttons.length instead of i <= buttons.length

Here is a nice little example using various JButtons ( here ). It includes adding ImageIcons to the buttons as well. ie An error button.

URL imgURL = JButtonDemo.class.getResource("Error.jpg");
ImageIcon icon = new createImageIcon(imgURL ); // should check that imgURL is not null
buttons[0] = new JButton("Error!",icon);

This makes them a bit more useful than a default JButton