Simple swing program, help please!

[N00Bie]

Hi,

This is a simple swing program with just a label and a button.
When I click the button I want the label to change to "a".

But instead it throws a null pointer exception at runtime (compiles fine)
please help!
The program looks big because i am also learning GridBagLayout, I have bolded the line that I want to change.

import java.awt.BorderLayout;


public class Test extends JFrame {

	private JPanel contentPane;

	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					Test frame = new Test();
					frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}

	/**
	 * Create the frame.
	 */
	public Test() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 450, 300);
		contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		setContentPane(contentPane);
		GridBagLayout gbl_contentPane = new GridBagLayout();
		gbl_contentPane.columnWidths = new int[]{0, 0, 0, 0, 0};
		gbl_contentPane.rowHeights = new int[]{0, 0, 0, 0, 0, 0, 0};
		gbl_contentPane.columnWeights = new double[]{0.0, 0.0, 0.0, 0.0, Double.MIN_VALUE};
		gbl_contentPane.rowWeights = new double[]{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, Double.MIN_VALUE};
		contentPane.setLayout(gbl_contentPane);
		
		[COLOR="Red"][B]JLabel lblNewLabel = new JLabel("New label");[/B][/COLOR]
		GridBagConstraints gbc_lblNewLabel = new GridBagConstraints();
		gbc_lblNewLabel.insets = new Insets(0, 0, 5, 0);
		gbc_lblNewLabel.gridx = 3;
		gbc_lblNewLabel.gridy = 2;
		contentPane.add(lblNewLabel, gbc_lblNewLabel);
		
		JButton btnNewButton = new JButton("New button");
		btnNewButton.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				but1ActionPerformed(arg0);
			}
		});
		GridBagConstraints gbc_btnNewButton = new GridBagConstraints();
		gbc_btnNewButton.insets = new Insets(0, 0, 0, 5);
		gbc_btnNewButton.gridx = 1;
		gbc_btnNewButton.gridy = 5;
		contentPane.add(btnNewButton, gbc_btnNewButton);
	}

	
	 private javax.swing.JLabel lblNewLabel;

		private void but1ActionPerformed(ActionEvent arg0) {
			lblNewLabel.setText("a");			
		}
}

[JosAH]

You both defined a local variable in your constructor and as a member variable. You initialize that local variable but leave the member variable equal to null. Remove that local variable lblNewLabel from your constructor.

kind regards,

Jos

[N00Bie]

if I remove this:

JLabel lblNewLabel = new JLabel("New label");

from the constructor I am getting more errors...

[JosAH]

if I remove this:

from the constructor I am getting more errors...

You should initialize your member variable in your constructor, just don't use a local variable with the same name. In your constructor do this:

lblNewLabel = new JLabel("New label");

kind regards,

Jos

[Fubarable]

What Jos is saying is don't declare the variable twice. Declare it once in the class:

This is variable declaration and initialization

MyFoo myFoo;  // this is variable declaration

myFoo = new MyFoo();  // this is variable initialization or construction

MyFoo myFoo = new MyFoo();  // this is both variable initialization and declaration combined

And initialize it once by calling new XXX() once either in the class or the constructor.

Think about why this matters -- when you declare it again in the constructor as you're doing, you're creating a completely new variable, one that is completely distinct from the one declared in the class itself. The variable declared in the constructor is only visible within the constructor and does not exist outside of it.


Edit: Did not see Jos' reply. Yeah, what he said.

[N00Bie]

Ah! That worked, thanks guys!