Null Pointer Exception in a very Simple Action Listener. Why ?

[fatabass]

I have a MainFrame class that extends JFrame and constructs itself and adds a Jbutton called rollDice.

Here is my Test Class:

public class MyApplication 
{
	private MainFrame mainFrame;
	private Brain brain;
	
	MyApplication()
	{
		this.mainFrame = new MainFrame();
		this.mainFrame.rollDice.addActionListener(new RollDiceListener());
	}
	
	class RollDiceListener implements ActionListener
	{
		@Override
		public void actionPerformed(ActionEvent e) 
		{
			System.out.println(brain.rollDie());
			mainFrame.validate();
		}
	}
}

and my Brain Class:

public class Brain {
	
	Random randomNumberGenerator;
	
	public int rollDie()
	{
		randomNumberGenerator = new Random();
		return (randomNumberGenerator.nextInt(6))+1;
	}

}

When I click the Roll Dice button I get:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at myPaackkage.MyApplication$RollDiceListener.actionP erformed(MyApplication.java:24)
at javax.swing.AbstractButton.fireActionPerformed(Unk nown Source)
at javax.swing.AbstractButton$Handler.actionPerformed (Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed (Unknown Source)

Any help ?

[doWhile]

You need to create an instance of Brain in the MyApplication class.

private Brain brain;

...brain is never initialized.

[fatabass]

Thanks.. So simple..

[Fubarable]

The key learning point is not to simply fix the NPE, but to understand that if you study the variables on the line the NPE is thrown, you often can back-track to find out why one of them is null. Also, learn to use System.out.println(...) debugging statements.