simple GUI, actionListener

[musclecode]

I'm testing swing, and I have error after I click on button

Exception in thread "AWT-EventQueue-0"
java.lang.NullPointerException
	at mojaGUI.actionPerformed(mojaGUI.java:35)

If, I change line 34 to something else it's working, but that method should change label of button (przycisk).

import java.awt.event.*;
import javax.swing.*;


public class mojaGUI implements ActionListener {

	JButton przycisk;
	
	public static void main(String[] args) {
		
		mojaGUI okno = new mojaGUI();
		okno.rob();
	}
	
		public void rob()
		{
			JFrame ramka = new JFrame();
			JButton przycisk = new JButton("potwierdz");
			
			przycisk.addActionListener(this);
			
			ramka.getContentPane().add(przycisk);
			ramka.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
			ramka.setSize(400,400);
			ramka.setVisible(true);
			
			
		}
		

	
	public void actionPerformed(ActionEvent event)
	{
		przycisk.setText("asdf");
	}

}

[Fubarable]

You have a classic case of variable shadowing. Since you have re-declared the przycisk variable in the class's constructor and have only initialized the przycisk that is local to the constructor, the class przycisk variable remains null. Solution: don't re-declare the variable in the constructor. Initialize and use the class przycisk variable instead.

[musclecode]

wow, I totally missed it. Thanks a lot ;)

[Fubarable]

wow, I totally missed it. Thanks a lot ;)

You're welcome. It's something that we all do, believe me. If you're using an IDE, you can often set it to flag you for these sorts of problems. I know that Eclipse has this ability.

[musclecode]

but how? I'm using eclipse.

[Fubarable]

Eclipse Menus:
window - preferences - Java - Compiler - Errors/Warnings

Then scroll down to Name shadowing and conflicts

[heatblazer]

Hello, since I am new to java too, I`d add an advice for events:
Always do this:

 public void actionPerformed(ActionEvent e) {
		
			screen += ((JButton)e.getSource()).getText();
			textFields.get("Screen1").setText(screen);
			System.out.println("dbg#"+screen);
	}

the line with e.getSource() will return the object that fired the event, thus getText(), in my case it`s the label of the buttons, will add the number to the screen of the calculator-like GUI, I`ve made. It was hard to me to understand for 2-3 hours today, since I am learning events self educationary.

[Fubarable]

Hello, since I am new to java too, I`d add an advice for events:
Always do this:

 public void actionPerformed(ActionEvent e) {
		
			screen += ((JButton)e.getSource()).getText();
			textFields.get("Screen1").setText(screen);
			System.out.println("dbg#"+screen);
	}

the line with e.getSource() will return the object that fired the event, thus getText(), in my case it`s the label of the buttons, will add the number to the screen of the calculator-like GUI, I`ve made. It was hard to me to understand for 2-3 hours today, since I am learning events self educationary.

I'm not sure how this advice applies to the original poster's code and problem.