Thread: can anyone help me what is wrong with my code?

Hello, i am just a beginner in java programming, can anyone help me how to figure out what is wrong with my codes??

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

public class colorslider extends JFrame{
private JSlider redSlider, greenSlider, blueSlider;
private JLabel redlabel, greenlabel, bluelabel;
private JPanel colorpanel, sliders, labels;

public colorslider(){

redSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
greenSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
blueSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);


colorpanel = new JPanel();
colorpanel.setBackground(Color.BLACK);

Container pane = this.getContentPane();
pane.setLayout(new GridLayout(1, 3, 3, 3));

sliders = new JPanel();
labels = new JPanel();

pane.add(sliders);
pane.add(labels);
pane.add(colorpanel);

sliders.setLayout(new GridLayout(3, 1, 2, 2));
sliders.add(redSlider);
sliders.add(greenSlider);
sliders.add(blueSlider);

labels.setLayout(new GridLayout(3, 1, 2, 2));
labels.add(redlabel);
labels.add(greenlabel);
labels.add(bluelabel);

event e = new event();
redSlider.addChangeListener(e);
greenSlider.addChangeListener(e);
blueSlider.addChangeListener(e);


}

public class event implements ChangeListener{
public void stateChanged(ChangeEvent e){
int r = redSlider.getValue();
int g = greenSlider.getValue();
int b = blueSlider.getValue();

redlabel.setText("Red = " + r);
greenlabel.setText("Greeb = " + g);
bluelabel.setText("Blue = " + b);

colorpanel.setBackground(new Color(r, g, b));

}
}

public static void main(String[] args){
colorslider cs = new colorslider();
cs.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
cs.setTitle("Color Slider");
cs.setVisible(true);
cs.setSize(300, 110);

}

}


This the warning what the eclipse compiler always says:

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at colorslider.<init>(colorslider.java:36)
at colorslider.main(colorslider.java:64

Please help me!
Thank You...

Welcome to the forum. Please read the Announcement topic to learn how to post code in code or highlight tags and other useful info for newcomers.

Originally Posted by andbin

First, this is not a compiler "warning" but an exception at runtime. Second, please learn how to interpret the stack trace, it says:

at colorslider.<init>(colorslider.java:36)

What is there at line 36? There is this:

labels.add(redlabel);

Sice labels was instantiated (line 24) the only possible problem for a NullPointerException is the argument. And if you see, redlabel, greenlabel, bluelabel are declared but not initialized/assigned.

thanks for answering, but it is still not clear to me on how to correct it... Can you tell me how to initialized/assigned those labels?

and if you don't mind can you please simplify what you have just said... i didn't totally understand it...

sorry for my ignorance, i am just a newbie in java...

thanks...

Originally Posted by andbin

You have declared (as instance variable) a:

JSlider redSlider
and you have created an instance of JSlider with
redSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);

then you have declared a

JLabel redlabel
but you have not created an instance of JLabel.
Do you see in your code something like redlabel = new JLabel("...label..."); ?

It works now!! Thank you very much !!!!