My first time posting a question here. Been coming here a while and enjoyed reading the threads. Was hoping someone on here could help me with a program I've been doing to learn Java. The program calls to implement sliders to change the background color in a gui background. It compiles fine, but when I run it, I get a few errors, which I commented in at the end of the code.
Code is as follows:
import java.awt.*;
import javax.swing.border.*;
import javax.swing.event.*;
import javax.swing.*;
public class sliderDemo extends JFrame
{
private JSlider redSlider, greenSlider, blueSlider;
private JPanel labels, sliders, colors;
private JLabel redlabel,greenlabel, bluelabel, colorlabel;
JTextArea colorPanel;
public sliderDemo()
{
setTitle("Slider Excercise");
setLayout(new BorderLayout(5, 5));
ChangeListener event = new eventListener();
colorlabel = new JLabel("Sliders to change colors:");
redlabel = new JLabel("Red slider");
greenlabel = new JLabel("Green slider");
bluelabel = new JLabel("Blue slider");
labels = new JPanel();
labels.setLayout(new GridLayout(3,1));
labels.add(redlabel);
labels.add(greenlabel);
labels.add(bluelabel);
redSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
redSlider.addChangeListener(event);
redSlider.setMaximum(255);
redSlider.setPaintLabels(true);
redSlider.setPaintTicks(true);
redSlider.setMajorTickSpacing(25);
redSlider.setMinorTickSpacing(5);
redSlider.setPaintTrack(false);
greenSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
greenSlider.addChangeListener(event);
greenSlider.setMaximum(255);
greenSlider.setPaintLabels(true);
greenSlider.setPaintTicks(true);
greenSlider.setMajorTickSpacing(25);
greenSlider.setMinorTickSpacing(5);
greenSlider.setPaintTrack(false);
blueSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
blueSlider.addChangeListener(event);
blueSlider.setMaximum(255);
blueSlider.setPaintLabels(true);
blueSlider.setPaintTicks(true);
blueSlider.setMajorTickSpacing(25);
blueSlider.setMinorTickSpacing(5);
blueSlider.setPaintTrack(false);
sliders = new JPanel();
sliders.setLayout(new GridLayout(3,1));
sliders.add(colorlabel);
sliders.add(redSlider);
sliders.add(greenSlider);
sliders.add(blueSlider);
colorPanel = new JTextArea(10, 10);
colorPanel.setEditable(false);
colorPanel.setBackground(Color.WHITE);
colorPanel.add(sliders, BorderLayout.CENTER);
colorPanel.add(colors, BorderLayout.NORTH);
colorPanel.add(labels, BorderLayout.WEST);
colors = new JPanel(new BorderLayout(5, 5));
colors.add(colorlabel);
colors.add(colorPanel, BorderLayout.NORTH);
}
public static void main(String[] args)
{
JFrame myFrame = new sliderDemo();
myFrame.setSize(500, 500);
myFrame.setVisible(true);
myFrame.setLocationRelativeTo(null);
myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public class eventListener implements ChangeListener
{
public void stateChanged(ChangeEvent e)
{
int r = redSlider.getValue();
int g = greenSlider.getValue();
int b = blueSlider.getValue();
colorPanel.setBackground(new Color(r, g, b));
}
}
}
/*
Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1090)
at java.awt.Container.add(Container.java:966)
at sliderDemo.<init>(sliderDemo.java:79)
at sliderDemo.main(sliderDemo.java:89)
Press any key to continue . . .
add JComponents
that are initialized
you tried to add JPanel colors
to JTextArea
(quite nonsence) and its intialization colors = new JPanel(new BorderLayout(5, 5));
is in the next code lines
rename JTextArea colorPanel;
to JTextArea textArea
then you miss 4th JPanel
, because JTextArea colorPanel
is called textArea
and JTextArea
isn't container for JPanels
, is designated for user keys input