Change Background Color of Jpanel in Java

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I Can't change JPanel Background colour.

Posted 27 November 2011 - 07:50 AM

I'm just started out learning about Swing and i'm trying to write a program with a simple button which will change the background colour of the panel.

here is what i have so far

class MyWindowListener extends WindowAdapter  {     @Override     public void windowClosing(WindowEvent e)     {         System.out.println("Closing window!");         System.exit(0);     } }        class MyButtonListener implements ActionListener  {     @Override     public void actionPerformed(ActionEvent e)      {         System.out.println("test");         panel.setBackground(Color.yellow);              } }  public class FredGui5 {          public static void main(String[] args) {                           JFrame frame = new JFrame("Fred plays with buttons...again!");         frame.setVisible(true);         frame.setSize(400, 400);         frame.addWindowListener(new MyWindowListener());                  JPanel panel = new JPanel();                  frame.getContentPane().add(panel, BorderLayout.NORTH);         GridBagConstraints c = new GridBagConstraints();                  JButton button = new JButton("Push me for yellow");         c.gridx = 0;         c.gridy = 1;                  button.addActionListener(new MyButtonListener());                  panel.add(button, c);         frame.add(panel);              } }            

in the actionPerformed method of the buttonlistener class there is an error on the "p.setBackground(Color.yellow);" line. It says that it cannot find variable panel in the mybuttonlistener class. How do i alter the code so panel can be seen by that class?

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#2 GregBrannon

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Re: I Can't change JPanel Background colour.

Posted 27 November 2011 - 09:18 AM

"panel" is local to your main() method, so is not visible to your MyButtonListener class. How might you make "panel" visible to your other classes?

#3 nexus490

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Re: I Can't change JPanel Background colour.

Posted 27 November 2011 - 10:01 AM

I can't declare it public can? I've also tried moving the "JPanel panel = new JPanel();" line to outside of the main and into the buttonlistenerclass but that causes other errors.

#4 GregBrannon

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Re: I Can't change JPanel Background colour.

Posted 27 November 2011 - 10:16 AM

I suggest you try something like:

public class FredGui5 {     // declare panel to be a class variable here     public static JPanel panel;      public static void main(String[] args)     {         JFrame frame = new JFrame("Fred plays with buttons...again!");         // . . .          panel = new JPanel();         // . . .      }      static class MyButtonListener implements ActionListener      {         @Override         public void actionPerformed(ActionEvent e)          {             System.out.println("test");             panel.setBackground(Color.yellow);         }     } }            

As your approach to coding GUIs evolves beyond running everything from the static main() method, your construction of a small app like this will change so that little, if anything has to be static.

This post has been edited by GregBrannon: 27 November 2011 - 10:17 AM

#5 nexus490

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Re: I Can't change JPanel Background colour.

Posted 27 November 2011 - 11:30 AM

The code is now as follows:

public class FredGui5 {      public static JPanel panel;                public static void main(String[] args)      {                          JFrame frame = new JFrame("Fred plays with buttons...again!");         frame.setVisible(true);         frame.setSize(400, 400);         frame.addWindowListener(new MyWindowListener());                  panel = new JPanel();                  frame.getContentPane().add(panel, BorderLayout.NORTH);         GridBagConstraints c = new GridBagConstraints();                  JButton button = new JButton("Push me for yellow");         c.gridx = 0;         c.gridy = 1;                  button.addActionListener(new MyButtonListener());                  panel.add(button, c);         frame.add(panel);              }          static class MyWindowListener extends WindowAdapter  {     @Override     public void windowClosing(WindowEvent e)     {         System.out.println("Closing window!");         System.exit(0);     } }          static class MyButtonListener implements ActionListener  {     @Override     public void actionPerformed(ActionEvent e)      {        JPanel panel = new JPanel();          System.out.println("test");         panel.setBackground(Color.yellow);              }  }              }            

Only problem is when i press the button the test String is output but the colour won't change. Any ideas?

#6 nexus490

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Re: I Can't change JPanel Background colour.

Posted 27 November 2011 - 12:14 PM

Its ok, no need to reply i found my mistake. For some reason i was creating a panel object twice.

Change Background Color of Jpanel in Java

Source: https://www.dreamincode.net/forums/topic/257359-i-cant-change-jpanel-background-colour/

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