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Thread: How to change JPanel color from another class?

  1. #1
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    Default How to change JPanel color from another class?

    I am trying to change the color of mainPanel from a different class. I have a Menu to select a radio button(black background or blue background) that is supposed to change the color of my mainPanel however, I am newer to Java and I have no idea what I am doing wrong. The code is below:


    import java.awt.BorderLayout;
    import java.awt.Color;
    import javax.swing.JFrame;
    import javax.swing.JPanel;
     
     
    public class MainBoard extends JFrame{
     
    	private JPanel mainPanel;
     
    	public static void main(String[] args) {
    		MainBoard mb = new MainBoard();
    		mb.setVisible(true);
    		mb.setDefaultCloseOperation(DISPOSE_ON_CLOSE);
    	}
     
    	public MainBoard() {
    		setTitle("Tic-Tac-Toe");
    		setSize(600,600);
     
    		mainPanel = new JPanel(new BorderLayout());
    		mainPanel.setBackground(Color.BLACK);
    		add(mainPanel);
     
    		Menu menu = new Menu();
    		setJMenuBar(menu.getMenu());
    	}
     
    	public JPanel getMainPanel() {
    		return mainPanel;
    	}
     
    	public void setMainPanel(Color c) {
    		this.mainPanel.setBackground(c);
    	}
    }


    import java.awt.Color;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
     
    import javax.swing.ButtonGroup;
    import javax.swing.JMenu;
    import javax.swing.JMenuBar;
    import javax.swing.JMenuItem;
    import javax.swing.JRadioButtonMenuItem;
     
    public class Menu extends JMenu{
    	private JMenuBar menu;
    	private JMenu file;
    	private JMenu editGUI;
    	private JMenuItem newGame;
    	private JRadioButtonMenuItem blackBackground;
    	private JRadioButtonMenuItem blueBackground;
    	private JMenuItem exit;
     
    	public Menu() {
    		menu = new JMenuBar();
     
    		file = new JMenu("File");
    		exit = new JMenuItem("Exit");
    		newGame = new JMenuItem("New Game");
    		menu.add(file);
    		file.add(newGame);
    		file.add(exit);
     
    		editGUI = new JMenu("Edit GUI");
    		ButtonGroup bg = new ButtonGroup();
    		blackBackground = new JRadioButtonMenuItem("Black Background",false);
    		blueBackground = new JRadioButtonMenuItem("Blue Background",false);
    		bg.add(blueBackground);
    		bg.add(blackBackground);
    		menu.add(editGUI);
    		editGUI.add(blackBackground);
    		editGUI.add(blueBackground);
     
    		MenuHandler mh = new MenuHandler();
    		exit.addActionListener(mh);
    		blueBackground.addActionListener(mh);
     
    	}
     
    	public JMenuBar getMenu() {
    		return this.menu;
    	}
     
    	private class MenuHandler implements ActionListener {
     
    	    public void actionPerformed(ActionEvent e){
    	        if(e.getSource() == exit) {
    	            System.exit(0);
    	        }else if(e.getSource() == blueBackground) {
    	            MainBoard mb = new MainBoard();
    	            mb.setMainPanel(Color.BLUE);
    	        }
    	    }
    	}
    }

    The exit menu item will work, but the blueBackground menu item will not. Thank you for any help!
    Last edited by uk1992; March 19th, 2019 at 09:58 AM.

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: How to change JPanel color from another class?

    Can you add the import statements so that the code can be compiled and executed for testing?

    The code needs to access the currently shown mainPanel object to change it. The code is creating a new MainBoard object with its own mainPanel object. The Menu object needs a reference to the existing MainBoard object so it can call its setMainPanel method.

    Also mainPanel should not be static.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: How to change JPanel color from another class?

    No problem!

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