Java JPanel object updated but update not visible on the screen

Question

I am quite new to GUI programming and I don't know how to update an object and show the change on the screen. I have a class Sudoku which extends JPanel. I have a JFrame and the sudoku is part of it. I have a button, which calls the function openFile. Here is how it is called:

private void openFileActionPerformed(java.awt.event.ActionEvent evt) {                                         
    sudoku = FileManagement.openFile(this, sudoku);
}

The sudoku object has a 9x9 grid and when I debug it it's okay, the new values are in the grid. But I don't know how to display the new values on the screen. I read a few comments and tried sudoku.validate(), sudoku.repaint() or just packing the JFrame again but it didn't work. So I would appreciate if someone tells me how to fix it. If my explanation is insufficient or unclear, please tell me what else to add in the description. Thanks!

Edit - added code:

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import javax.swing.JFileChooser;
import javax.swing.JFrame;

public class Sample extends javax.swing.JFrame {
    public Sample() {
        initComponents();
    }

    private void initComponents() {
        sudoku = new sudoku.Sudoku();
        openFile = new javax.swing.JButton();

        openFile.setText("Open");
        openFile.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                openFileActionPerformed(evt);
            }
        });
        /*sorry for that part - automaticlly generated, don't know how to make it less*/
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                        .addComponent(sudoku)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(openFile)))));
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addComponent(sudoku)
                .addComponent(openFile)));
        pack();
    }                     

    private void openFileActionPerformed(java.awt.event.ActionEvent evt) {                                         
        sudoku = openFile(this, sudoku);
    }                                        

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Sample().setVisible(true);
            }
        });
    }

    private javax.swing.JButton openFile;
    private sudoku.Sudoku sudoku;                  

    public static Sudoku openFile(JFrame frame, Sudoku sudoku) {
        JFileChooser fileChooser = new JFileChooser();
        fileChooser.setFileSelectionMode(JFileChooser.FILES_ONLY);
        int result = fileChooser.showOpenDialog(frame);
        if (result == JFileChooser.CANCEL_OPTION) {
            return sudoku;
        }

        File fileName = fileChooser.getSelectedFile();

        try (ObjectInputStream input = new ObjectInputStream(new FileInputStream(fileName))) {
            sudoku = (Sudoku) input.readObject();
        } catch (IOException | ClassNotFoundException ioException) {
            System.out.println(ioException);
        }
        return sudoku;
    }
}

1) For better help sooner, post a [MCVE] or [Short, Self Contained, Correct Example](http://www.sscce.org/). 2) BTW - it's likely you could use a [`CardLayout`](http://download.oracle.com/javase/8/docs/api/java/awt/CardLayout.html) as shown in [this answer](http://stackoverflow.com/a/5786005/418556). — Andrew Thompson, Feb 18 '17 at 09:05

score 1 · Accepted Answer · edited May 23 '17 at 12:24

1

A quick fix for the problem would be to remove the JPanel from the JFrame, change its value, and then re-add the JPanel to the JFrame. This can be seen in the example below.

import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

@SuppressWarnings("serial")
public class PanelFix extends JFrame {
    JFrame frame = this;
    private JPanel view = a();

    PanelFix() {
        super("Panel Fix");
        setSize(400, 400);
        setLocationRelativeTo(null);
        getContentPane().add(view);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    private JPanel a() {
        JPanel panel = new JPanel();
        JButton but = new JButton("Press Me");
        but.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                getContentPane().remove(view);
                view = b();
                getContentPane().add(view);
                repaint();
                validate();
            }
        });
        panel.add(but);
        return panel;
    }

    private JPanel b() {
        JPanel panel = new JPanel();
        panel.add(new JLabel("Second Panel"));
        return panel;
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(() -> new PanelFix());
    }
}

If this does not fix the problem, like Andrew Thompson said, add an MCVE. This should allow us to help fix the exact problem you have

edited May 23 '17 at 12:24

Community

1
1

answered Feb 18 '17 at 09:32

Dan

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That worked! Thank you very much :) – vixenn Feb 18 '17 at 09:45
@vixenn No problem – Dan Feb 18 '17 at 09:46
1

From the sounds of `revalidate` and `repaint` would have been simpler solution, but since the OP failed to deliver an example of their work, well never know – MadProgrammer Feb 18 '17 at 10:51
@MadProgrammer Where would you have put those two functions in the code? I tried placing them after `view = b();` on both `view` and the `JFrame` and it still had no effect – Dan Feb 18 '17 at 12:07
@Dan `invalidate` and `repaint` should follow any change which might require the layout to be changed or the UI to be updated – MadProgrammer Feb 18 '17 at 21:36
@MadProgrammer I agree, it's just that your last comment made it sound like you could mitigate `getContentPane().remove(view);` and `getContentPane().add(view);` by just using `revalidate()` and `repaint()`. – Dan Feb 18 '17 at 21:49
@dan Basically, yes, revalidate and repaint will do what remove/add finally does, but with less overhead, because you're going have revalidate the layout and repaint the container anyway – MadProgrammer Feb 18 '17 at 23:06
@MadProgrammer, thank you for the comment. If I understand right you suggest `openFile()` then `revalidate` then `repaint` without removing the sudoku from the JFrame (although in you next comment you say something about `invalidate` so maybe I didn't get it right). I tried it now but it did not work. – vixenn Feb 19 '17 at 20:09
@vixenn Take a look at [this](http://stackoverflow.com/a/9511578/4601149). That explains the difference between the different validations – Dan Feb 19 '17 at 20:12
1

@Dan Thanks! That clears things up! Does the code now satisfy the MCVE or should I add the code for the Sudoku class too? – vixenn Feb 19 '17 at 20:16
@vixenn *"Does the code now satisfy the MCVE"* is it a self contained example which demonstrates your problem? Does it compile? Does it run? Does it demonstrate your problem? What is `Sudoku`? - My point is, in most cases, if you're updating the contents of a container, you should only need to call `revalidate` and `repaint` to have it update, but with no context to what you are actually doing, it's impossible to know what solution is right for you – MadProgrammer Feb 19 '17 at 20:27

Java JPanel object updated but update not visible on the screen

1 Answers1