JPanel does not appear in JDialog

Question

I am solving an activity with the following statement: Create an application to display in a JDialog window eight JPanel components. Each panel should be colored in one of the eight colors in figure 1. Figure 1

On each panel should be written the word that translates the meaning of the color. Use font size 18. Each panel should be colored using a color from Table 1, specifying the amount of each RGB (Red, Green, Blue) component that corresponds to the meaning of the color. Use the java.awt.Color class. Should be implementing just a single paintComponent method to paint the 8 panels and write the meaning of each color. Table 1

The problem is that my JPanel does not appear in JDialog. And I have no idea how to make it appear.

Follows the code:

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.GridLayout;

import javax.swing.JDialog;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class SigCoresGUI extends JDialog {

    private static final long serialVersionUID = 1L;
    private Color[] cores = { new Color(255, 255, 255), new Color(249, 206, 137), new Color(255, 128, 0),
            new Color(255, 0, 0), new Color(244, 102, 174), new Color(5, 120, 203), new Color(116, 186, 160),
            new Color(0, 0, 0) };
    private String[] sig = { "Paz", "Energia", "Criatividade", "Paixão", "Ternura", "Tranquilidade", "Harmonia",
            "Elegância" };
    private Font font = new Font("Arial", Font.BOLD, 18);

    public SigCoresGUI() {
        super();
        Desenha desenha = new Desenha();
        add(desenha);
        setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
        setLocationByPlatform(true);
        setLayout(new GridLayout(8, 8));
        setSize(400, 500);
        setVisible(true);
    }

    public class Desenha extends JPanel {
        private static final long serialVersionUID = 1L;

        @Override
        public void paintComponents(Graphics g) {
            super.paintComponents(g);
            for (int i = 0; i < 8; i++) {
                JPanel panel = new JPanel();
                panel.setBackground(cores[i]);
                panel.setFont(font);
                JLabel label = new JLabel(sig[i]);
                label.setFont(font);
                if (i > 0)
                    label.setForeground(Color.WHITE);
                panel.add(label);
                add(panel);
            }
        }
    }
}

Answer 1

Never add or remove components from a container (here your JPanel) within a painting method. Painting methods should be for painting and painting only, not for changing the component structure of a container. Understand that you do not have direct control over when or even if a painting method is called, and it can be called many times -- adding many unnecessary components to the container -- and you also never want to slow painting down.

You are overriding paintComponents, a method that (per the API)

Paints each of the components in this container

and since the JPanel has no components to begin with, the method is likely never called.

Instead, add your components in the SigCoresGUI constructor.

Also:

• Avoid using setSize(...)
• Instead pack() the JDialog before displaying it to let the components and containers size themselves using their preferred sizes.

So this will work although I don't know what layout you're looking for:

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

public class SigCoresGUI extends JDialog {

    private static final long serialVersionUID = 1L;
    private Color[] cores = { new Color(255, 255, 255), new Color(249, 206, 137), new Color(255, 128, 0),
            new Color(255, 0, 0), new Color(244, 102, 174), new Color(5, 120, 203), new Color(116, 186, 160),
            new Color(0, 0, 0) };
    private String[] sig = { "Paz", "Energia", "Criatividade", "Paixão", "Ternura", "Tranquilidade", "Harmonia",
            "Elegância" };
    private Font font = new Font("Arial", Font.BOLD, 18);

    public SigCoresGUI() {
        super();
        Desenha desenha = new Desenha();
        add(desenha);
        pack();
        setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
        setLocationByPlatform(true);
        // setLayout(new GridLayout(8, 8));
        // setSize(400, 500);
        setVisible(true);
    }

    public class Desenha extends JPanel {
        private static final long serialVersionUID = 1L;

        public Desenha() {
            setLayout(new GridLayout(0, 1));
            for (int i = 0; i < 8; i++) {
                JPanel panel = new JPanel(new GridBagLayout());
                panel.setBackground(cores[i]);
                panel.setFont(font);
                panel.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
                JLabel label = new JLabel(sig[i]);
                label.setFont(font);
                if (i > 0)
                    label.setForeground(Color.WHITE);
                panel.add(label);
                add(panel);
            }
        }

        //@Override
        //public void paintComponents(Graphics g) {
        //    super.paintComponents(g);
        //}
    }

    public static void main(String[] args) {
        new SigCoresGUI();
    }
}