Question

I’m trying to display a frame using GridLayout, but one of my panels isn’t displaying.

The JPanel that I’m having trouble with (gridPanel) is supposed to have a 50 by 50 GridLayout and each cell in that grid is supposed to have a Square object added to it. Then that panel is supposed to be added to the frame, but it doesn’t display.

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

public class Gui extends JFrame{

    JPanel buttonPanel, populationPanel, velocityPanel, gridPanel;
    JButton setupButton, stepButton, goButton;
    JLabel populationLabel, velocityLabel;
    JSlider populationSlider, velocitySlider;
    Square [] [] square;

    public Gui() {

        setLayout(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();

        //Set up JButtons
        buttonPanel = new JPanel();
        setupButton = new JButton("Setup");
        stepButton = new JButton("Step");
        goButton = new JButton("Go");

        buttonPanel.add(setupButton);
        buttonPanel.add(stepButton);
        buttonPanel.add(goButton);

        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 0;
        c.gridy = 1;
        c.gridwidth = 2;   //2 columns wide
        add(buttonPanel, c);


        //Set up populationPanel    
        populationPanel = new JPanel();
        populationLabel = new JLabel("Population");
        populationSlider = new JSlider(JSlider.HORIZONTAL,0, 200, 1);

        populationPanel.add(populationLabel);
        populationPanel.add(populationSlider);

        c.fill = GridBagConstraints.LAST_LINE_END;
        c.gridx = 0;
        c.gridy = 2;
        c.gridwidth = 2;   //2 columns wide
        add(populationPanel, c);        


        //Set up velocityPanel
        velocityPanel = new JPanel();
        velocityLabel = new JLabel("    Velocity");
        velocitySlider = new JSlider(JSlider.HORIZONTAL,0, 200, 1);

        velocityPanel.add(velocityLabel);
        velocityPanel.add(velocitySlider);

        c.fill = GridBagConstraints.LAST_LINE_END;
        c.gridx = 0;
        c.gridy = 3;
        c.gridwidth = 2;   //2 columns wide
        add(velocityPanel, c);  


        //Set up gridPanel
        JPanel gridPanel = new JPanel(new GridLayout(50, 50));
        square = new Square[50][50];

        for(int i = 0; i < 50; i++){
            for(int j = 0; j < 50; j++){
                    square[i][j] = new Square();
                    gridPanel.add(square[i][j]);
            }
        }

        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 0;
        c.gridy = 0;
        c.gridwidth = 2;   //2 columns wide
        add(gridPanel, c);  
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        Gui frame = new Gui();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

Square class

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

public class Square extends JComponent{

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLACK);
        g.fillRect(10, 10, 10, 10);
    }

}

Answer 1

The reason you are not seeing anything are two-fold (though not entirely sure if the second is intentional, guessing on that part 🙂

• JComponent is a bare-bone container, it has no ui delegate and no inherent size – it’s up to your code to return something reasonable for min/max/pref
• the painting rect is hard-coded to “somewhere” inside, which might or might be inside the actual component area

Following is showing (border just to see the boundaries)

public static class Square extends JComponent {

    public Square() {
        setBorder(BorderFactory.createLineBorder(Color.RED));
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLACK);
        g.fillRect(0, 0, getWidth(), getHeight());
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(10, 10);
    }
    @Override
    public Dimension getMaximumSize() {
        return getPreferredSize();
    }
    @Override
    public Dimension getMinimumSize() {
        return getPreferredSize();
    }

}