Question

This is my current RectangleComponent class and I add it to a panel in my main JFrame but it never appears. I thought it wasn’t drawing so I decided to call the paintComponent method in the Rectangle’s constructor, and after sorting through 4-5 nullPointerExceptions, nothing has changed. I’ve read multiple guides on how to draw rectangles and I have seen multiple code examples, but I can never get the panels to work with more than one JComponent. If you could, please take a brief look at my code and see if you can devise a solution.
Thank you for your time. Also listed is the Frame I call the rectangle constructor in.

public class GameFrame extends JFrame
{
    private SpellBarComponent bar;
    private JPanel mainPanel = new JPanel();
    private JPanel buttonPanel = new JPanel();
    private JPanel healthPanel = new JPanel();
    Color green = new Color(29, 180, 29);
    Color red = new Color(255, 0, 0);
    private RectangleComponent life;
    private RectangleComponent death;
    private JFrame frame = new JFrame();

    public GameFrame(char x)
    {
        frame.setSize(1024, 768);
        frame.setTitle("Game");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        FlowLayout layout = new FlowLayout();
        createPanels(x);
        healthPanel.setLayout(layout);
        buttonPanel.setLayout(layout);
        mainPanel.setLayout(layout);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        repaint();
    }

    public RectangleComponent getLife()
    {
        return life;
    }

    private void createHealth()
    {
        life = new RectangleComponent(green, healthPanel);
        death = new RectangleComponent(red, healthPanel);
    }

    private void createPanels(char x)
    {
        add(healthPanel);
        pack();
        createBar(x);
        createHealth();
        mainPanel.add(buttonPanel);
        mainPanel.add(healthPanel);
        healthPanel.add(death);
        healthPanel.add(life);
        buttonPanel.add(bar.getSpell1());
        buttonPanel.add(bar.getSpell2());
        buttonPanel.add(bar.getSpell3());
        add(mainPanel);
    }

    private void createBar(char x)
    {
        bar = new SpellBarComponent(x, mainPanel);
    }
}


public class RectangleComponent extends JComponent
{
    Color color;
    int width;
    int height = 18;
    RoundRectangle2D roundedRectangle;
    private JPanel panel;
    public RectangleComponent(Color color, JPanel panel)
    {
        this.panel = panel;
        this.color = color;
        paintComponent(panel.getGraphics());
    }

    public void paintComponent(Graphics g)
    {
        Graphics2D graphics2 = (Graphics2D) g;
        width = 125;
        roundedRectangle = new RoundRectangle2D.Float(10, 10, width, height, 10, 10);
        graphics2.setPaint(color);
        graphics2.fill(roundedRectangle);
        graphics2.draw(roundedRectangle); 
    }

    public void subtractLife(int amount)
    {
        width -= amount;
        roundedRectangle.setRoundRect(10, 10, width, height, 10, 10);
        repaint();
    }
}

Answer 1

In order for your Swing Application to work as expected, there are many a things you need to keep in mind. There are always certain steps that one must follow in order to escape certain hurdles, that might can arise, since you coded in the wrong way. For this stick to the basics of Swing Programming Strictly, and follow them.

• Like as mentioned by @HovercraftFullOfEels , you calling to your
  Graphics directly, which one should never do.

• Secondly, look at your GameFrame() constructor, you set it to
  visible, even before you had added any components to it and much
  before it’s real size has been established

Such loop holes inside your coding might can give rise to many a headaches, as you sit down to write huge programs, so better to be on the safe road from the beginning, then to curse yourself at the later stage. As they say Prevention is better than Cure.

Now coming to your program, you missed the main thingy, since you failed to specify the size of your CustomComponent i.e. JComponent, hence you are not been able to see it on your screen. As you extends a JCompoent to your class, make it a customary habbit to override it’s getPreferredSize(), in the same manner you override it’s paintComponent(...) method.

Have a look at this small program, I had crafted for you, might be this be able to help you out, to understand the logic a bit more.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.RoundRectangle2D;
import javax.swing.*;

public class CustomPainting {

    private RectangleComponent life;
    private RectangleComponent death;

    private void createAndDisplayGUI() {
        JFrame frame = new JFrame("Custom Painting");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel centerPanel = new JPanel();
        centerPanel.setLayout(new GridLayout(0, 2, 5, 5));
        // Specifying the WIDTH, HEIGHT and Colour for this JComponent.
        life = new RectangleComponent(Color.GREEN.darker(), 20, 20);
        death = new RectangleComponent(Color.RED.darker(), 20, 20);
        centerPanel.add(life);
        centerPanel.add(death);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout(FlowLayout.LEFT, 5, 5));
        JButton incLifeButton = new JButton("INCREASE LIFE");
        incLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                life.addLife(1);
            }
        });

        JButton decLifeButton = new JButton("DECREASE LIFE");
        decLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                life.subtractLife(1);
            }
        });

        JButton incDeathButton = new JButton("INCREASE DEATH");
        incDeathButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                death.addLife(1);
            }
        });

        JButton decDeathButton = new JButton("DECREASE DEATH");
        decDeathButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                death.subtractLife(1);
            }
        }); 

        buttonPanel.add(incLifeButton);
        buttonPanel.add(decLifeButton);
        buttonPanel.add(incDeathButton);
        buttonPanel.add(decDeathButton);

        frame.getContentPane().add(centerPanel, BorderLayout.CENTER);
        frame.getContentPane().add(buttonPanel, BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String\u005B\u005D args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new CustomPainting().createAndDisplayGUI();
            }
        });
    }
}

class RectangleComponent extends JComponent {

    private Color colour;
    private static final int MARGIN = 10;
    private int width;
    private int height;
    private int originalWidth;
    private RoundRectangle2D roundedRectangle;

    public RectangleComponent(Color c, int w, int h) {
        colour = c;
        width = w;
        height = h;
        originalWidth = width;
    }

    /*
     * Overriding this method, so that
     * the size of the JComponent
     * can be determined, on the screen
     * or by the LayoutManager concern.
     */
    @Override 
    public Dimension getPreferredSize() {
        return (new Dimension(width, height));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        roundedRectangle = new RoundRectangle2D.Float(MARGIN, MARGIN,
                                        width, height, MARGIN, MARGIN);
        g2d.setPaint(colour);
        g2d.draw(roundedRectangle);
        g2d.fill(roundedRectangle);
    }

    public void subtractLife(int amount) {
        width -= amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width : " + width);
        if (width > 0) {
            roundedRectangle.setRoundRect(MARGIN, MARGIN, width, height,
                                            MARGIN, MARGIN);
            /*
             * This repaint() will call the paintComponent(...)
             * by itself, so nothing else to be done.
             */
            repaint();
        } else {
            width += amount;
        }
    }

    public void addLife(int amount) {
        width += amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width : " + width);
        if (width < originalWidth) {
            roundedRectangle.setRoundRect(MARGIN, MARGIN, width, height,
                                            MARGIN, MARGIN);
            repaint();
        } else {
            width -= amount;
        }
    }
}

Do ask any question, that might can arise as you go through this program :-), I be HAPPY to help on that 🙂

**LATEST EDIT WITH TWO COLOURS : **

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.RoundRectangle2D;
import javax.swing.*;

public class CustomPainting {

    private RectangleComponent lifeDeath;

    private void createAndDisplayGUI() {
        JFrame frame = new JFrame("Custom Painting");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel centerPanel = new JPanel();
        centerPanel.setLayout(new GridLayout(0, 2, 5, 5));
        // Specifying the WIDTH, HEIGHT and Colour for this JComponent.
        lifeDeath = new RectangleComponent(Color.GREEN, Color.RED, 20, 20);
        centerPanel.add(lifeDeath);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(1, 2, 5, 5));
        JButton incLifeButton = new JButton("INCREASE LIFE");
        incLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                lifeDeath.addLife(1);
            }
        });

        JButton decLifeButton = new JButton("DECREASE LIFE");
        decLifeButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                lifeDeath.subtractLife(1);
            }
        });

        buttonPanel.add(incLifeButton);
        buttonPanel.add(decLifeButton);

        frame.getContentPane().add(centerPanel, BorderLayout.CENTER);
        frame.getContentPane().add(buttonPanel, BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String\u005B\u005D args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new CustomPainting().createAndDisplayGUI();
            }
        });
    }
}

class RectangleComponent extends JComponent {

    private Color lifeColour;
    private Color deathColour;
    private static final int MARGIN = 10;
    private int widthLife;
    private int widthDeath;
    private int height;
    private int originalWidth;
    private RoundRectangle2D roundedRectangle;

    public RectangleComponent(Color lc, Color dc, int w, int h) {
        lifeColour = lc;
        deathColour = dc;
        widthLife = w;
        height = h;
        originalWidth = widthLife;
        widthDeath = 0;     
    }

    /*
     * Overriding this method, so that
     * the size of the JComponent
     * can be determined, on the screen
     * or by the LayoutManager concern.
     */
    @Override 
    public Dimension getPreferredSize() {
        return (new Dimension(originalWidth, height));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;

        roundedRectangle = new RoundRectangle2D.Float((MARGIN + widthDeath), MARGIN,
                                        widthLife, height, MARGIN, MARGIN);
        g2d.setPaint(lifeColour);
        g2d.draw(roundedRectangle);
        g2d.fill(roundedRectangle);

        roundedRectangle.setRoundRect(MARGIN, MARGIN,
                                        widthDeath, height, MARGIN, MARGIN);
        g2d.setPaint(deathColour);
        g2d.draw(roundedRectangle);
        g2d.fill(roundedRectangle);
    }

    public void subtractLife(int amount) {
        widthLife -= amount;
        widthDeath += amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width Life : " + widthLife);
        System.out.println("Width Death : " + widthDeath);
        if (widthLife > 0 && widthDeath < originalWidth) {
            /*
             * This repaint() will call the paintComponent(...)
             * by itself, so nothing else to be done.
             */
            repaint();
        } else {
            widthLife += amount;
            widthDeath -= amount;
        }
    }

    public void addLife(int amount) {
        widthLife += amount;
        widthDeath -= amount;
        System.out.println("ORIGINAL Width : " + originalWidth);
        System.out.println("Width Life : " + widthLife);
        System.out.println("Width Death : " + widthDeath);
        if (widthLife < originalWidth && widthDeath > 0) {
            repaint();
        } else {
            widthLife -= amount;
            widthDeath += amount;
        }   
    }
}