如何在JPanel中使用RectangularShape绘制JPanel

Question

I'm having an issue with drawing RectangularShapes Or Shapes. I need to have this within the actual Panel instead of within the Frame because I need that kind of flexibility. But I keep on getting stuck. I haven't been well trained in GUI with any language so feedback is much appreciated.

Code is below, Please help me find out why this isn't working, and how I can get it to work.

public class GUI_Test 
{
    public static void main(String[] args)
    {
        GUI_Test gui = new GUI_Test();
        gui.tryBasic();
    }


    public void tryBasic()
    {
        JFrame MyFrame = new JFrame();
        MyFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        MyFrame.setSize(800, 500);
        MyFrame.setLayout(new BorderLayout());

        // This won't work and can't figure out why!!!!
        // The button appears but not the Ellipse.
        JPanel JP = new JPanel();
        JButton btn3 = new JButton("This is a button");
        JP.add(new MyGUIObject(new Ellipse2D.Double(70, 70, 100, 100), Color.red));
        JP.add(btn3);
        MyFrame.getContentPane().add(JP);

        // This works but I need it in a panel for easier control 
        //MyFrame.add(new MyGUIObject(new Ellipse2D.Double(70, 70, 100, 100), Color.blue));

        // This of course works.
        //MyFrame.getContentPane().add(new MyGUIObject(new Ellipse2D.Double(70, 70, 100, 100), Color.green));

        MyFrame.setVisible(true);

    }
    public class MyGUIObject extends JPanel
    {
        protected RectangularShape RecObj;
        protected Color myColor;    
        public MyGUIObject(RectangularShape SetShape, Color setColor)
        {
            RecObj = SetShape;
            myColor = setColor;
        }
        @Override
        protected void paintComponent(Graphics g)
        {
            Graphics2D g2d = (Graphics2D) g;
            g2d.setPaint(myColor);
            g2d.fill(RecObj);
        }
    }
}

Answer 1

There are three main problems I can see.

1. You're not calling super.paintComponent from your paintComponent method, this is going to cause some interesting, but nasty paint artefacts
2. JPanel use a FlowLayout by default, where the JFrame uses a BorderLayout , this is further complicated by...
3. You don't provide any sizing hints for the MyGUIObject class, so the FlowLayout will use the JPanel 's default preferredSize , which is 0x0 when it calculates the layout for the JPanel .

First, call super.paintComponent from your paintComponent method and override the getPreferredSize method

public class MyGUIObject extends JPanel {

    protected RectangularShape RecObj;
    protected Color myColor;

    public MyGUIObject(RectangularShape SetShape, Color setColor) {
        RecObj = SetShape;
        myColor = setColor;
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400, 400);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setPaint(myColor);
        g2d.fill(RecObj);
    }
}

Next, change the layout that the JPanel is using to something like BorderLayout (although the default FlowLayout will work, BorderLayout will give you a little more control)

JPanel JP = new JPanel(new BorderLayout());
JButton btn3 = new JButton("This is a button");
JP.add(new MyGUIObject(new Ellipse2D.Double(70, 70, 100, 100), Color.red));
JP.add(btn3, BorderLayout.SOUTH);
MyFrame.getContentPane().add(JP);

Have a look at Painting in AWT and Swing , Performing Custom Painting , Laying Out Components Within a Container and How to Use Borders for more details

Answer 2

How about this:

public class GUI_Test {
    public static void main(String[] args) {
        GUI_Test gui = new GUI_Test();
        gui.tryBasic();
    }

    public void tryBasic() {
        JFrame frame = createFrame();
        JPanel panel = new JPanel(new BorderLayout());
        JButton button = new JButton("This is a button");
        JPanel buttonPanel = new JPanel(new FlowLayout());
        buttonPanel.add(button);
        MyGUIObject guiObject = new MyGUIObject(new Ellipse2D.Double(70, 70, 100, 100), Color.red);
        panel.add(buttonPanel, BorderLayout.PAGE_START);
        panel.add(guiObject,BorderLayout.CENTER);
        frame.getContentPane().add(panel, BorderLayout.CENTER);
        frame.setVisible(true);
    }

    private JFrame createFrame() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(800, 500);
        frame.setLayout(new BorderLayout());
        return frame;
    }

    public class MyGUIObject extends JPanel {
        protected RectangularShape RecObj;
        protected Color myColor;

        public MyGUIObject(RectangularShape SetShape, Color setColor) {
            RecObj = SetShape;
            myColor = setColor;
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g;
            g2d.setPaint(myColor);
            g2d.fill(RecObj);
        }
    }
}