JFrame not showing a picture

Following is the code I have so far: All the imports are correct. I'm sure. :D

When I run the program, all I get is a blank frame, without the picture. It should show up.

public class WindowPractice extends JFrame {

   final static int width= 800;
   final static int height= 400;
   int x;
   int y;
   Image steve;
   Dimension gamesize= new Dimension (width, height);
    public WindowPractice(){
        setTitle ("Hangman");
        setSize (gamesize);
        setVisible (true);
        setResizable (false);
        setLocationRelativeTo (null);
        setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);


    }
    public static void main (String[] args) {
        new WindowPractice();
        ImageIcon steve= new ImageIcon ("Libraries/Pictures/ba190cd951302bcebdf216239e156a4.jpg");
        JLabel imageLabel = new JLabel(steve);

    }
    public void paint(Graphics g){

        g.setColor(Color.red);
        //g.fillRect(  x, y, 100, 20);
        g.drawImage(steve, x, y,this);


        x= 150;
        y= 250;
    }

}

There are so many things wrong with this I'm not sure where to start...

Let's start at the beginning...

Problem #1

You declare a instance field called steve in you WindowPractice class, this is fine, but in your main method, you declare ANOTHER variable called steve which you are using the reference to the loaded image...

public static void main(String[] args) {
    new WindowPractice();
    ImageIcon steve = new ImageIcon("C:/Users/shane/Dropbox/issue459.jpg");
    JLabel imageLabel = new JLabel(steve);
}

This means that the class instance variable is never initialised and remains null.

Problem #2

While not directly related, you never call super.paint from your paint method. This is a BIG NO, NO. You are obligated to maintain the paint chain. The paint methods are complex and very, very important.

Problem #3

You should never override a top level container (such as JFrame) nor you should you override any of it's paint methods. There are lots of reasons for this, but among the top two are, most top level containers actually contain a number of components (JRootPane, which houses the glass pane, content pane, layer pane and menu bar) which can sit over your painting efforts and, generally, they're not double buffered, meaning you paint updates will flicker and look horrible ;)

You should also avoid using paint, instead, you should look towards using paintComponent where it's available.

Problem #4

ImageIcon is not you best choice for loading images. The main reason I don't use them is that you have no idea of when the image being loaded will actually become available (actually there are ways, but to be frank, ImageIO is just simpler). This was a great feature back in 1999 when dial-up speeds where around the 14.4k mark, but now days...

ImageIO supports a wider range of picture formats, supports reading and writing of images and guarantees that when the method returns (successfully), the image pixel data is available to your application.

Example

Here's a better (IMHO) approach...

public class BetterDrawing {

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

    public BetterDrawing() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new PaintPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class PaintPane extends JPanel {

        private BufferedImage background;

        public PaintPane() {
            try {
                background = ImageIO.read(new File("/path/to/image"));
                // Use this instead to load embedded resources instead
                //background = ImageIO.read(getClass().getResource("/path/to/image"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return background == null ? super.getPreferredSize() : new Dimension(background.getWidth(), background.getHeight());
        }

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

            if (background != null) {

                int x = (getWidth() - background.getWidth()) / 2;
                int y = (getHeight() - background.getHeight()) / 2;

                g.drawImage(background, x, y, this);

            }

        }
    }
}

Have a read of

For some more information.

来源：https://stackoverflow.com/questions/13871307/jframe-not-showing-a-picture

标签

java

swing

jframe