是否通过validate（），revalidate（）或paint（）调用JComponent的paint（Graphics g）方法？

Question

I haven't made a Java Swing app for a while (2+ years), so excuse me for any silly mistake.

I want to upload some artwork , that will be displayed as full screen image.

The problem I'm having is that the JComponent, that I have extended to represent my Image object , does not call its paint () method & does not draw itself on the existing JPanel.

Below is the relevant snippet & event that uploads an Image & instantiates by “art” class which extends JComponent Class JFrameMainArt's actionPerformed() method (relevant section only) , when the OPEN action fies , the image object should be drawn on the fullscreenJPanel

@Override
    public void actionPerformed(ActionEvent arg0) {

        if (arg0.getActionCommand().equalsIgnoreCase("Fullscreen Mode")) {
            isScreenFullscreen = true;
            // full screen mode settings
            fullscreenFrame.invalidate();
            fullscreenFrame = null;
            // create new fullscreen frame
            fullscreenFrame = new JFrame();
            fullscreenFrame
                    .setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            fullscreenFrame.setUndecorated(true);
            fullscreenFrame.setResizable(false);
            // add label to Panel
            fullscreenPanel.add(new JLabel("ALT + F4 to exit .",
                    SwingConstants.CENTER), BorderLayout.CENTER);
            fullscreenFrame.invalidate();
            GraphicsEnvironment.getLocalGraphicsEnvironment()
                    .getDefaultScreenDevice()
                    .setFullScreenWindow(fullscreenFrame);
        } else if (arg0.getActionCommand().equalsIgnoreCase("Upload")) {
            System.out.println("Upload Pictures");
            fileChooser = new JFileChooser();
            fileChooser
                    .setDialogTitle("Choose Your Image File (PNG or JPEG only)");
            // below codes for select the file
            int returnval = fileChooser.showOpenDialog(null);
            File file = fileChooser.getSelectedFile();
            System.out.println("Upload Pictures, selected file path: "
                    + file.getAbsolutePath());

            // if OPEN
            if (returnval == JFileChooser.APPROVE_OPTION) {

                String name = file.getName();
                if (name != null) {
                    int i = name.indexOf('.');
                    // get the extension such as jpeg, or png
                    String extension = name.substring(i + 1);
                    System.out.println("extension: " + extension);
                    if (extension.equalsIgnoreCase("jpeg")
                            || extension.equalsIgnoreCase("jpg")
                            || extension.equalsIgnoreCase("png")) {

                        Art newArt = new Art(file, fullscreenPanel.getSize());
                        //test dimension 
                        System.out.println("Dimension of image "+name+"- Dimensions "+newArt.scaleImageToJPanelDimensions(newArt.getImageFromFile()));


                        //add Art Jcomponent to existing JPanel
                        fullscreenPanel.add(newArt);
                        //revalidate & paint
                        fullscreenPanel.revalidate();
                        fullscreenPanel.repaint();
                        //pack & revalidate the Jframe
                        fullscreenFrame.revalidate();


                    } else {
                        System.out
                                .println("Error file/image type is not supported: "
                                        + extension);
                    }

                }// end name not null
                else {
                    System.out.println("Error file name: " + name);
                }

            } else if (returnval == JFileChooser.ERROR_OPTION) {
                System.out.println("Error selecting file");
            } else {
                System.out.println("returnval: " + returnval);
            }

        }// end else if upload
        else if (arg0.getActionCommand().equalsIgnoreCase("Choose Effect")) {

        } else {

        }

    }//end method actionPerformed()

Art.java extends JComponent and is responsible for loading the image, scaling image, drawing of the image object

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.beans.Transient;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JComponent;

public class Art extends JComponent {

    private File file = null;
    private BufferedImage image = null;
    private Dimension dimensionToScale = null;

    public Art(File file, Dimension dimensionToScale) {
        super();
        this.file = file;
        this.dimensionToScale = dimensionToScale;
        this.setVisible(true);

    }

    @Override
    @Transient
    public Dimension getPreferredSize() {
        // TODO Auto-generated method stub
        return this.dimensionToScale;
    }

    public Art(File file) {
        super();
        this.file = file;
        this.setVisible(true);
    }

    public BufferedImage getImageFromFile() {
        BufferedImage  image = null;
        if (this.file != null) {
            try {
                image = ImageIO.read(file);
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        return image;
    }

    public Dimension scaleImageToJPanelDimensions(BufferedImage image) {
        Dimension dimension = null;
        // scaling factors

        int imageWidth = image.getWidth();
        int imageHeight = image.getHeight();


        // scale based on largest side
        int largestImageSide = Math.min(imageWidth, imageHeight);

        double scaledHeight =  this.dimensionToScale.getHeight()
                / largestImageSide;
        double scaledWidth =  this.dimensionToScale.getWidth()
                / largestImageSide;
        // compute new image dimensions
        imageWidth = (int) (imageWidth * scaledWidth);
        imageHeight = (int) (imageHeight * scaledHeight);

        dimension = new Dimension(imageWidth, imageHeight);

        return dimension;

    }

    @Override
    public void paintComponent(Graphics g) {

        System.out.println("paintComponent(Graphics g) ");

        super.paintComponent (g);

        // repaint background
        g.setColor(getBackground());
        g.fillRect(0, 0, getWidth(), getHeight());
        g.setColor(getForeground());


        image = getImageFromFile();
        if (image != null) {

            // compute dimensions of image to draw
            Dimension imageDimension = scaleImageToJPanelDimensions(image);
    /*public abstract boolean drawImage(Image img,
            int x,
            int y,
            int width,
            int height,
            ImageObserver observer)*/

            g.drawImage(image, 0,0, (int) imageDimension.getWidth() ,(int)imageDimension.getHeight(),this);

        } else {
            System.out.println("paintComponent(Graphics g)  image:"+image);
        }

    }

}/end Art class

Any help will be appreciated.

Thanks.

Actual working method ,image is now drawn , issue is solved.

public void loadArtToScreen(Art art)
    {


        //add Art JComponent to existing JPanel
        JPanelGroupLayout.setHorizontalGroup(JPanelGroupLayout.createParallelGroup(
                Alignment.LEADING).addGroup(
                        JPanelGroupLayout.createSequentialGroup().addGap(0)
                        .addComponent(art )
                        .addContainerGap((int)mainJFrameSize.getWidth(), Short.MAX_VALUE)));

        JPanelGroupLayout.setVerticalGroup(JPanelGroupLayout.createParallelGroup(
                Alignment.LEADING).addGroup(
                        JPanelGroupLayout.createSequentialGroup().addGap(0)
                        .addComponent(art )
                        .addContainerGap((int)mainJFrameSize.getHeight(), Short.MAX_VALUE)));

        //revalidate & paint
        fullscreenPanel.revalidate();
        fullscreenPanel.repaint();
        //pack & revalidate the Jframe
        mainJFrame.revalidate();

    }//end loadArtToScreen(Art art)

Answer 1

1. You never load the image , so your paint method is going to throw a NullPointerException
2. Graphics#drawImage(Image, int, int, ImageObserver) refers to the position the image should be painted, not it's size (strangely, your scale algorithm is return 0x0 , but I didn't spend much time looking into it)
3. In this context, you should be passing this to the last parameter of Graphics#drawImage
4. It is recommended that custom painting be done within the context of the paintComponent method and not the paint method. See Performing Custom Painting for more details

nb- I also tried adding the pane to a full screen window without issues. I even introduced a small delay between the frame becoming visible and adding the pane and had it work - after I corrected for the NullPointerException .

Try removing the full screen requirement while you test it...