当我为JFrame使用FlowLayout时，为什么我的图像没有出现？

Question

There is an issue which I do not understand: Why I am able to paint the image with the drawImage() method if the Layout of the JFrame is a BorderLayout or GridLayout but NOT GridbagLayout , FlowLayout or BoxLayout ? Can somebody please explain it to me?

Here is the code:

package footballQuestioner;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.RenderingHints;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.BorderFactory;
import javax.swing.ImageIcon;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.border.EtchedBorder;
import javax.swing.border.TitledBorder;




public class attempter {

    public static void main(String[] args) {
        JFrame frame = new Beispielfenster();

    }

}

class Beispielfenster extends JFrame {

    private class TransparentBG extends JLabel {

        BufferedImage image;

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

            try {
                image = ImageIO
                        .read(TransparentBG.class
                                .getClassLoader()
                                .getResourceAsStream(
                                        "footballQuestioner/rightAnswerSign.png"));

            } catch (IOException e) {
                e.printStackTrace();
            }

            Graphics2D g2d = (Graphics2D) g;

            g2d.drawImage(image, 0, 0, null);
            g2d.dispose();


        }

    }



    public Beispielfenster() {



             //setLayout(new FlowLayout());
        JPanel panel=new JPanel(new BorderLayout());

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);



        JLabel label=new TransparentBG();

        panel.add(label);

        add(panel);

        pack();

        centeringWindow();
        setVisible(true);

    }

    public void centeringWindow() {
        Dimension dimension = Toolkit.getDefaultToolkit().getScreenSize();
        int x;
        int y;

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

        setLocation(x, y);
    }



}

Answer 1

Because with the first two layouts, the JLabel will fill the container, here the JPanel. With the other layouts, it will size to its preferredSize which is 0. Consider overriding getPreferredSize if you wish to use other layouts.

Also note:

• You should not read in image files without a paintComponent method.
• You shouldn't be re-reading image files if possible (which will happen if you read in the images in a method that gets repeatedly called). Read them in once, such as in a constructor, and store them in a field.
• You should not dispose of a Graphics object that the JVM gives you, only one that you create yourself (such as from a BufferedImage).

---

Edit
For example, something like this:

private class TransparentBG extends JLabel {
  BufferedImage image;

  public TransparentBG() throws IOException {
     image = ImageIO.read(TransparentBG.class.getClassLoader()
           .getResourceAsStream("footballQuestioner/rightAnswerSign.png"));
  }

  @Override
  protected void paintComponent(Graphics g) {
     super.paintComponent(g);
     Graphics2D g2d = (Graphics2D) g;
     if (image != null) {
        g2d.drawImage(image, 0, 0, null);
     }
     // g2d.dispose();
  }

  @Override
  public Dimension getPreferredSize() {
     if (image != null) {
        int w = image.getWidth();
        int h = image.getHeight();
        return new Dimension(w, h);
     }
     return super.getPreferredSize();
  }
}