Resizing an image when resizing JPanel at the same time

Question

The problem with my code is that my image only gets the beginning size of the JFrame - but I want it to change size every time the user changes the size of the window.

Also should the image size be relative to JFrame size or its JPanel? If so, how would I achieve that?

Here is my code:

import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;

public class MainFrame extends JFrame {

private JPanel card1;
private JPanel principalPanel,centerPanel;
private BufferedImage image;

public MainFrame() {

    setSize(800,600);
    setLocationRelativeTo(null);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setVisible(true);

    URL url = null;
    try {
        url = new URL("https://i.sstatic.net/7bI1Y.jpg");
    } catch (MalformedURLException e) {

    }
    try {
        image = ImageIO.read(url);
    } catch (IOException e) {
        System.out.println("Can not find image");
    }


    Image scaledImage = image.getScaledInstance(this.getWidth(),
            this.getHeight(),Image.SCALE_SMOOTH);

    card1 = new JPanel() {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawImage(scaledImage,0,0,this);
        }
    };

    centerPanel = new JPanel();
    centerPanel.setLayout(new CardLayout());
    centerPanel.add(card1,"1");

    principalPanel = new JPanel();
    principalPanel.setLayout(new BorderLayout());
    principalPanel.add(centerPanel,BorderLayout.CENTER);

    setContentPane(principalPanel);
}

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            new MainFrame();
        }
    });
}
}

Answer 1

The image could (indeed should) be scaled in the paintComponent(Graphics) method. That way it can change to whatever size the panel is.

See also the comments in the code:

import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.net.URL;

public class MainFrame2 extends JFrame {

    private JPanel card1;
    private BufferedImage image;

    public MainFrame2() {

        setSize(800, 600);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        try {
            // if there is a problem with URL - no point trying to load
            URL url = new URL("https://i.sstatic.net/7bI1Y.jpg");
            image = ImageIO.read(url);
        } catch (Exception e) {
            System.out.println("Problem loading image");
            e.printStackTrace();
        }

        /* The frame will actually be larger than the panel. 
        Better to scale the image to the SIZE OF THE PANEL ...
        Image scaledImage = image.getScaledInstance(this.getWidth(),
                this.getHeight(), Image.SCALE_SMOOTH); */

        card1 = new JPanel() {
            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
                // .. this is how.
                g.drawImage(image, 0, 0, getWidth(), getHeight(), this);
            }
        };

        setContentPane(card1);

        setVisible(true); // this should be last! 
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new MainFrame2();
            }
        });
    }
}