“扩展JFrame”和JPanel？

Question

The following code, as you see below:

public class app extends javax.swing.JFrame implements Runnable {

extends JFrame. But I also need it to extend JPanel, in order to make a transparent JPanel. The problem is that I can't extend both, java throws a mistake:

If I extend JPanel I'm able to make a transparent JPanel, but the program can't run because there's a mistake in a few lines of code (mistake that disappears if I extend JFrame).

However, if I extend JFrame the program will run just fine, but it keeps me away from doing a transparent JPanel. How can I solve this?

Answer 1

Basically, create your custom class extending from a JPanel , use setOpaque to false to make it transparent.

Create an instance of JFrame , set it to undecorated and adjust it's opacity separately.

Add the custom panel to the frame...

Example

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.Image;
import java.awt.LayoutManager;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TransparentPanel {

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

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

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
            setLayout(new BorderLayout());
            try {
                BufferedImage img = ImageIO.read(new File("/Users/swhitehead/Dropbox/MegaTokyo/issue459.jpg"));
                final JLabel label = new JLabel(new ImageIcon(img.getScaledInstance(-1, 200, Image.SCALE_SMOOTH)));
                label.setLayout(new CardLayout());

                JPanel menu = new JPanel(new GridBagLayout());
                JButton button = new JButton("Show");
                menu.add(button);

                JPanel transparent = new JPanel(new GridBagLayout());
                transparent.setOpaque(false);
                transparent.add(new JLabel("Look, I'm see through"));

                label.add(menu, "menu");
                label.add(transparent, "transparent");
                CardLayout layout = (CardLayout) label.getLayout();
                layout.show(label, "menu");

                button.addActionListener(new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        CardLayout layout = (CardLayout) label.getLayout();
                        layout.show(label, "transparent");
                    }
                });

                add(label);
            } catch (IOException exp) {
                exp.printStackTrace();
            }
        }
    }    
}

Answer 2

A class can only ever extend one other class, that's why it won't allow you to extend more than 1 class. here is an explanation why that is the case. Perhaps you should try reading this for more information about such window modifications (window transparency and shapes)

If you want specifically a transparent JPanel , perhaps you should look at this answer as it could explain it better than I could.