Component layering Java Swing, Layers showing on hover

问题

I have two JPanels layered on top of each other in the same container. I am using container.add(jpanel,0); and container.add(otherjpanel, 1). It works fine however in order for the top layer to show I have to hover over the components with the mouse.

Here is some executable code showing my problem.

Just hover mouse on upper part of screen.

import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import javax.swing.event.*;
import javax.swing.*;
import javax.swing.text.*;
import java.io.*;
import java.util.*;

public class test {


    public static void main(String args[]) {
    try {
     UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
    } catch (Exception e) {}



    JFrame frame = new GUIframe();
    frame.setVisible(true);
    frame.setResizable(false);

  }
}



class GUIframe extends JFrame{


public GUIframe(){

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setPreferredSize(new Dimension(300,400));
      setSize(300, 400);

      JLayeredPane content = new JLayeredPane();

      content.setPreferredSize(new Dimension(300,400));

      content.setSize(300,400);

      JPanel board = new JPanel();

      for (int i = 0;i<5;i++){

      JButton button = new JButton("button");
      board.add(button);

      }

      content.add(new ImagePanel());
      this.add(content);
      this.add(board);


}
} 

class ImagePanel extends JPanel {

  private Image img;
  String imageLocation = "image location here";

  ImagePanel() {
    img = new ImageIcon(imageLocation).getImage();
    setPreferredSize(new Dimension(300,400));
    setSize(300,400);
    setLayout(null);
    setOpaque(false);

  }

  public void paint(Graphics g){

  super.paint(g);
  g.drawImage(img, 0, 0, this);

  }

}

回答1:

A contentPane's layout is by default BorderLayout. Have you changed this? Perhaps you should set your contentPane to be a JLayeredPane instead.

If anything about this recommendation is unclear, please leave a comment.

Edit 1: Example of JLayeredPane
You could solve this sort of thing with a layered pane as I described, something like listed below, but you must take care to set size and to make overlying JPanels non-opaque:

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;

import javax.imageio.ImageIO;
import javax.swing.*;

public class Test2 {
   private static final int LP_WIDTH = 450;
   private static final int LP_HEIGHT = 600;
   private static final String IMAGE_SITE = "http://upload.wikimedia.org/wikipedia/"
         + "commons/thumb/b/b8/Laser_Towards_Milky_Ways_Centre.jpg/"
         + "660px-Laser_Towards_Milky_Ways_Centre.jpg";
   private JLayeredPane layeredPanel = new JLayeredPane();

   public Test2() {
      layeredPanel.setPreferredSize(new Dimension(LP_WIDTH, LP_HEIGHT));
      try {
         URL url = new URL(IMAGE_SITE);
         BufferedImage image = ImageIO.read(url);
         ImagePanel2 imagePanel2 = new ImagePanel2(image);
         imagePanel2.setSize(layeredPanel.getPreferredSize());

         JPanel buttonPanel = new JPanel();
         buttonPanel.setOpaque(false);
         for (int i = 0; i < 8; i++) {
            buttonPanel.add(new JButton("Button"));
         }
         buttonPanel.setSize(layeredPanel.getPreferredSize());

         layeredPanel.add(imagePanel2, JLayeredPane.DEFAULT_LAYER);
         layeredPanel.add(buttonPanel, JLayeredPane.PALETTE_LAYER);
      } catch (MalformedURLException e) {
         e.printStackTrace();
         System.exit(-1);
      } catch (IOException e) {
         e.printStackTrace();
         System.exit(-1);
      }
   }

   private JComponent getMainComponent() {
      return layeredPanel;
   }

   private static void createAndShowGui() {
      Test2 test2 = new Test2();

      JFrame frame = new JFrame("Test2");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(test2.getMainComponent());
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

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

class ImagePanel2 extends JPanel {
   private Image image;

   public ImagePanel2(Image image) {
      this.image = image;
   }

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

}

However if all you want is a background image, then that's what I'd do, create a JPanel that uses a background image, and then add stuff to it.

import java.awt.*;
import java.awt.image.*;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;

import javax.imageio.ImageIO;
import javax.swing.*;

public class Test3 extends JPanel {
   private static final int LP_WIDTH = 450;
   private static final int LP_HEIGHT = 600;
   private static final String IMAGE_SITE = "http://upload.wikimedia.org/wikipedia/"
         + "commons/thumb/b/b8/Laser_Towards_Milky_Ways_Centre.jpg/"
         + "660px-Laser_Towards_Milky_Ways_Centre.jpg";

   private BufferedImage image;

   public Test3() {
      try {
         URL url = new URL(IMAGE_SITE);
         image = ImageIO.read(url);

         for (int i = 0; i < 8; i++) {
            add(new JButton("Button"));
         }
      } catch (MalformedURLException e) {
         e.printStackTrace();
         System.exit(-1);
      } catch (IOException e) {
         e.printStackTrace();
         System.exit(-1);
      }
   }

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(LP_WIDTH, LP_HEIGHT);
   }

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

   private static void createAndShowGui() {
      Test3 mainPanel = new Test3();

      JFrame frame = new JFrame("Test3");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

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

来源：https://stackoverflow.com/questions/7629473/component-layering-java-swing-layers-showing-on-hover