java - Placing component on Glass Pane

Question

I have a subclass of JLabel that forms a component of my GUI. I have implemented the ability to drag and drop the component from one container to another, but without any visual effects. I want to have this JLabel follow the cursor during the drag of the item from one container to another. I figured that I could just create a glass pane and draw it on there. However, even after I add the component to the glass pane, set the component visible, and set the glass pane visible, and set the glass pane as opaque, I still so not see the component. I know the component works because I can add it to the content pane and have it show up.

How do I add a component to the glass pane?

---

Finally figured how to get the simple example working. Thanks, @akf. I was able to adapt this solution to my original problem, allowing me to remove ~60 lines of Java2D code that manually rendered a representation of the JLabel.

package test;

import java.awt.Color;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.border.LineBorder;

public class MainFrame extends JFrame {

    /**
     * @param args
     */
    public static void main(String[] args) {
        MainFrame mf = new MainFrame();
        mf.setSize(400, 400);
        mf.setLocationRelativeTo(null);
        mf.setDefaultCloseOperation(DISPOSE_ON_CLOSE);
        mf.setGlassPane(new JPanel());

        JLabel l = new JLabel();
        l.setText("Hello");
        l.setBorder(new LineBorder(Color.BLACK, 1));
        l.setBounds(10, 10, 50, 20);
        l.setBackground(Color.RED);
        l.setOpaque(true);
        l.setPreferredSize(l.getSize());

        //mf.add(l);
        ((JPanel)mf.getGlassPane()).add(l);
        mf.getGlassPane().setVisible(true);

        mf.setVisible(true);
    }
}

Answer 1

The example code below shows how to drag a chess piece around a chess board. It uses JLayeredPane instead of a glass pane, but I'm sure the concepts would be the same. That is:

a) add the glass pane to the root pane
b) make the glass pane visible
c) add the component to the glass pane making sure the bounds are valid
d) use setLocation() to animate the dragging of the component

Edit: added code to fix SSCCE

JLabel l = new JLabel();
l.setText("Hello");
l.setBorder(new LineBorder(Color.BLACK, 1));
// l.setPreferredSize(l.getSize());
// l.setBounds(10, 10, 50, 20);
((JPanel)mf.getGlassPane()).add(l);

mf.setVisible(true);
mf.getGlassPane().setVisible(true);

When using layout managers you never use the setSize() or setBounds() methods. In your case you just set the preferred size to (0, 0) since this is the default size of all components.

It works when you add the label to the frame because the default layout manger for the content pane of the frame is a border layout, therefore the preferred size of the label is ignored and the label is made the size of the frame.

However, by default a JPanel uses a FlowLayout which does respect the preferred size of the component. Since the preferred size is 0, there is nothing to paint.

Also, the glass pane needs to made visible in order for it to be painted.

I suggest you read the Swing tutorial. There are section on how layout managers work and on how glass panes work and each section has working examples.

Edit: Example code added below:

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;

public class ChessBoard extends JFrame implements MouseListener, MouseMotionListener
{
    JLayeredPane layeredPane;
    JPanel chessBoard;
    JLabel chessPiece;
    int xAdjustment;
    int yAdjustment;

    public ChessBoard()
    {
        Dimension boardSize = new Dimension(600, 600);

        //  Use a Layered Pane for this this application

        layeredPane = new JLayeredPane();
        layeredPane.setPreferredSize( boardSize );
        layeredPane.addMouseListener( this );
        layeredPane.addMouseMotionListener( this );
        getContentPane().add(layeredPane);

        //  Add a chess board to the Layered Pane

        chessBoard = new JPanel();
        chessBoard.setLayout( new GridLayout(8, 8) );
        chessBoard.setPreferredSize( boardSize );
        chessBoard.setBounds(0, 0, boardSize.width, boardSize.height);
        layeredPane.add(chessBoard, JLayeredPane.DEFAULT_LAYER);

        //  Build the Chess Board squares

        for (int i = 0; i < 8; i++)
        {
            for (int j = 0; j < 8; j++)
            {
                JPanel square = new JPanel( new BorderLayout() );
                square.setBackground( (i + j) % 2 == 0 ? Color.red : Color.white );
                chessBoard.add( square );
            }
        }

        // Add a few pieces to the board

        ImageIcon duke = new ImageIcon("dukewavered.gif"); // add an image here

        JLabel piece = new JLabel( duke );
        JPanel panel = (JPanel)chessBoard.getComponent( 0 );
        panel.add( piece );
        piece = new JLabel( duke );
        panel = (JPanel)chessBoard.getComponent( 15 );
        panel.add( piece );
    }

    /*
    **  Add the selected chess piece to the dragging layer so it can be moved
    */
    public void mousePressed(MouseEvent e)
    {
        chessPiece = null;
        Component c =  chessBoard.findComponentAt(e.getX(), e.getY());

        if (c instanceof JPanel) return;

        Point parentLocation = c.getParent().getLocation();
        xAdjustment = parentLocation.x - e.getX();
        yAdjustment = parentLocation.y - e.getY();
        chessPiece = (JLabel)c;
        chessPiece.setLocation(e.getX() + xAdjustment, e.getY() + yAdjustment);

        layeredPane.add(chessPiece, JLayeredPane.DRAG_LAYER);
        layeredPane.setCursor(Cursor.getPredefinedCursor(Cursor.MOVE_CURSOR));
    }

    /*
    **  Move the chess piece around
    */
    public void mouseDragged(MouseEvent me)
    {
        if (chessPiece == null) return;

        //  The drag location should be within the bounds of the chess board

        int x = me.getX() + xAdjustment;
        int xMax = layeredPane.getWidth() - chessPiece.getWidth();
        x = Math.min(x, xMax);
        x = Math.max(x, 0);

        int y = me.getY() + yAdjustment;
        int yMax = layeredPane.getHeight() - chessPiece.getHeight();
        y = Math.min(y, yMax);
        y = Math.max(y, 0);

        chessPiece.setLocation(x, y);
     }

    /*
    **  Drop the chess piece back onto the chess board
    */
    public void mouseReleased(MouseEvent e)
    {
        layeredPane.setCursor(null);

        if (chessPiece == null) return;

        //  Make sure the chess piece is no longer painted on the layered pane

        chessPiece.setVisible(false);
        layeredPane.remove(chessPiece);
        chessPiece.setVisible(true);

        //  The drop location should be within the bounds of the chess board

        int xMax = layeredPane.getWidth() - chessPiece.getWidth();
        int x = Math.min(e.getX(), xMax);
        x = Math.max(x, 0);

        int yMax = layeredPane.getHeight() - chessPiece.getHeight();
        int y = Math.min(e.getY(), yMax);
        y = Math.max(y, 0);

        Component c =  chessBoard.findComponentAt(x, y);

        if (c instanceof JLabel)
        {
            Container parent = c.getParent();
            parent.remove(0);
            parent.add( chessPiece );
            parent.validate();
        }
        else
        {
            Container parent = (Container)c;
            parent.add( chessPiece );
            parent.validate();
        }
    }

    public void mouseClicked(MouseEvent e) {}
    public void mouseMoved(MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited(MouseEvent e) {}

    public static void main(String[] args)
    {
        JFrame frame = new ChessBoard();
        frame.setDefaultCloseOperation( DISPOSE_ON_CLOSE );
        frame.setResizable( false );
        frame.pack();
        frame.setLocationRelativeTo( null );
        frame.setVisible(true);
     }
}