JLabel mouse events for Drag and Drop

Question

I want to enable the drag and drop feature over a JLabel by overriding mouse events over it , but when I define the drag and drop in mousePressed event ,the mouseReleased does not take effect on that JLabel. Am I doing something wrong ?

            Thumbnails[I_Loop].setText("1");
            Thumbnails[I_Loop].setTransferHandler(new TransferHandler("text"));
            Thumbnails[I_Loop].addMouseListener( new MouseAdapter() {
                public void  mouseReleased(MouseEvent me) {
                       System.out.println("here mouse released");
                  }
                public void mousePressed(MouseEvent me) {
                    System.out.println("here mouse pressed");
                    JComponent comp = (JComponent) me.getSource();
                    TransferHandler handler = comp.getTransferHandler();
                    handler.exportAsDrag(comp, me, TransferHandler.COPY);
            });

*Thumbnails is array of JLabel

When running the program , the drag and drop works but the statement "here mouse released" does not get printed. However, When I remove the code responsible for DND from the mousePressed() method, "here mouse released" is printed.

What is the wrong in this code?

Answer 1

@Thomas is correct, but two alternatives are worth noting:

• This example shows how to drag a component using JLayeredPane; this variation expands on the concept; this more recent example uses a similar approach.

• The code below shows how to use a MouseMotionListener; this more complex example uses the same principle.

Code:

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import javax.swing.JFrame;
import javax.swing.JPanel;

/** @see https://stackoverflow.com/a/5312702/230513 */
public class MouseDragTest extends JPanel {

    private static final String TITLE = "Drag me!";
    private static final int W = 640;
    private static final int H = 480;
    private Point textPt = new Point(W / 2, H / 2);
    private Point mousePt;

    public MouseDragTest() {
        this.setFont(new Font("Serif", Font.ITALIC + Font.BOLD, 32));
        this.addMouseListener(new MouseAdapter() {

            @Override
            public void mousePressed(MouseEvent e) {
                mousePt = e.getPoint();
                repaint();
            }
        });
        this.addMouseMotionListener(new MouseMotionAdapter() {

            @Override
            public void mouseDragged(MouseEvent e) {
                int dx = e.getX() - mousePt.x;
                int dy = e.getY() - mousePt.y;
                textPt.setLocation(textPt.x + dx, textPt.y + dy);
                mousePt = e.getPoint();
                repaint();
            }
        });
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(W, H);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        int w2 = g.getFontMetrics().stringWidth(TITLE) / 2;
        g.drawString(TITLE, textPt.x - w2, textPt.y);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame f = new JFrame(TITLE);
                f.add(new MouseDragTest());
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.pack();
                f.setLocationRelativeTo(null);
                f.setVisible(true);
            }
        });
    }
}

Answer 2

Well, if I remember correctly, the drag and drop machinery catches all mouse events and processes them itself. Thus, the normal MouseEvents are not thrown anymore. You'd need to register a DropTargetListener on the JLabel's DropTarget.

Answer 3

Does it have to be a JLabel? I made a class with a string that might get you started..

import java.awt.Graphics;
import java.awt.MouseInfo;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

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


public class mainProgram extends JPanel implements Runnable
{
    private static final long serialVersionUID = 1L;
    public static boolean MOUSE_DOWN = false;
    public static String str;

    public mainProgram() 
    {
        JFrame win = new JFrame("Window");

        win.add(this);

        win.setSize(700,500);
        win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        win.setVisible(true);
        str = "Drag me!";
        new Thread(this).start();
    }

    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);

        if(MOUSE_DOWN)
        {
            g.drawString(str, MouseInfo.getPointerInfo().getLocation().x, MouseInfo.getPointerInfo().getLocation().y);
        }
    }

    @Override
    public void run() 
    {
        Thread t = Thread.currentThread();

        this.addMouseListener(new MouseListener()
        {

            @Override
            public void mouseClicked(MouseEvent arg0) 
            {

            }

            @Override
            public void mouseEntered(MouseEvent arg0) 
            {

            }

            @Override
            public void mouseExited(MouseEvent arg0) 
            {

            }

            @Override
            public void mousePressed(MouseEvent arg0) 
            {
                MOUSE_DOWN = true;
            }

            @Override
            public void mouseReleased(MouseEvent arg0) 
            {
                MOUSE_DOWN = false;
            }

        });

        while(t==Thread.currentThread())
        {
            if(MOUSE_DOWN)
                repaint();

            try {Thread.sleep(10);}
            catch (InterruptedException e) {e.printStackTrace();}
        }   
    }
}