Drag and Drop from JButton to JComponent in Java

Question

I searched on the internet for examples how to Drag and Drop JButtons to an Object but I could not make it work.

What my program does, is that when I click on a button, the object updated a field (with a selectedobject.setField()). I want to be able to do this not by clicking, but by dragging the JButton.

How can I do this ? I found this, and I tried to put in my code:

btn.setTransferHandler(new ImageHandler());
btn.addMouseListener(new MouseAdapter() {
    public void mousePressed(MouseEvent e) {
        JComponent c = (JComponent)e.getSource();
        TransferHandler handler = c.getTransferHandler();
        handler.exportAsDrag(c, e, TransferHandler.COPY);
    }
});

I took the ImageHandler class from here.

Garbage

What do you mean by "gets a new value"? Do you want to drag the "value" of the button onto something else? – Natividad

The button adds an animation to the selected object. I want to be able to add the same animation to a random object by drag and drop. – Garbage

Answer 1

Drag'n'drop is a fun bag of crunchy, munchy carrots...not helped by the fact that there is a "core" API and the newer "transfer" API, so it's really easy to get confused

The following example uses the "transfer" API and basically transfers a String value from a button to a JLabel.

import java.awt.Color;
import java.awt.Component;
import java.awt.EventQueue;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.StringSelection;
import java.awt.datatransfer.Transferable;
import java.awt.dnd.DnDConstants;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.TransferHandler;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.CompoundBorder;
import javax.swing.border.EmptyBorder;
import javax.swing.border.LineBorder;

public class Test {

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

    public Test() {
        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 GridLayout(1, 2));
            add(createLeftPanel());
            add(createRightPanel());

        }

        protected JPanel createLeftPanel() {
            JPanel panel = new JPanel(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;
            gbc.fill = GridBagConstraints.HORIZONTAL;
            gbc.weightx = 1;
            for (int index = 0; index < 10; index++) {
                JButton btn = new JButton(Integer.toString(index + 1));
                panel.add(btn, gbc);
                btn.setTransferHandler(new ValueExportTransferHandler(Integer.toString(index + 1)));

                btn.addMouseMotionListener(new MouseAdapter() {
                    @Override
                    public void mouseDragged(MouseEvent e) {
                        JButton button = (JButton) e.getSource();
                        TransferHandler handle = button.getTransferHandler();
                        handle.exportAsDrag(button, e, TransferHandler.COPY);
                    }
                });
            }
            return panel;
        }

        protected JPanel createRightPanel() {
            JPanel panel = new JPanel(new GridBagLayout());
            JLabel label = new JLabel("Drop in");
            label.setBorder(new CompoundBorder(new LineBorder(Color.DARK_GRAY), new EmptyBorder(20, 20, 20, 20)));
            label.setTransferHandler(new ValueImportTransferHandler());
            panel.add(label);
            return panel;
        }

    }

    public static class ValueExportTransferHandler extends TransferHandler {

        public static final DataFlavor SUPPORTED_DATE_FLAVOR = DataFlavor.stringFlavor;
        private String value;

        public ValueExportTransferHandler(String value) {
            this.value = value;
        }

        public String getValue() {
            return value;
        }

        @Override
        public int getSourceActions(JComponent c) {
            return DnDConstants.ACTION_COPY_OR_MOVE;
        }

        @Override
        protected Transferable createTransferable(JComponent c) {
            Transferable t = new StringSelection(getValue());
            return t;
        }

        @Override
        protected void exportDone(JComponent source, Transferable data, int action) {
            super.exportDone(source, data, action);
            // Decide what to do after the drop has been accepted
        }

    }

    public static class ValueImportTransferHandler extends TransferHandler {

        public static final DataFlavor SUPPORTED_DATE_FLAVOR = DataFlavor.stringFlavor;

        public ValueImportTransferHandler() {
        }

        @Override
        public boolean canImport(TransferHandler.TransferSupport support) {
            return support.isDataFlavorSupported(SUPPORTED_DATE_FLAVOR);
        }

        @Override
        public boolean importData(TransferHandler.TransferSupport support) {
            boolean accept = false;
            if (canImport(support)) {
                try {
                    Transferable t = support.getTransferable();
                    Object value = t.getTransferData(SUPPORTED_DATE_FLAVOR);
                    if (value instanceof String) {
                        Component component = support.getComponent();
                        if (component instanceof JLabel) {
                            ((JLabel) component).setText(value.toString());
                            accept = true;
                        }
                    }
                } catch (Exception exp) {
                    exp.printStackTrace();
                }
            }
            return accept;
        }
    }
}

I've gone out my way to separate the TransferHandlers allowing for a "drag" and "drop" version. You don't "have" to do this and you "could" use a single TransferHandler to perform both operations, that's up to you.

You will have to modify the ValueExportTransferHandler to accept different values and modify the SUPPORTED_DATE_FLAVOR accordingingly, but those are the basics

You could also have a look at Drag and Drop custom object from JList into JLabel as another example...

Natividad

Something's wrong there. I mean, "exportAsDrag" should not be called for every single drag event, right? Even if it may work in practice. EDIT: I just ran it. It works, and the mouseDragged method is, for some reason, only executed once, so all seems to be well. – Kreit

@DreamspacePresident Why not? exportAsDrag is been called on mouseDragged, if not then, then when should it be called? ps- I think you're thinking of mouseMoved ;) – Natividad

Because the docs of exportAsDrag say "Causes the Swing drag support to be initiated." and "The transfer will not necessarily have been completed at the return of this call (i.e. the call does not block waiting for the drop)." which, to me, clearly implies that this thing is to be called once per D&D. But see my edit of my other comment, all seems to be well. – Kreit