Question

I have a panel with lists and buttons. The lists set MouseAdapter with mouseClick(). I added to the panel MouseAdapter with mousePressed() and mouseReleased() and MouseMotionAdapter with mouseDragged.

Drag and drop only works if you click on the panel.

How to make the drag work even if I clicked on the list?

Simple examlpe:

public class DragTest extends JFrame{
private boolean drag;
private Point btnCoord;
private Point startPoint;

public DragTest(){
    setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    setSize(500,500);
    setLayout(null);
    final JPanel panel = new JPanel();
    final JButton button = new JButton();
    button.setText("Button");
    button.setSize(30,60);
    button.setLocation(50, 50);
    panel.setLayout(null);
    setContentPane(panel);
    panel.addMouseMotionListener(new MouseMotionAdapter() {
        @Override
        public void mouseDragged(MouseEvent e) {
            if (drag){
                panel.setLocation(btnCoord.x-(startPoint.x-e.getX()),btnCoord.y-(startPoint.y-e.getY()));
            }
        }
    });
    panel.addMouseListener(new MouseAdapter() {
        @Override
        public void mousePressed(MouseEvent e) {
            drag = true;
            startPoint = e.getPoint();
            btnCoord = panel.getLocation();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            drag = false;
        }
    });
    getContentPane().add(button);
}

public static void main(String[] args) {
    JFrame frame = new DragTest();
    frame.setVisible(true);
}
}

If you drag panel, all work fine, if start drag button, then button intercept event.

Answer 1

The above code has many errors…

• Dont use null/Absolute LayoutManager. Have a look at Laying Out Components Within a Container.
• Dont call setSize on components and JFrame (if correct Layout Manager is implemented you can simply add components and call pack() on JFrame.
• Create and manipulate Swing components on EDT. Read more on Concurrency in Swing.
• You should call revalidate() and repaint() on container after adding/removing components from a visible container.

I have been working on my own little ComponentDrag API which will allow us to drag Swing components across a JComponent or over multiple JComponents. It is by no means error free though (especially the multiple components part just finished it today so you never know)

You would create JButton and add to JPanel:

    JButton button = new JButton("Text");

    JPanel panel1 = new JPanel() {
        @Override
        public Dimension getPreferredSize() {
            return new Dimension(300, 300);
        }
    };
    panel1.add(button);

Than create an instance of ComponentDrag class by passing argument of you container i.e JFrame/JDialog or JWindow and register the draggable components and call setName("Droppable") on your JPanel to allow dragging and dropping to take place correctly especially across multiple JComponents etc, you would only do this for the JPanels etc that you want the ability to drop Swing components too:

    JFrame frame=..;//has access to setGlassPane same applies for JDialog and JWindow
    frame.add(panel1);

    ComponentDrag cd=new ComponentDrag(frame);
    cd.registerComponent(button);
    panel1.setName("Droppable");//needed for the dragging and dropping to work correctly especailly over multiple components

and your good to go :).

Here are 2 full examples for analysis purposes:

1) Show the logic to move JButtons and re-position the accordingly using ComponentDrag and overriding its componentDropped method:

Before drag:

After 4 dragged to 1 position:

import java.awt.Component;
import java.awt.Container;
import java.awt.Cursor;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import javax.swing.AbstractAction;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class DragButtonsTest {

    ArrayList<JButton> buttons = new ArrayList<>();

    public DragButtonsTest() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final JPanel panel = new JPanel(new GridLayout(2, 2));

        ComponentDrag cd = new ComponentDrag(frame) {
            @Override
            protected void componentDropped(MouseEvent me) {
                HashMap<Integer, JButton> collisions = new HashMap<>();
                JButton draggedButton = (JButton) me.getSource();

                for (JButton btn : buttons) {//iterate through all buttons and get the number of collsions of each
                    if (btn != draggedButton) {//dont chck button we were dragging
                        int col = checkPerPixelCollision(draggedButton, btn);
                        System.out.println("Button " + btn.getText());
                        System.out.println(col);
                        collisions.put(col, btn);
                    }
                }

                //lets get the button which had most collisions
                int maxCollisions = 0;
                JButton intersectingButton = null;
                for (Map.Entry<Integer, JButton> entry : collisions.entrySet()) {
                    Integer collisionCount = entry.getKey();
                    JButton button = entry.getValue();
                    if (collisionCount > maxCollisions) {
                        maxCollisions = collisionCount;
                        intersectingButton = button;
                    }
                }

                boolean reLayout = false;

                if (maxCollisions > 0) {//check if there was any
                    System.out.println("Button " + draggedButton.getText() + " is intersecting more of Button " + intersectingButton.getText());
                    System.out.println("Collisions: " + maxCollisions);
                    reLayout = true;
                } else {
                    System.out.println("No change made");
                    reLayout = false;
                }

                ArrayList<JButton> tmpButtons = (ArrayList<JButton>) buttons.clone();//create clone of buttons

                if (reLayout) {//a button as moved and panel needs to be layed out
                    buttons.clear();//clear old buttons

                    for (JButton b : tmpButtons) {//re-order jbuttons
                        if (b == intersectingButton) {
                            buttons.add(draggedButton);
                        } else if (b == draggedButton) {
                            buttons.add(intersectingButton);
                        } else {
                            buttons.add(b);
                        }
                    }
                    panel.removeAll();//remove all buttons
                    for (JButton btn : buttons) {//iterate through all buttons and get the number of collsions of each
                        panel.add(btn);//re-add buttons according to arraylist
                    }
                    panel.revalidate();
                    panel.repaint();
                }
                super.componentDropped(me);

            }
        };

        for (int i = 0; i < 4; i++) {
            JButton b = new JButton(String.valueOf(i + 1));
            panel.add(b);
            buttons.add(b);
            cd.registerComponent(b);
        }

        frame.add(panel);

        frame.pack();
        frame.setVisible(true);
    }

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

    public HashSet<String> getMask(JButton e) {
        HashSet<String> mask = new HashSet<>();
        int pixel, a;
        BufferedImage bi = componentToImage(e); //gets the current image being shown

        for (int i = 0; i < bi.getWidth(); i++) { // for every (x,y) component in the given box, 
            for (int j = 0; j < bi.getHeight(); j++) {
                pixel = bi.getRGB(i, j); // get the RGB value of the pixel
                a = (pixel >> 24) & 0xff;
                if (a != 0) {  // if the alpha is not 0, it must be something other than transparent
                    mask.add((e.getX() + i) + "," + (e.getY() - j)); // add the absolute x and absolute y coordinates to our set
                }
            }
        }
        return mask;  //return our set
    }

    public static BufferedImage componentToImage(Component component) {
        BufferedImage img = new BufferedImage(component.getWidth(), component.getHeight(), BufferedImage.TRANSLUCENT);
        Graphics g = img.getGraphics();
        component.paintAll(g);
        return img;
    }

    // Returns true if there is a collision between object a and object b   
    public int checkPerPixelCollision(JButton b, JButton b2) {
        // This method detects to see if the images overlap at all. If they do, collision is possible
        int ax1 = (int) b2.getX();
        int ay1 = (int) b2.getY();

        int ax2 = ax1 + (int) b2.getWidth();
        int ay2 = ay1 + (int) b2.getHeight();

        int bx1 = (int) b.getX();
        int by1 = (int) b.getY();

        int bx2 = bx1 + (int) b.getWidth();

        int by2 = by1 + (int) b.getHeight();

        if (by2 < ay1 || ay2 < by1 || bx2 < ax1 || ax2 < bx1) {
            return 0; // Collision is impossible.
        } else { // Collision is possible.
            // get the masks for both images
            HashSet<String> maskPlayer1 = getMask(b2);
            HashSet<String> maskPlayer2 = getMask(b);
            maskPlayer1.retainAll(maskPlayer2);  // Check to see if any pixels in maskPlayer2 are the same as those in maskPlayer1
            if (maskPlayer1.size() > 0) {  // if so, than there exists at least one pixel that is the same in both images, thus
                return maskPlayer1.size();
            }
        }
        return 0;
    }
}

class ComponentDrag {

    private MouseAdapter ma;
    private ArrayList<JComponent> components = new ArrayList<>();
    private int startingX, startingY;
    private boolean autoLayout = true;
    private final JFrame container;
    private JPanel glassPane;
    boolean firstTime = true;
    private JComponent lastClickedContainer;

    public ComponentDrag(final JFrame container) {
        this.container = container;
        glassPane = new JPanel();
        glassPane.setOpaque(false);
        glassPane.setLayout(null);
        this.container.setGlassPane(glassPane);

        new Timer(10, new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                if (container.isVisible()) {
                    glassPane.setVisible(true);
                    ((Timer) ae.getSource()).stop();
                }
            }
        }).start();

        ma = new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent me) {
                super.mousePressed(me);
                componentPressed(me);
            }

            @Override
            public void mouseReleased(MouseEvent me) {
                super.mouseReleased(me);
                componentDropped(me);
            }

            @Override
            public void mouseDragged(MouseEvent me) {
                super.mouseDragged(me);
                componentDragged(me);
            }
        };
    }

    public JComponent findDroppableUnderGlassPane(Point p, Container container, Component source) {
        JComponent c = null;
        Component[] comps = container.getComponents();
        for (Component com : comps) {
            if (com.getName() != null) {
                if (com.getName().equals("Droppable") && com instanceof JComponent) {
                    if (com.contains(SwingUtilities.convertPoint(source, p, com))) {
                        return (JComponent) com;
                    }
                } else if (com instanceof Container) {
                    findDroppableUnderGlassPane(p, (Container) com, source);
                }
            }
        }
        return c;
    }

    public boolean isAutoLayout() {
        return autoLayout;
    }

    public void setAutoLayout(boolean autoLayout) {
        this.autoLayout = autoLayout;
    }

    protected void componentDropped(MouseEvent me) {
        firstTime = true;
        Component droppedComponent = (Component) me.getSource();
        droppedComponent.setCursor(null);

        JComponent jc = findDroppableUnderGlassPane(me.getPoint(), container.getContentPane(), (Component) me.getSource());

        if (jc != null) {
            glassPane.removeAll();
            glassPane.revalidate();
            glassPane.repaint();

            jc.add(droppedComponent);
            System.out.println("Removed from glasspane and added to: " + jc);

            if (autoLayout) {
                if (lastClickedContainer != null) {
                    lastClickedContainer.revalidate();
                    lastClickedContainer.repaint();
                }
                droppedComponent.revalidate();
                droppedComponent.repaint();
                jc.revalidate();
                jc.repaint();
            }

        } else {
            glassPane.removeAll();
            glassPane.revalidate();
            glassPane.repaint();
            if (lastClickedContainer != null) {
                lastClickedContainer.add(droppedComponent);
                lastClickedContainer.revalidate();
                lastClickedContainer.repaint();
            }
        }
    }

    protected void componentPressed(MouseEvent me) {
        JComponent jc = findDroppableUnderGlassPane(me.getPoint(), container.getContentPane(), (Component) me.getSource());

        if (jc != null && jc.getName().equals("Droppable")) {
            lastClickedContainer = jc;
            System.out.println("Pressed: " + lastClickedContainer);
        }

        boolean clickedRegisteredComponent = false;

        Component clickedComponent = (Component) me.getSource();

        for (Component component : ComponentDrag.this.components) {
            if (component.equals(clickedComponent)) {
                clickedRegisteredComponent = true;
                break;
            }
        }

        if (clickedRegisteredComponent) {
            startingX = me.getX();
            startingY = me.getY();
            clickedComponent.setCursor(new Cursor(Cursor.MOVE_CURSOR));
        }
    }

    protected void componentDragged(MouseEvent me) {
        Component draggedComponent = (Component) me.getSource();

        if (firstTime && lastClickedContainer != null) {
            firstTime = false;
            lastClickedContainer.remove(draggedComponent);
            lastClickedContainer.revalidate();
            lastClickedContainer.repaint();
            glassPane.add(draggedComponent);
            glassPane.revalidate();
            glassPane.repaint();
            System.out.println("Removed from: " + lastClickedContainer + " \nAnd added to glasspane for dragging");
        }

        //MouseEvent will refire on each drag with the position being relative to the firing Component
        draggedComponent.setLocation((me.getX() - startingX) + draggedComponent.getLocation().x, (me.getY() - startingY) + draggedComponent.getLocation().y);
    }

    void registerComponent(JComponent draggableComp) {
        draggableComp.addMouseListener(ma);
        draggableComp.addMouseMotionListener(ma);
        components.add(draggableComp);
    }

    void deregisterComponent(JComponent draggableComp) {
        draggableComp.removeMouseListener(ma);
        draggableComp.removeMouseMotionListener(ma);
        components.remove(draggableComp);
    }
}

2) Simply shows how to drag 2 JLabels across JPanels

Before Drag me label dragged:

After label dragged:

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Container;
import java.awt.Cursor;
import java.awt.Dimension;
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import javax.swing.AbstractAction;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSeparator;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

/**
 *
 * @author David
 */
public class DragOverMultipleComponentsTest {

    public DragOverMultipleComponentsTest() {
        createAndShowUI();
    }

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

    private void createAndShowUI() {
        final JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);


        JLabel draggableLabel = new JLabel("<- Drag me");
        JLabel draggableLabel2 = new JLabel("<- Drag me too");

        JLabel labelPanel1 = new JLabel("Drag 'em here");
        JPanel panel1 = new JPanel() {
            @Override
            public Dimension getPreferredSize() {
                return new Dimension(300, 300);
            }
        };
        panel1.add(labelPanel1);
        panel1.setName("Droppable");

        JLabel labelPanel2 = new JLabel("Drag 'em here");
        JPanel panel2 = new JPanel();
        panel2.add(labelPanel2);
        panel2.add(draggableLabel);
        panel2.add(draggableLabel2);
        panel2.setName("Droppable");

        JSeparator js = new JSeparator(JSeparator.VERTICAL);

        ComponentDrag cd = new ComponentDrag(frame);

        cd.registerComponent(draggableLabel);
        cd.registerComponent(draggableLabel2);

        frame.add(panel1, BorderLayout.WEST);
        frame.add(js);
        frame.add(panel2, BorderLayout.EAST);

        frame.pack();
        frame.setVisible(true);
    }
}
class ComponentDrag {

    private MouseAdapter ma;
    private ArrayList<JComponent> components = new ArrayList<>();
    private int startingX, startingY;
    private boolean autoLayout = true;
    private final JFrame container;
    private JPanel glassPane;
    boolean firstTime = true;
    private JComponent lastClickedContainer;

    public ComponentDrag(final JFrame container) {
        this.container = container;
        glassPane = new JPanel();
        glassPane.setOpaque(false);
        glassPane.setLayout(null);
        this.container.setGlassPane(glassPane);

        new Timer(10, new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                if (container.isVisible()) {
                    glassPane.setVisible(true);
                    ((Timer) ae.getSource()).stop();
                }
            }
        }).start();

        ma = new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent me) {
                super.mousePressed(me);
                componentPressed(me);
            }

            @Override
            public void mouseReleased(MouseEvent me) {
                super.mouseReleased(me);
                componentDropped(me);
            }

            @Override
            public void mouseDragged(MouseEvent me) {
                super.mouseDragged(me);
                componentDragged(me);
            }
        };
    }

    public JComponent findDroppableUnderGlassPane(Point p, Container container, Component source) {
        JComponent c = null;
        Component[] comps = container.getComponents();
        for (Component com : comps) {
            if (com.getName() != null) {
                if (com.getName().equals("Droppable") && com instanceof JComponent) {
                    if (com.contains(SwingUtilities.convertPoint(source, p, com))) {
                        return (JComponent) com;
                    }
                } else if (com instanceof Container) {
                    findDroppableUnderGlassPane(p, (Container) com, source);
                }
            }
        }
        return c;
    }

    public boolean isAutoLayout() {
        return autoLayout;
    }

    public void setAutoLayout(boolean autoLayout) {
        this.autoLayout = autoLayout;
    }

    protected void componentDropped(MouseEvent me) {
        firstTime = true;
        Component droppedComponent = (Component) me.getSource();
        droppedComponent.setCursor(null);

        JComponent jc = findDroppableUnderGlassPane(me.getPoint(), container.getContentPane(), (Component) me.getSource());

        if (jc != null) {
            glassPane.removeAll();
            glassPane.revalidate();
            glassPane.repaint();

            jc.add(droppedComponent);
            System.out.println("Removed from glasspane and added to: " + jc);

            if (autoLayout) {
                if (lastClickedContainer != null) {
                    lastClickedContainer.revalidate();
                    lastClickedContainer.repaint();
                }
                droppedComponent.revalidate();
                droppedComponent.repaint();
                jc.revalidate();
                jc.repaint();
            }

        } else {
            glassPane.removeAll();
            glassPane.revalidate();
            glassPane.repaint();
            if (lastClickedContainer != null) {
                lastClickedContainer.add(droppedComponent);
                lastClickedContainer.revalidate();
                lastClickedContainer.repaint();
            }
        }
    }

    protected void componentPressed(MouseEvent me) {
        JComponent jc = findDroppableUnderGlassPane(me.getPoint(), container.getContentPane(), (Component) me.getSource());

        if (jc != null && jc.getName().equals("Droppable")) {
            lastClickedContainer = jc;
            System.out.println("Pressed: " + lastClickedContainer);
        }

        boolean clickedRegisteredComponent = false;

        Component clickedComponent = (Component) me.getSource();

        for (Component component : ComponentDrag.this.components) {
            if (component.equals(clickedComponent)) {
                clickedRegisteredComponent = true;
                break;
            }
        }

        if (clickedRegisteredComponent) {
            startingX = me.getX();
            startingY = me.getY();
            clickedComponent.setCursor(new Cursor(Cursor.MOVE_CURSOR));
        }
    }

    protected void componentDragged(MouseEvent me) {
        Component draggedComponent = (Component) me.getSource();

        if (firstTime && lastClickedContainer != null) {
            firstTime = false;
            lastClickedContainer.remove(draggedComponent);
            lastClickedContainer.revalidate();
            lastClickedContainer.repaint();
            glassPane.add(draggedComponent);
            glassPane.revalidate();
            glassPane.repaint();
            System.out.println("Removed from: " + lastClickedContainer + " \nAnd added to glasspane for dragging");
        }

        //MouseEvent will refire on each drag with the position being relative to the firing Component
        draggedComponent.setLocation((me.getX() - startingX) + draggedComponent.getLocation().x, (me.getY() - startingY) + draggedComponent.getLocation().y);
    }

    void registerComponent(JComponent draggableComp) {
        draggableComp.addMouseListener(ma);
        draggableComp.addMouseMotionListener(ma);
        components.add(draggableComp);
    }

    void deregisterComponent(JComponent draggableComp) {
        draggableComp.removeMouseListener(ma);
        draggableComp.removeMouseMotionListener(ma);
        components.remove(draggableComp);
    }
}