Java drawing a line between the centre of components

Question

I am trying to have a line drawn between the centre of two JLabels when the user clicks on one label, drags and releases on top of another. Which should work no matter what

Answer 1

So, the basic problem is, the location of your components are relative to panel, which is offset by the frame's decorations, but you are using the frame's existing Graphics context to paint the line, so the lines are misaligned.

Apart from NOT using getGraphics, EVER, you could achieve the expected results by using the frame's glassPane, for example

import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.geom.Line2D;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test extends JFrame implements MouseListener {

    private JPanel panel = new JPanel();

    public Test() {
        ConnectTheDots dots = new ConnectTheDots();
        setGlassPane(dots);
        dots.setVisible(true);
        panel.setLayout(new GridLayout(2, 2));

        panel.add(createLabel(Color.RED));
        panel.add(createLabel(Color.BLUE));
        panel.add(createLabel(Color.GREEN));
        panel.add(createLabel(Color.ORANGE));

        panel.addMouseListener(this);

        this.add(panel);
    }

    private Component pressComponent;
    private Component releaseComponent;

    public void mousePressed(MouseEvent e) {
        pressComponent = panel.getComponentAt(new Point(e.getX(), e.getY()));
    }

    public void mouseReleased(MouseEvent e) {
        releaseComponent = panel.getComponentAt(new Point(e.getX(), e.getY()));
        joinTheDots();
    }

    @Override
    public void mouseClicked(MouseEvent arg0) {
    }

    @Override
    public void mouseEntered(MouseEvent arg0) {
    }

    @Override
    public void mouseExited(MouseEvent arg0) {
    }

    protected void joinTheDots() {

        Rectangle bounds = pressComponent.getBounds();
        Point startPoint = centerOf(bounds);
        bounds = releaseComponent.getBounds();
        Point endPoint = centerOf(bounds);

        ((ConnectTheDots) getGlassPane()).drawLine(startPoint, endPoint);

    }

    protected Point centerOf(Rectangle bounds) {

        return new Point(
                        bounds.x + (bounds.width / 2),
                        bounds.y + (bounds.height / 2));

    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                Test frame = new Test();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    protected JLabel createLabel(Color background) {
        JLabel label = new JLabel() {
            @Override
            public Dimension getPreferredSize() {
                return new Dimension(100, 100);
            }
        };
        label.setOpaque(true);
        label.setBackground(background);
        return label;
    }

    public class ConnectTheDots extends JPanel {

        private Point startPoint;
        private Point endPoint;

        public ConnectTheDots() {
            setOpaque(false);
        }

        public void drawLine(Point startPoint, Point endPoint) {
            this.startPoint = startPoint;
            this.endPoint = endPoint;
            repaint();
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (startPoint != null && endPoint != null) {
                Graphics2D g2d = (Graphics2D) g.create();
                Line2D line = new Line2D.Double(startPoint, endPoint);
                g2d.setColor(Color.BLACK);
                g2d.draw(line);
                g2d.dispose();
            }
        }

    }

}

Now, this will only work if the content covers the entire visible area of the contentPane, while you could mess around with converting the location information from one component context to another, a simpler solution would be to use JXLayer.

The reason I would avoid overriding paint in this case, is Swing components can be updated without requiring the parent component to be painted, this could wipe out what ever the parent component painted the last time it was painted...

Take a look at How to Use Root Panes for more details