How to Draw Lines in a JPanel Using Mouse Clicks and Dragging?

Drawing Lines in a JPanel with Button Click

Problem:

Someone wants to draw lines in a JPanel when a button is clicked but faces difficulties due to a misunderstanding of the drawing process.

Solution:

To draw lines in a JPanel, a different approach is suggested:

1. Mark the first endpoint with a click.
2. Drag the mouse to display the line in progress.
3. Release the mouse to mark the second endpoint.

Implementation:

The following code demonstrates the implementation:

<br>import java.awt.BasicStroke;<br>import java.awt.Color;<br>import java.awt.Dimension;<br>import java.awt.Graphics;<br>import java.awt.Graphics2D;<br>import java.awt.Point;<br>import java.awt.RenderingHints;<br>import java.awt.event.ActionEvent;<br>import java.awt.event.KeyEvent;<br>import java.awt.event.MouseAdapter;<br>import java.awt.event.MouseEvent;<br>import javax.swing.AbstractAction;<br>import javax.swing.JButton;<br>import javax.swing.JFrame;<br>import javax.swing.JPanel;<br>import javax.swing.KeyStroke;</p>
<p>public class LinePanel extends JPanel {</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">private MouseHandler mouseHandler = new MouseHandler();
private Point p1 = new Point(100, 100);
private Point p2 = new Point(540, 380);
private boolean drawing;

public LinePanel() {
    this.setPreferredSize(new Dimension(640, 480));
    this.addMouseListener(mouseHandler);
    this.addMouseMotionListener(mouseHandler);
}

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D) g;
    g2d.setColor(Color.blue);
    g2d.setRenderingHint(
        RenderingHints.KEY_ANTIALIASING,
        RenderingHints.VALUE_ANTIALIAS_ON);
    g2d.setStroke(new BasicStroke(8,
        BasicStroke.CAP_ROUND, BasicStroke.JOIN_BEVEL));
    g.drawLine(p1.x, p1.y, p2.x, p2.y);
}

private class MouseHandler extends MouseAdapter {

    @Override
    public void mousePressed(MouseEvent e) {
        drawing = true;
        p1 = e.getPoint();
        p2 = p1;
        repaint();
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        drawing = false;
        p2 = e.getPoint();
        repaint();
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        if (drawing) {
            p2 = e.getPoint();
            repaint();
        }
    }
}

private class ControlPanel extends JPanel {

    private static final int DELTA = 10;

    public ControlPanel() {
        this.add(new MoveButton("\u2190", KeyEvent.VK_LEFT, -DELTA, 0));
        this.add(new MoveButton("\u2191", KeyEvent.VK_UP, 0, -DELTA));
        this.add(new MoveButton("\u2192", KeyEvent.VK_RIGHT, DELTA, 0));
        this.add(new MoveButton("\u2193", KeyEvent.VK_DOWN, 0, DELTA));
    }

    private class MoveButton extends JButton {

        KeyStroke k;
        int dx, dy;

        public MoveButton(String name, int code,
                final int dx, final int dy) {
            super(name);
            this.k = KeyStroke.getKeyStroke(code, 0);
            this.dx = dx;
            this.dy = dy;
            this.setAction(new AbstractAction(this.getText()) {

                @Override
                public void actionPerformed(ActionEvent e) {
                    LinePanel.this.p1.translate(dx, dy);
                    LinePanel.this.p2.translate(dx, dy);
                    LinePanel.this.repaint();
                }
            });
            ControlPanel.this.getInputMap(WHEN_IN_FOCUSED_WINDOW)
                .put(k, k.toString());
            ControlPanel.this.getActionMap()
                .put(k.toString(), new AbstractAction() {

                @Override
                public void actionPerformed(ActionEvent e) {
                    MoveButton.this.doClick();
                }
            });
        }
    }
}

private void display() {
    JFrame f = new JFrame("LinePanel");
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.add(this);
    f.add(new ControlPanel(), BorderLayout.SOUTH);
    f.pack();
    f.setLocationRelativeTo(null);
    f.setVisible(true);
}

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

        @Override
        public void run() {
            new LinePanel().display();
        }
    });
}

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}

This code provides the functionality to draw lines using mouse clicks, drag, and release, as well as a panel of buttons for controlling the movement of the line.

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