swing - Java make a directed line and make it move

Question

I want to make a directed line and make it move. I am able to make a directed line and move the line but the arrow get displaced while i move the line

This is my paint method

 Line2D.Double line = new Line2D.Double(startX, startY, endX, endY);
                    g2d.draw(line);
                    tx.setToIdentity();                    
                  double angle = Math.atan2(line.y2 - line.y1, line.x2 - line.x1);
                   tx.translate(line.x2, line.y2);
                    tx.rotate((angle - Math.PI / 2d));
                    Graphics2D gClone = (Graphics2D) g2d.create();
                    gClone.setTransform(tx);
 Polygon arrowHead = new Polygon();
        arrowHead.addPoint(0, 15);
        arrowHead.addPoint(-15, -15);
        arrowHead.addPoint(15, -15);
                    Area area = new Arear(arrowHead );
                    Area lineArea = new Area(line);
                    lineArea.subtract(area);
                    gClone.fill(area);
                    gClone.dispose();

and i have change the value of startx and start y at mouse pressedd and endx and enny at mouse drag

Answer 1

This is a basic example that demonstrates the use of a Path2D, to define a custom shape and an AffineTransformation to transform the shape.

This example will cause the arrow to point towards the mouse as it moves over the panel

import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Shape;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.AffineTransform;
import java.awt.geom.Path2D;
import javax.swing.JFrame;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class RotateArrow {

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

    public RotateArrow() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new RotatePane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class RotatePane extends javax.swing.JPanel {

        private Point mousePoint;
        private PointyThing pointyThing;

        public RotatePane() {

            pointyThing = new PointyThing();

            addMouseMotionListener(new MouseAdapter() {
                @Override
                public void mouseMoved(MouseEvent e) {
                    mousePoint = e.getPoint();
                    repaint();
                }
            });

        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            Graphics2D g2d = (Graphics2D) g.create();

            double rotation = 0f;

            int width = getWidth() - 1;
            int height = getHeight() - 1;

            if (mousePoint != null) {
                int x = width / 2;
                int y = height / 2;

                int deltaX = mousePoint.x - x;
                int deltaY = mousePoint.y - y;

                rotation = -Math.atan2(deltaX, deltaY);
                rotation = Math.toDegrees(rotation) + 180;
            }

            Rectangle bounds = pointyThing.getBounds();

            AffineTransform at = new AffineTransform();
            at.translate((width - bounds.width) / 2, (height - bounds.height) / 2);
            at.rotate(Math.toRadians(rotation), bounds.width / 2, bounds.height / 2);
            Shape shape = new Path2D.Float(pointyThing, at);

            g2d.setStroke(new BasicStroke(3));
            g2d.setColor(Color.RED);

            g2d.fill(shape);
            g2d.draw(shape);
            g2d.dispose();
        }
    }

    public class PointyThing extends Path2D.Float {

        public PointyThing() {
            moveTo(15, 0);
            lineTo(30, 15);
            lineTo(0, 15);
            lineTo(15, 0);
            moveTo(15, 15);
            lineTo(15, 60);
        }

    }
}

Update with example two

Basically, this uses the PointyThing as the arrow head only, and draws the line separately.

The arrow is orientated so it will point down the line (towards the mouse)

import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Shape;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.AffineTransform;
import java.awt.geom.Line2D;
import java.awt.geom.Path2D;
import javax.swing.JFrame;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class RotateArrow {

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

    public RotateArrow() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new RotatePane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }

        });
    }

    public class RotatePane extends javax.swing.JPanel {

        private PointyThing pointyThing;
        private Point mouseStart;
        private Point mouseEnd;

        public RotatePane() {

            pointyThing = new PointyThing();

            MouseAdapter ma = new MouseAdapter() {
                @Override
                public void mouseMoved(MouseEvent e) {
                    mouseEnd = e.getPoint();
                    repaint();
                }

                @Override
                public void mouseClicked(MouseEvent e) {
                    mouseStart = e.getPoint();
                    repaint();
                }
            };

            addMouseListener(ma);
            addMouseMotionListener(ma);

        }

        @Override
        public Dimension getPreferredSize() {

            return new Dimension(200, 200);

        }

        @Override
        protected void paintComponent(Graphics g) {

            super.paintComponent(g);

            Graphics2D g2d = (Graphics2D) g.create();
            if (mouseStart != null && mouseEnd != null) {

                double rotation = 0f;

                int width = getWidth() - 1;
                int height = getHeight() - 1;

                if (mouseEnd != null) {

                    int x = mouseStart.x;
                    int y = mouseStart.y;

                    int deltaX = mouseEnd.x - x;
                    int deltaY = mouseEnd.y - y;

                    rotation = -Math.atan2(deltaX, deltaY);
                    rotation = Math.toDegrees(rotation) + 180;

                }

                Rectangle bounds = pointyThing.getBounds();

                g2d.setStroke(new BasicStroke(3));
                g2d.setColor(Color.RED);
                g2d.draw(new Line2D.Float(mouseStart, mouseEnd));

                AffineTransform at = new AffineTransform();

                at.translate(mouseEnd.x - (bounds.width / 2), mouseEnd.y - (bounds.height / 2));
                at.rotate(Math.toRadians(rotation), bounds.width / 2, bounds.height / 2);
                Shape shape = new Path2D.Float(pointyThing, at);

                g2d.fill(shape);
                g2d.draw(shape);
            }
            g2d.dispose();

        }

    }

    public class PointyThing extends Path2D.Float {

        public PointyThing() {
            moveTo(15, 0);
            lineTo(30, 15);
            lineTo(0, 15);
            lineTo(15, 0);
        }

    }

}