如何使用java2d绘制箭头？

Question

I'm trying to draw an arrow in a circle(like a clock pointer), but I can't align the tip of the arrow with the rest of the line.

I made the "arrow" based on this answer , but I can't make it is correctly positioned with the line drawing.

The arrow is more to the left of the line, as follows in the image:

Here my mcve:

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.Polygon;
import java.awt.geom.AffineTransform;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;

public class LineArrowTest extends JFrame {

    private static final long serialVersionUID = 1L;
    private JPanel contentPane;
    private JPanel DrawPanel;

    public static void main(String[] args) {
        EventQueue.invokeLater(() -> {
            new LineArrowTest().setVisible(true);
        });
    }

    public LineArrowTest() {
        initComponents();
        pack();
    }

    private void initComponents() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setPreferredSize(new Dimension(400, 300));
        this.contentPane = new JPanel(new BorderLayout(0, 0));
        this.contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(this.contentPane);

        this.DrawPanel = new JPanel() {

            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
                LineArrow line = new LineArrow(getWidth() / 2, getHeight() / 2, getWidth() / 2, getHeight(),
                        Color.black, 3);
                line.draw(g);
            }
        };
        this.contentPane.add(this.DrawPanel, BorderLayout.CENTER);
    }

    class LineArrow {

        int x;
        int y;
        int endX;
        int endY;
        Color color;
        int thickness;

        public LineArrow(int x, int y, int x2, int y2, Color color, int thickness) {
            super();
            this.x = x;
            this.y = y;
            this.endX = x2;
            this.endY = y2;

            this.color = color;
            this.thickness = thickness;
        }

        public void draw(Graphics g) {
            Graphics2D g2 = (Graphics2D) g.create();

            g2.setColor(color);
            g2.setStroke(new BasicStroke(thickness));
            g2.drawLine(x, y, endX, endY);
            ;
            drawArrowHead(g2);
            g2.dispose();
        }

        private void drawArrowHead(Graphics2D g2) {

            Polygon arrowHead = new Polygon();
            AffineTransform tx = new AffineTransform();

            arrowHead.addPoint(0, 5);
            arrowHead.addPoint(-5, -5);
            arrowHead.addPoint(5, -5);

            tx.setToIdentity();
            double angle = Math.atan2(endY - y, endX - x);
            tx.translate(endX, endY);
            tx.rotate(angle - Math.PI / 2d);

            g2.setTransform(tx);
            g2.fill(arrowHead);
        }

    }

}

Answer 1

You are resetting the transformation.

Change AffineTransform tx = new AffineTransform(); to AffineTransform tx = g2.getTransform(); and remove the tx.setToIdentity(); call.

The below code was also rearranged to keep related statements together.

private void drawArrowHead(Graphics2D g2) {
    double angle = Math.atan2(endY - y, endX - x);
    AffineTransform tx = g2.getTransform();
    tx.translate(endX, endY);
    tx.rotate(angle - Math.PI / 2d);
    g2.setTransform(tx);

    Polygon arrowHead = new Polygon();
    arrowHead.addPoint(0, 5);
    arrowHead.addPoint(-5, -5);
    arrowHead.addPoint(5, -5);
    g2.fill(arrowHead);
}