当我拖动鼠标以绘制圆时，形状在某些情况下会移动

Question

 @Override
    public Shape getShape() {
        final Ellipse2D.Double result = new Ellipse2D.Double();
        final double px = Math.min(getStart().getX(), getEnd().getX());
        final double py = Math.min(getStart().getY(), getEnd().getY());
        final double pw = Math.abs(getStart().getX() - getEnd().getX());
        result.setFrame(px, py, pw, pw);
        return result;
    }

So this getShape() is returning the shape to a class that draws the shape. getStart() gets the starting Point of the mouse on click, and getEnd() gets the Point at mouse release. Now, when I drag to draw a circle, if i drag to the right or down the circle works as intended and expands to the mouse, if I drag up or left of the cursor the circle expands as it should BUT the circle shape moves up and down with the cursor and I am not sure why.

Answer 1

Shapes in Java are based on the top/left corner as the anchor and the width/height been drawn down/right.

You need to calculate the bounding box between the click point and the drag point

public Shape getShape() {
    final Ellipse2D.Double result = new Ellipse2D.Double();
    final double px = Math.min(getStart().getX(), getEnd().getX());
    final double py = Math.min(getStart().getY(), getEnd().getY());
    final double pw = Math.abs(getStart().getX() - getEnd().getX());
    result.setFrame(px, py, pw, pw);
    return result;
}

The problem is, you're still using the difference between the click point and the drag point to calculate the width, pw should be the difference between the maxX and minX values

So this example shows you how to calculate the anchor and size properties

I read through that post, but it doesn't solve the dragging expansion of the circle, and how it moves up and down.

Then you're doing something wrong

I am trying to get a circle drawn not a circle without the same width and height(ellipse).

Okay, so it should always appear that the circle is been draw from the anchor point, so when the minX or minY is less than the clickPoint 's x / y points, then you need to adjust them as a difference of the clickPoint and the size

So, this will make it "appear" as if the circle is always been drawn from the intial click point

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.Ellipse2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class SelectionExample {

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

    public SelectionExample() {
        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 TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private Point clickPoint;

        private Shape shape;
        private Rectangle box;

        public TestPane() {
            MouseAdapter ma = new MouseAdapter() {

                @Override
                public void mouseDragged(MouseEvent e) {
                    int minX = Math.min(e.getX(), clickPoint.x);
                    int minY = Math.min(e.getY(), clickPoint.y);
                    int maxX = Math.max(e.getX(), clickPoint.x);
                    int maxY = Math.max(e.getY(), clickPoint.y);

                    box = new Rectangle(minX, minY, maxX - minX, maxY - minY);
                    int size = Math.min(maxX - minX, maxY - minY);
                    if (minX < clickPoint.x) {
                        minX = clickPoint.x - size;
                    }
                    if (minY < clickPoint.y) {
                        minY = clickPoint.y - size;
                    }

                    shape = new Ellipse2D.Double(minX, minY, size, size);
                    repaint();
                }

                @Override
                public void mousePressed(MouseEvent e) {
                    clickPoint = new Point(e.getPoint());
                }

            };

            addMouseListener(ma);
            addMouseMotionListener(ma);
        }

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

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (shape != null) {
                Graphics2D g2d = (Graphics2D) g.create();
                g2d.setColor(new Color(0, 0, 255, 64));
                g2d.fill(shape);
                g2d.setColor(Color.BLUE);
                g2d.draw(shape);
                g2d.draw(box);
                g2d.dispose();
            }
        }
    }

}