您需要知道两件事，您需要知道
鼠标被按下的原始点和形状的原始边界

private Point clickPoint;
private Rectangle originalBounds;
//...
@Override
public void mousePressed(MouseEvent e) {
    if (hovering) {
        resizing = true;
        System.out.println("Starting to resize");
        clickPoint = e.getPoint();
        originalBounds = new Rectangle(shapes.get(currentIndex).getBounds());
    }
}

With this information you can calculate the delta between the original click
point and the drag point, armed with this information, you can modify the the
shape by adding the difference to the originalBounse

@Override
public void mouseDragged(MouseEvent e) {
    if (resizing && clickPoint != null) {

        int mouseX = e.getX();
        int mouseY = e.getY();

        int xDelta = mouseX - clickPoint.x;
        int yDelta = mouseY - clickPoint.y;

        int delta = Math.max(xDelta, yDelta);

        Rectangle bounds = shapes.get(currentIndex).getBounds();
        int shapeX = bounds.x;
        int shapeY = bounds.y;

        int shapeWidth = originalBounds.width + delta;
        int shapeHeight = originalBounds.height + delta;

        if (shapeWidth < 0) {
            shapeWidth *= -1;
            shapeX = originalBounds.x - shapeWidth;
        }
        if (shapeHeight < 0) {
            shapeHeight *= -1;
            shapeY = originalBounds.y - shapeHeight;
        }

        System.out.printf("%d %dx%dx%dx%d%n", delta, shapeX, shapeY, shapeWidth, shapeHeight);

        shapes.get(currentIndex).reset();
        shapes.get(currentIndex).append(
                        new Ellipse2D.Double(shapeX, shapeY, shapeWidth, shapeHeight), true);
        repaint();
    }
}

Just don’t forget to reset the clickPoint and originalBounds when the user
releases the button ;)

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Cursor;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Panel;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Path2D;
import java.awt.geom.Point2D;
import java.awt.geom.Rectangle2D;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import static javax.swing.JFrame.EXIT_ON_CLOSE;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Editor {

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

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

                JFrame frame = new UMLWindow();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setBounds(30, 30, 1000, 700);
                frame.getContentPane().setBackground(Color.white);
                frame.setVisible(true);
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public static class UMLWindow extends JFrame {

        Shapes shapeList = new Shapes();
        Panel panel;

        private static final long serialVersionUID = 1L;

        public UMLWindow() {
            addMenus();
            panel = new Panel();
        }

        public void addMenus() {

            getContentPane().add(shapeList);

            setSize(300, 200);
            setLocationRelativeTo(null);
            setDefaultCloseOperation(EXIT_ON_CLOSE);

            shapeList.addCircle(100, 100);
        }
    }

    // Shapes class, used to draw the shapes on the panel
    // as well as implements the MouseListener for dragging
    public static class Shapes extends JPanel {

        private static final long serialVersionUID = 1L;

        private List<Path2D> shapes = new ArrayList<Path2D>();
        int currentIndex;

        private Point mousePoint;

        public Shapes() {
            MyMouseAdapter myMouseAdapter = new MyMouseAdapter();
            addMouseListener(myMouseAdapter);
            addMouseMotionListener(myMouseAdapter);
        }

        public void addCircle(int width, int height) {
            Path2D circ = new Path2D.Double();
            circ.append(new Ellipse2D.Double(442, 269, width, height), true);
            shapes.add(circ);
            repaint();
        }

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

            Graphics2D g2 = (Graphics2D) g;
            g2.setStroke(new BasicStroke(2));
            for (Path2D shape : shapes) {
                g2.setColor(Color.BLACK);
                g2.draw(shape);
            }
        }

        public Rectangle2D getActiveBounds(float angel, Rectangle bounds) {

            Point2D p = getPointOnEdge(angel, bounds);

            return new Rectangle2D.Double(p.getX() - 4, p.getY() - 4, 8, 8);

        }

        public Point2D getPointOnEdge(float angel, Rectangle bounds) {

            float radius = Math.max(bounds.width, bounds.height) / 2;

            float x = radius;
            float y = radius;

            double rads = Math.toRadians((angel + 90));

            // Calculate the outter point of the line
            float xPosy = (float) (x + Math.cos(rads) * radius);
            float yPosy = (float) (y + Math.sin(rads) * radius);

            return new Point2D.Float(xPosy + bounds.x, yPosy + bounds.y);

        }

        class MyMouseAdapter extends MouseAdapter {

            Boolean hovering = false;
            Boolean resizing = true;

            private Point clickPoint;
            private Rectangle originalBounds;

            @Override
            public void mouseMoved(MouseEvent e) {
                mousePoint = e.getPoint();
                for (int i = 0; i < shapes.size(); i++) {
                    Path2D shape = shapes.get(i);
                    Rectangle2D bottomRight = getActiveBounds(-45,
                                    shape.getBounds());

                    if (mousePoint != null) {

                        if (bottomRight.contains(mousePoint)) {
                            Cursor cursor = Cursor
                                            .getPredefinedCursor(Cursor.SE_RESIZE_CURSOR);
                            setCursor(cursor);
                            hovering = true;
                        } else {
                            Cursor cursor = Cursor
                                            .getPredefinedCursor(Cursor.DEFAULT_CURSOR);
                            setCursor(cursor);
                            hovering = false;
                        }
                    }
                }

                repaint();
            }

            @Override
            public void mousePressed(MouseEvent e) {
                if (hovering) {
                    resizing = true;
                    System.out.println("Starting to resize");
                    clickPoint = e.getPoint();
                    originalBounds = new Rectangle(shapes.get(currentIndex).getBounds());
                }
            }

            @Override
            public void mouseDragged(MouseEvent e) {
                if (resizing && clickPoint != null) {

                    int mouseX = e.getX();
                    int mouseY = e.getY();

                    int xDelta = mouseX - clickPoint.x;
                    int yDelta = mouseY - clickPoint.y;

                    int delta = Math.max(xDelta, yDelta);

                    Rectangle bounds = shapes.get(currentIndex).getBounds();
                    int shapeX = bounds.x;
                    int shapeY = bounds.y;

                    int shapeWidth = originalBounds.width + delta;
                    int shapeHeight = originalBounds.height + delta;

                    if (shapeWidth < 0) {
                        shapeWidth *= -1;
                        shapeX = originalBounds.x - shapeWidth;
                    }
                    if (shapeHeight < 0) {
                        shapeHeight *= -1;
                        shapeY = originalBounds.y - shapeHeight;
                    }

                    System.out.printf("%d %dx%dx%dx%d%n", delta, shapeX, shapeY, shapeWidth, shapeHeight);

                    shapes.get(currentIndex).reset();
                    shapes.get(currentIndex).append(
                                    new Ellipse2D.Double(shapeX, shapeY, shapeWidth, shapeHeight), true);
                    repaint();
                }
            }

            @Override
            public void mouseReleased(MouseEvent e) {
                if (resizing) {
                    System.out.println("Done resizing");
                    resizing = false;
                    clickPoint = null;
                    originalBounds = null;
                }
            }

        }
    }

}