请记住，
Rectangle（和
Graphics#fillRect和
Graphics#drawRect）不会渲染宽度/高度为负的矩形
你需要两件事…
- 当前鼠标点（或您的情况下的拖动点）
- 首次按下鼠标的点（锚点或原点）
您应该从
mousePressed事件中获取锚点…
public void mousePressed(MouseEvent e) { clickPoint = new Point(e.getPoint());}然后，您需要确定哪个点最小，并将其作为起点，哪个点最大，并将其用于尺寸。
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); selection.x = minX; selection.y = minY; selection.width = maxX - minX; selection.height = maxY - minY; repaint();}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.event.MouseAdapter;import java.awt.event.MouseEvent;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 Rectangle selection = new Rectangle(); private Point clickPoint; 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); selection.x = minX; selection.y = minY; selection.width = maxX - minX; selection.height = maxY - minY; 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); Graphics2D g2d = (Graphics2D) g.create(); if (selection.width > 0 && selection.height > 0) { g2d.setColor(new Color(0, 0, 255, 64)); g2d.fill(selection); g2d.setColor(Color.BLUE); g2d.draw(selection); } g2d.dispose(); } }}
