在JPanel绘画

Question

我有一个JFrame，其中有JToolBar和JPanel。我必须通过MouseListener捕获鼠标点击在面板中以交互方式绘制线条。捕获鼠标单击并执行paintComponent方法（我使用对话框测试了这些方法），但面板中没有绘制任何内容。我认为repaint()（在 mousePressed（）中调用）或方法paintComponent()存在问题，但我不知道它是什么。代码有什么问题？

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Line2D;
import javax.swing.*;

public class Clipping extends JPanel implements MouseListener, ActionListener
{
    static JFrame frame;
    static JComboBox cboDraw;
    static JButton btnClear;
    static JButton btnClip;

    double x1, y1, x2, y2;
    boolean FirstPoint;

    public static void main(String[] args) 
    {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                CreateFrame();
            }
        });
    }

    private static void CreateFrame()
    {
        frame = new JFrame("Clipping");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new Clipping());
        frame.setSize(500,500);
        frame.setVisible(true);
    }

    public Clipping()
    {
        setLayout(new BorderLayout());

        JToolBar toolbar = new JToolBar(JToolBar.VERTICAL);
        PopulateToolBar(toolbar);
        add(toolbar, BorderLayout.WEST);

        addMouseListener(this);
        cboDraw.addMouseListener(this);
        btnClip.addActionListener(this);
        btnClear.addActionListener(this);

    }

    private static void PopulateToolBar(JToolBar toolbar)
    {
        String[] cboList = {"Line", "Polygon"};
        cboDraw = new JComboBox(cboList);
        cboDraw.setMaximumSize(new Dimension(70,30));
        btnClip = new JButton("Set clip area");
        btnClear = new JButton("Clear");

        toolbar.add(cboDraw);
        toolbar.addSeparator();
        toolbar.add(btnClip);
        toolbar.addSeparator();
        toolbar.add(btnClear);

        cboDraw.setAlignmentX(Component.CENTER_ALIGNMENT);
        btnClip.setAlignmentX(Component.CENTER_ALIGNMENT);
        btnClear.setAlignmentX(Component.CENTER_ALIGNMENT);

        toolbar.setMargin(new Insets(10,10,10,10));
        toolbar.setFloatable(false);
        toolbar.setBackground(Color.black);
    }

    public void paintComponent(Graphics g)
    {
        Graphics2D g2 = (Graphics2D) g;
        if (cboDraw.getSelectedIndex() == 0) //draw line
        {
            g2.draw(new Line2D.Double(x1, y1, x2, y2));
        }
    }

    public void mousePressed(MouseEvent e)
    {
        if (e.getSource() != cboDraw) //to prevent line coordinates from being saved when selecting from combobox
        {
            if (cboDraw.getSelectedIndex() == 0) //user wants to draw line
            {
                if (FirstPoint == false) //first coordinates
                {
                    x1 = e.getX();
                    y1 = e.getY();
                    FirstPoint = true;
                }
                else //second coordinates
                {
                    x2 = e.getX();
                    y2 = e.getY();
                    repaint();
                    FirstPoint = false;
                }
            }
        }
    }

    public void actionPerformed(ActionEvent e) {}
    public void mouseClicked(MouseEvent e) {}
    public void mouseReleased(MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited(MouseEvent e) {}
}