公共无效paintComponent（Graphics g）无法正常工作

Question

I am a beginner to java Swing. My Code is this:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.applet.*;

public class LineDrawing extends JComponent
{   
    public static void main(String[] args)
    {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();

        int startX = 0;
        int startY = 0;
        int endX = 0;
        int endY = 0;

        addMouseListener(new MouseListener(){
            @Override
            public void mousePressed(MouseEvent e)
            {
                startX = e.getXOnScreen();
                startY = e.getYOnScreen();
            }

            @Override
            public void mouseReleased(MouseEvent e)
            {
                endX = e.getXOnScreen();
                endY = e.getYOnScreen();
                Draw d = new Draw();
                d.lineDraw(startX,startY,endX,endY);
                panel.add(d);
                frame.add(panel);
            }

            @Override
            public void mouseClicked(MouseEvent e){}

            @Override
            public void mouseEntered(MouseEvent e){}

            @Override
            public void mouseExited(MouseEvent e){}
        });

        addMouseMotionListener(new MouseMotionListener(){
            @Override
            public void mouseDragged(MouseEvent e)
            {
                endX = e.getXOnScreen();
                endY = e.getYOnScreen();
            }

            @Override
            public void mouseMoved(MouseEvent e){}
        });

        frame.setSize(500,400);
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    public class Draw extends JPanel
    {
        public void lineDraw(int x1,int y1,int x2,int y2)
        {
            @Override
            protected void paintComponent(Graphics g)
            {
                super.paintComponent(g);
                g.setColor(Color.black);
                g.drawLine(x1,y1,x2,y2);
            }
        }
     }
}

While I execute this it gives me this error:

How can I enhance this code and solve the above mentioned error.

I have seen other codes too but something wrong occurs in paintComponent.

If I leave putting @Override before paintComponent ... It gives this error:

1) ';' statement missing. 2) illegal start of expression.

Answer 1

Like BackSlash commented you can't declare a method inside of an existing.

Try this:

public class Draw extends JPanel {

    private int[] line = new int[4];

    public void lineDraw(int x1,int y1,int x2,int y2) {
        line[0] = x1;
        line[1] = y1;
        line[2] = x2;
        line[3] = y2;
        repaint();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.black);
        g.drawLine(line[0],line[1],line[2],line[3]);
    }  
}

To draw more than one line you need to add the line array into another data structure like an ArrayList and in the paintComponent method you iterate through this list and paint everything.