后面的类不可以单拿出来么？为什么拿出来就错？ - JAVA论坛

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后面的类不可以单拿出来么？为什么拿出来就错？

package cao;
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;

public class cao4 extends JFrame
{
JPanel jp;
public Color color;
public JLabel jl;
public int x1=0,y1=0;
public cao4()
{
super("鼠标电击事件");
jl=new JLabel();
getContentPane().add(jl,BorderLayout.SOUTH);
jp=new JPanel(new FlowLayout());
getContentPane().add(jp,BorderLayout.CENTER);
addMouseMotionListener(new mc());
setSize(500,500);
setVisible(true);
}
public void paint(Graphics g)
{
super.paint(g);

g.setColor(color);
g.fillOval(x1-50,y1-50,100,100);
}

public static void main(String[] args)
{
cao4 mycao4=new cao4();
mycao4.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

class mc extends MouseMotionAdapter
{
public void mouseDragged(MouseEvent event)
{
x1=event.getX();
y1=event.getY();
color=Color.red;
repaint();
}
public void mouseMoved(MouseEvent event)
{
x1=event.getX();
y1=event.getY();
color=Color.black;
repaint();

}
}
}
////////////////////////////////////////////////////////////////////////////////////////////////////
就是写成这样就错
package cao;
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;

g.setColor(color);
g.fillOval(x1-50,y1-50,100,100);
}

public static void main(String[] args)
{
cao4 mycao4=new cao4();
mycao4.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

}
class mc extends MouseMotionAdapter
{
public void mouseDragged(MouseEvent event)
{
x1=event.getX();
y1=event.getY();
color=Color.red;
repaint();
}
public void mouseMoved(MouseEvent event)
{
x1=event.getX();
y1=event.getY();
color=Color.black;
repaint();

}
}
为什么？