//DrawPanel.java
import java.awt.Graphics;
import javax.swing.JPanel;
import java.awt.Color;
public class DrawPanel extends JPanel
{
public void paintComponent(Graphics g)
{
super.paintComponent(g);
int width=getWidth();
int height=getHeight();
g.setColor(Color.red);
g.drawLine(0,0,width,height);
g.drawLine(0,height,width,0);
}
}
//TestDrawLine.java
import javax.swing.JFrame;
public class TestDrawLine
{
public static void main(String args[])
{
DrawPanel myPanel=new DrawPanel();
JFrame myWindow=new JFrame();
myWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
myWindow.add(myPanel);
myWindow.setSize(300,500);
myWindow.setVisible(true);
}
}

别沉啊

木有人么？

救命啊

你两个public class放一起编译了吧

必须两个把java文件（实际上是class文件）放在同一个路径下面

import javax.swing.JFrame;