import java.awt.Graphics;
import javax.swing.JApplet;
import javax.swing.JOptionPane;
public class DrawOval extends JApplet{
 int x,y,length,width;
 public void init()
 {
  String xs,ys,lengths,widths;
  
  xs = JOptionPane.showInputDialog("Enter posX");
  ys = JOptionPane.showInputDialog("Enter posY");
  widths = JOptionPane.showInputDialog("Enter width");
  lengths = JOptionPane.showInputDialog("Enter length");
  
  x = Integer.parseInt(xs);
  y = Integer.parseInt(ys);
  width = Integer.parseInt(widths);
  length = Integer.parseInt(lengths);
 }
 public void paint(Graphics g)
 {
  super.paint(g);
  g.drawOval(x,y,width,length);
 }
}

//怎么我运行的时候，只是画了半个椭圆？

解决了！