graphics - Java: draw a rectangular spiral with drawLine -

i'm working on java programming exercise i'm supposed draw rectangular spiral drawline method. here's have far, spiral comes looking incomplete.

import java.awt.graphics;  import javax.swing.jpanel; import javax.swing.jframe;  public class rectspiral extends jpanel {     public void paintcomponent(graphics g)     {         int width = getsize().width;         int height = getsize().height;          int widthcenter = width / 2;         int heightcenter = height / 2;          (int = 0; < 4 ; i++)         {             g.drawline(widthcenter + (20 * i), heightcenter + (20 * i), widthcenter + (20 * i), heightcenter + 20 + (20 * i));             g.drawline(widthcenter + (20 * i), heightcenter + 20 + (20 * i), widthcenter - 20 - (20 * i), heightcenter + 20 + (20 * i));             g.drawline(widthcenter - 20 - (20 * i), heightcenter + 20 + (20 * i), widthcenter - 20 - (20 * i), heightcenter - 20 - (20 * i));             g.drawline(widthcenter - 20 - (20 * i), heightcenter - 20 - (20 * i), widthcenter + 20 + (20 * i), heightcenter - 20 - (20 * i));         }     }      public static void main(string[] args)     {         rectspiral panel = new rectspiral();          jframe application = new jframe();          application.setdefaultcloseoperation(jframe.exit_on_close);         application.add(panel);               application.setsize(300, 300);          application.setvisible(true);          } } 

this output:

i've tried adding fifth line withing loop, result still comes incomplete. appreciated, thank you!

in first g.drawline need subtract (20 * i) center of y, not start

| | | center | |<-center + (20 * i) | 

but here:

| |<-center - (20 * i) | center | | | 

so use

g.drawline(widthcenter + (20 * i), heightcenter - (20 * i), //                                              ^---------------change            widthcenter + (20 * i), heightcenter + 20 + (20 * i));