java - Why is rectangle not rendering to screen -

public class rec extends jframe {        public rec (){           jframe jframe = new jframe();           jframe.setsize(500, 500);    jframe.setvisible(true);   }  public void render (graphics g){    g.setcolor(color.red);    g.fillrect(0,0,50,50);           }  public static void main(string[] args) {     rec frame = new rec();     frame.render(g);              }       } 

why not work? aware may need paintcomponent, if how go doing this? great, thank you!

the thing painting in jframe not should doing. better (in instance) set contentpane jpanel, , paint inside jpanel.

take short snippet example:

import java.awt.*; import javax.swing.*;  public class rec {     public static void main(string[] args) {         swingutilities.invokelater(new runnable() {             public void run() {                 jframe rec = new jframe();                 rec.setsize(50, 150);                 rec.setcontentpane(new jpanel() {                     @override                     protected void paintcomponent(graphics g) {                         super.paintcomponent(g);                         g.setcolor(color.red);                         g.fillrect(0, 0, 50, 50);                     }                            });                 rec.setvisible(true);             }         });     } } 

result: