java - Change JFrame's background color -

i following video tutorial tries make java game in 2d. have found course's author's approach not working because of changing jframe's color doesn't show up!

his approach, window class:

package modelos; import java.awt.color; import java.awt.event.*; import javax.swing.jframe;  public class ventana extends jframe implements windowlistener  {      public ventana(string titulo){         super(titulo);         this.setsize(800,600);         addwindowlistener(this);         setbackground(color.black);     }      @override     public void windowopened(windowevent we) {     }      @override     public void windowclosing(windowevent we) {         system.exit(0);     }      @override     public void windowclosed(windowevent we) {     }      @override     public void windowiconified(windowevent we) {     }      @override     public void windowdeiconified(windowevent we) {     }      @override     public void windowactivated(windowevent we) {     }      @override     public void windowdeactivated(windowevent we) {     } } 

panel class supposed graph rectangles, triangles, circles represent game's objects:

package modelos; import java.awt.*; import java.util.arraylist; import javax.swing.jpanel;  public class panelfg extends jpanel {      arraylist adibujar;      public panelfg(arraylist dibujar){         this.adibujar=dibujar;     }      public void print(graphics g){         dibujable dib;         (int = 0; <adibujar.size(); i++) {             dib = (dibujable) adibujar.get(i);             dib.dibujar(g);         }     } } 

and main

package ejecuciones; import modelos.*; import java.awt.button; import java.awt.color; import java.util.arraylist;  public class main {      /**      * @param args command line arguments      */     public static void main(string[] args) {         ventana nuestraventana = new ventana("juego de naves");         arraylist arreglodeobjetos = new arraylist();          panelfg nuestropanel = new panelfg(arreglodeobjetos);          nuestraventana.add(nuestropanel);          nuestraventana.setsize(800,600);         nuestraventana.setvisible(true);     } } 

result:

however approach:

window:

package modelos; import java.awt.color; import java.awt.event.*; import javax.swing.jframe;  public class ventana extends jframe implements windowlistener  {      public ventana(string titulo){         super(titulo);         this.setsize(800,600);         addwindowlistener(this);         this.getcontentpane().setbackground(color.black);         this.setvisible(true);     }      @override     public void windowopened(windowevent we) {     }      @override     public void windowclosing(windowevent we) {         system.exit(0);     }      @override     public void windowclosed(windowevent we) {     }      @override     public void windowiconified(windowevent we) {     }      @override     public void windowdeiconified(windowevent we) {     }      @override     public void windowactivated(windowevent we) {     }      @override     public void windowdeactivated(windowevent we) {     } } 

panel class equal.

main:

package ejecuciones; import modelos.*; import java.awt.button; import java.awt.color; import java.util.arraylist;  public class main {      /**      * @param args command line arguments      */     public static void main(string[] args) {         ventana nuestraventana = new ventana("juego de naves");         arraylist arreglodeobjetos = new arraylist();          panelfg nuestropanel = new panelfg(arreglodeobjetos);     } } 

think due author did not used getcontentpane() when creating main jframe set background black, , did.

i have followed topic: jframe.setbackground() not working -- why?

also not understand how code work , other not, considering both using netbeans 8.1.

any opinion/suggestion/explanation?

you never add control jpanel container .... gray color default color background de jpanel , black set jframe. want do?