java - JPanel on JWindow is not transparent if `set setOpaque(false);` - why? -


how make jpanel transparent in example? gradient background not visible:

package test;  import java.awt.color; import java.awt.dimension; import java.awt.gradientpaint; import java.awt.graphics; import java.awt.graphics2d; import javax.swing.boxlayout; import javax.swing.jlabel; import javax.swing.jpanel; import javax.swing.jwindow;  public class paintjpanelonjwindow extends jwindow {      public paintjpanelonjwindow() {          jpanel panel = new jpanel();          panel.setpreferredsize(new dimension(350, 120));         panel.setminimumsize(new dimension(350, 120));         panel.setmaximumsize(new dimension(350, 120));          panel.setlayout(new boxlayout(panel, boxlayout.y_axis));         panel.setopaque(false);          jlabel sometext = new jlabel("i'm not transparent , jpanel :(");         sometext.setopaque(false);         panel.add(sometext);         add(panel);          pack();         setlocationrelativeto(null);         setvisible(true);     }      @override     public void paint(graphics g) {          graphics2d g2d = (graphics2d) g.create();         try {             int w = getwidth(), h = getheight();             g2d.setpaint(new gradientpaint(0, 0, color.red, 0, h, color.white));             g2d.fillrect(0, 0, w, h);         } {             g2d.dispose();         }         super.paint(g);     } }

the immediate problem 

super.paint(g);

is being called after custom painting code in paint method cause previous painting lost. calling panel.setopaque(false) has no effect done in paint method. calling setopaque of components in question unnecessary - default backgrounds displayed when custom painting correctly implemented.

this should done overriding paintcomponent method. means creating new jpanel , placing custom painting functionality there rather in top level container such jwindow.

example:

enter image description here

public class paintjpanelapp {       public static void main(string[] args) {         swingutilities.invokelater(new runnable() {              @override             public void run() {                 jframe frame = new jframe("gradient app");                 frame.setdefaultcloseoperation(jframe.exit_on_close);                 frame.setundecorated(true);                 frame.setlocationrelativeto(null);                  jlabel sometext = new jlabel("i transparent , jpanel :)");                 gradientpanel gradientpanel = new gradientpanel();                 gradientpanel.add(sometext);                 frame.add(gradientpanel);                  frame.pack();                 frame.setvisible(true);             }         });     }      static class gradientpanel extends jpanel {           @override         protected void paintcomponent(graphics g) {             super.paintcomponent(g);             int w = getwidth();              int h = getheight();             graphics2d g2d = (graphics2d) g;             g2d.setpaint(new gradientpaint(0, 0, color.red, 0, h, color.white));             g2d.fillrect(0, 0, w, h);         }     } }

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