java - what calls paintComponent()? -

i attempting draw sprites out of sprite sheet.

i have following class

public class gtcomponent extends jcomponent {      graphics2d g2;      @override     public void paintcomponent(graphics g){         g2 = (graphics2d)g;     }      public void drawspriteframe(image source, int x, int y, int frame) {         int framex = (frame % 12) * 32;         int framey = (frame / 12) * 32;         g2.drawimage(source, x, y, x + 32, y + 32,                 framex, framey, framex + 32, framey + 32, this);     } } 

that created object in main class so

    jframe f = new jframe();     gtcomponent img = new gtcomponent();      f.add(img);      f.setdefaultcloseoperation(jframe.exit_on_close);     f.setsize((int)(i.length * 8.1), (int)(i[0].length * 8.5));     f.setvisible(true);     f.setlocationrelativeto(null);      bufferedimage test = null;     try {         test = imageio.read(new file( /*image file path*/));     }     catch (ioexception e) {         system.out.println("error");         system.exit(0);     }      img.drawspriteframe(test, (u * 32 + 1), (z * 32 + 1), c); 

the problem im facing following error gets thrown

exception in thread "awt-eventqueue-0" java.lang.nullpointerexception 

after doing several debugs, setting breakpoints @ paintcomponent , drawspriteframe, found out drawspriteframe method gets called before paintcomponent method, meaning g2 = null resulting in error being thrown.

the question here triggers paintcomponent method allows me initialise g2 variable?

paintcomponent() called automatically event dispatch thread. if want custom component painted part of ordinary swing painting process, should override paintcomponent() call drawspriteframe() method, not call drawspriteframe() directly.

if want control drawing operation yourself, need use "active rendering" as described in full-screen exclusive mode tutorial -- note technique described there works windowed applications. need ask window graphics instance (instead of waiting 1 passed paintcomponent() , draw that.

a basic example using double buffering:

// initial setup frame mainframe = new frame(); mainframe.setvisible(true); // you'll want set size, location, etc. mainframe.createbufferstrategy(2); bufferstrategy bufferstrategy = mainframe.getbufferstrategy();  //....   // inside draw loop (call once each frame) graphics2d g2 = (graphics2d) bufferstrategy.getdrawgraphics(); g2.drawimage(...) // etc. g2.dispose(); bufferstrategy.show();