Hey,
check out the following code. See if u get any clue from
this. Your display needs to be initialised first..hope ur
doin tat in ur code.. unless u do tat ur paint wudn't be
called at all. try to display a form instead of a canvas and
see if display is causing the problem. hope this was helpful
to u.



public class Mainscreen extends MIDlet implements
CommandListener{
Display display;
public Mainscreen(){
  display = Display.getDisplay(this);
  setcommandListener(this);
}
public void startApp(){
 Form f = new Form("ShowCanvas"){
 f.addCommand(Command.OK);
 f.setcommandListener(this);
 display.setcurrent(f);
 }
}
public void DestroyApp(boolean t){
notifyDestroyed();
}
public void pauseApp(){
}
 public void commandAction(Command c ,Displayable d){
 if(c==Command.OK){
   showDetails();
   }
 }
}
public void showDetails() {
		
	Canvas canvas = new Canvas() {
		protected void paint(Graphics g) {
				
		g.setColor(255, 255, 255);
		g.fillRect(0, 0, getWidth(), getHeight());
		g.setColor(0, 0, 0);
		g.setFont(font_med);
		g.drawString("HI anonymous",10, 0, 0);
				
		}
	};
		canvas.addCommand(Commands.Back);
		canvas.setCommandListener(this);
		display.setcurrent(canvas);
}

With the same flow,the paint(Graphics g) will execute.
If not executes then do not display.setCurrent(canobj) here.
Go to the Canvas class & take a Display object & initialize
it with the MIDlet call display object.Then from the Canvas
class constructor call a method,where first call repaint()
method,then serviceRepaint() method,then call
display.setCurrent(this).It will work like bellow..


import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class TestView extends MIDlet {
    CanvasView canObj;
    static Display DISPLAY;
    public void startApp() {
        String word = "Hi Susanta";
        DISPLAY = Display.getDisplay(this);
        canObj = new CanvasView(word,this);
    }
    
    public void pauseApp() {
    }
    
    public void destroyApp(boolean unconditional) {
        notifyDestroyed();
    }
}

class CanvasView extends Canvas implements CommandListener{
    Display display;
    String str="";
    Command exit;
    TestView midlet;
    public CanvasView(String str,TestView midlet){
        this.midlet = midlet;
        display = TestView.DISPLAY;
        this.str = str;
        exit = new Command("Exit",Command.EXIT,0);
        addCommand(exit);
        setCommandListener(this);
        callPaint();
    }
    
    protected void paint(Graphics g){
        setFullScreenMode(true);
        g.setColor(0,0,200);
        g.fillRect(getWidth()/2-40,getHeight()/2-20,80,40);
        g.setColor(255,255,255);
       
g.drawString(str,getWidth()/2,getHeight()/2-10,g.HCENTER|g.TOP);
    }
    
    public void callPaint(){
        repaint();
        serviceRepaints();
        display.setCurrent(this);
    }

    public void commandAction(Command command,Displayable disp){
        if(command == exit){
            midlet.destroyApp(true);
        }
    }
}