Drawing a special cursor: completely repaint everytime or only a region?

Question

Hello, I'm currently building an application which displays a BufferedImage. When I move the cursor in the the component that displays the BufferedImage my app draws a special cursor. It's a square which is moved only if my current position is dividable by 16 (if there was a grid, it would change it's position when the mouse moves to another square in the grid, it is around the square that the mouse occupies). At first, I was only using on thread so the application was a little laggy and my cursor thing didn't work too well. So I decided to use another thread to take care of my cursor. The drawing is in another class (external to the drawing class), therefore, I use getGraphics (since my drawing class subclasses JComponent). I signal a draw by changing a variable. I have to erase my old cursor to draw the new one. The problem is that my component "'flashes" gray instead of staying the way it should be (which is black, for now). Here are my questions: Why does it flash gray ? and what should I do to change that?
Here's my code:
import java.awt.*;
import javax.swing.*;
import java.awt.geom.*;
import java.awt.image.*;
class Cursor implements Runnable {
     public boolean draw;
     public int[] iCursorCoord;
     public JComponent drawSurface;
     private Thread t;
     private Rectangle cursor;
     Cursor(){
          cursor = new Rectangle(0,0,15,15);
          t = new Thread(this);
          iCursorCoord = new int[2];
          iCursorCoord[0] = 0;
          iCursorCoord[1] = 0;
          draw = false;
          drawSurface = null;
     Cursor(int[] initCoord){
          cursor = new Rectangle(initCoord[0],initCoord[1],15,15);
          t = new Thread(this);
          iCursorCoord = new int[2];
          iCursorCoord[0] = 0;
          iCursorCoord[1] = 0;
          draw = false;
          drawSurface = null;
     Cursor(Rectangle rect){
          cursor = rect;
          t = new Thread(this);
          iCursorCoord = new int[2];
          iCursorCoord[0] = rect.x;
          iCursorCoord[1] = rect.y;
          draw = false;
          drawSurface = null;
     Cursor(JComponent drawSurf){
          cursor = new Rectangle(0,0,15,15);
          t = new Thread(this);
          iCursorCoord = new int[2];
          iCursorCoord[0] = 0;
          iCursorCoord[1] = 0;
          draw = false;
          drawSurface = drawSurf;
     Cursor(JComponent drawSurf, Rectangle rect){
          cursor = rect;
          t = new Thread(this);
          iCursorCoord = new int[2];
          iCursorCoord[0] = rect.x;
          iCursorCoord[1] = rect.y;
          draw = false;
          drawSurface = drawSurf;
     Cursor(JComponent drawSurf, int[] initCoord){
          cursor = new Rectangle(initCoord[0], initCoord[1], 15, 15);
          t = new Thread(this);
          iCursorCoord = new int[2];
          iCursorCoord[0] = initCoord[0];
          iCursorCoord[1] = initCoord[1];
          draw = false;
          drawSurface = drawSurf;
     public void init(){
          t.start();
     public void run(){
          while(true){
               if(!draw) continue;
               draw = false;
               cursor.x = iCursorCoord[0];
               cursor.y = iCursorCoord[1];
               System.out.println("Inside Cursor::run()");
               Graphics2D gfx = (Graphics2D) drawSurface.getGraphics();
               this.completePaint(gfx);
               Color current = gfx.getColor();
               gfx.setColor(new Color(84,255,3));
               gfx.draw(cursor);
               gfx.setColor(current);
     private void completePaint(Graphics2D gfx){
          int layer = MapMaker.getLayerMode();
          if(layer != 4){
               BufferedImage bi = MapMaker.getCurrentMap().getData(MapMaker.getLayerMode());
               gfx.setPaint(new TexturePaint(bi, new Rectangle(0,0,MapMaker.getCurrentMap().getWidth()*16, MapMaker.getCurrentMap().getHeight()*16)));
               gfx.fillRect(0, 0, MapMaker.getCurrentMap().getWidth()*16, MapMaker.getCurrentMap().getHeight()*16);
               //gfx.drawImage(bi, null, 0,0);     
          } else {//Will be implemented later, render will be modified
               System.out.println("Will be implemented later");
}P.S.: This is only part of my code. There are other classes that I do not post because I believe it is not necessary. I will post the full code if needed.

Answer 1

You need to override the paint(Graphics g) method. E.g.:
public class MyPanel extends JPanel {
     public void paint(Graphics g) {
               super.paint(g);
                // ... do draw code here
}The reason your app is flashing is because your code only executes
when the thread is requests it to. Inbetween the time your code is
executing, other threads are calling the paint(Graphics g) method.
Gray is the default color used in painting.
You can use a thread to call the repaint() method, but no drawing
should be done in the thread.