Draw to an image using graphics object

Question

ive been trying to this. create a starfield 800px wide and twice the height of the screen 2x600. this image would be scrolling down the screen giving the appearance of moving through space. what i did was create an image the called "starField", and created a graphics object "starFieldObj" to draw dots (stars) to this image.
is this the correct way to do this? my program seems to work differently depending on where i call "generateStarField()" which draws to "starFieldObj"
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class Alpha1 extends Canvas implements Runnable, MouseMotionListener, MouseListener, KeyListener {
     private Image buffer; //bufer image
     private Graphics gfx;
     private Graphics starFieldObj;
     private Image playerShip;
     private int frames = 60; //frames per second
     private MediaTracker mediaTracker; //tracks loaded images
     private boolean ready; //ready to  start animation
     private int [] keys = new int[256];
     private int playerShipX = 100;
     private int playerShipY = 100;
     private static Random random = new Random();
     private Image starField;
     private int starFieldSpeed = 5;
     private int starFieldX = 0;
     private int starFieldY = -600;
     public static void main(String[] args)
          Alpha1 t = new Alpha1();
          Frame f = new Frame("blah");
          f.setSize(800, 600);
          f.add(t, BorderLayout.CENTER);
          f.show();
          t.start();
          t.requestFocus();
          f.addWindowListener(new WindowAdapter(){
               public void windowClosing(WindowEvent e){
                    System.exit(0);}});
     public Alpha1()
          addKeyListener(this);
          addMouseListener(this);
          ready = false;
          mediaTracker = new MediaTracker(this);
          playerShip = getToolkit().getImage("testship1.png");
          //starField = getToolkit().getImage("testbg.png");
          //mediaTracker.addImage(playerShip, 0);
          mediaTracker.addImage(starField, 0);
          try {
               mediaTracker.waitForID(0);
          catch (InterruptedException e) {
               e.printStackTrace();
     public void start()
          buffer = createImage(getSize().width,getSize().height);
          //starField = createImage(getSize().width,getSize().height * 2);
          gfx = buffer.getGraphics();
          //starFieldObj = starField.getGraphics();
          generateStarfield();
          ready = true;
          new Thread(this).start();
     public void mouseDragged(MouseEvent e)     {}
     public void mouseMoved(MouseEvent e)     
          playerShipX = e.getX();
          playerShipY = e.getY();
     public void mouseReleased(MouseEvent e)     {}
     public void mousePressed(MouseEvent e)     {}
     public void mouseExited(MouseEvent e)     {}
     public void mouseEntered(MouseEvent e)  {}
     public void mouseClicked(MouseEvent e)     {}
     public void keyTyped(KeyEvent keyevent)     {}
     public void keyPressed(KeyEvent keyevent)
          keys[keyevent.getKeyCode()] = 1;
          System.out.println(keyevent.getKeyCode());
     public void keyReleased(KeyEvent keyevent)     
          keys[keyevent.getKeyCode()] = 0;
     public void run()
          while (true)
               if (isVisible())
                    repaint();
                    updatePositions();
               }//end if
               try 
               { Thread.sleep(1000/frames); }
               catch (InterruptedException e)
               { e.printStackTrace(); }
          }//end while
     public void paint(Graphics g) {     /*empty etc*/ }
     public void updatePositions()
          if (keys[38] == 1 & keys[40] == 0) //IF UP IS PRESSED
          { playerShipY -= 3; }
          if (keys[40] == 1 & keys[38] == 0) //IF DOWN IS PRESSED
          { playerShipY += 6; }
          if (keys[37] == 1 & keys[39] == 0) //IF LEFT IS PRESSED
          { playerShipX -= 5; }
          if (keys[39] == 1 & keys[37] == 0) //IF RIGHT IS PRESSED
          { playerShipX += 5; }
          //starFieldY = starFieldY + starFieldSpeed;
          //if (starFieldY == 0)
          //     starFieldY = -600;
          //     System.out.println("dadssa");
     public int getRandom(int from, int to)
          if (from > to)
               int randTemp;
               randTemp = from;
               from = to;
               to = randTemp;
          return random.nextInt(to-from+1)+from;
     public void generateStarfield()
          starFieldObj.setColor(Color.black);
          starFieldObj.fillRect (0, 0, 800, 600 * 2);
          for (int td=0; td < 900 ; td++) //draw 900x2 dots on 800x1200 image
                    starFieldObj.fillRect (getRandom(1,800), getRandom(1,600), 1, 1);
                    starFieldObj.fillRect (getRandom(1,800) + 800, getRandom(1,600) + 800, 1, 1);
     public void update(Graphics g)
          if (ready)
               gfx.setColor(Color.black);
               gfx.fillRect (0, 0, this.getSize().width, this.getSize().height);
               //gfx.drawImage(starField,starFieldX,starFieldY,null);
               gfx.drawImage(playerShip,playerShipX,playerShipY,null);
               //gfx.drawString(Integer.toString(showFps),5,5);
               g.drawImage(buffer,0,0,null);
}

Answer 1

updated code, i cant get starField to be 1200px in height
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class Alpha1 extends Canvas implements Runnable, MouseMotionListener, MouseListener, KeyListener {
     private Image buffer; //bufer image
     private Graphics gfx;
     private Graphics starFieldObj;
     private Image playerShip;
     private int frames = 60; //frames per second
     private MediaTracker mediaTracker; //tracks loaded images
     private boolean ready; //ready to  start animation
     private int [] keys = new int[256];
     private int playerShipX = 100;
     private int playerShipY = 100;
     private static Random random = new Random();
     private Image starField;
     private int starFieldSpeed = 5;
     private int starFieldX = 0;
     private int starFieldY = -600;
     public static void main(String[] args)
          Alpha1 t = new Alpha1();
          Frame f = new Frame("blah");
          f.setSize(800, 600);
          f.add(t, BorderLayout.CENTER);
          f.show();
          t.start();
          t.requestFocus();
          f.addWindowListener(new WindowAdapter(){
               public void windowClosing(WindowEvent e){
                    System.exit(0);}});
     public Alpha1()
          addKeyListener(this);
          addMouseListener(this);
          ready = false;
          mediaTracker = new MediaTracker(this);
          playerShip = getToolkit().getImage("testship1.png");
          //starField = getToolkit().getImage("testbg.png");
          //mediaTracker.addImage(playerShip, 0);
          mediaTracker.addImage(starField, 0);
          try {
               mediaTracker.waitForID(0);
          catch (InterruptedException e) {
               e.printStackTrace();
     public void start()
          buffer = createImage(getSize().width,getSize().height);
          starField = createImage(getSize().width,1200);
          gfx = buffer.getGraphics();
          starFieldObj = starField.getGraphics();
          ready = true;
          new Thread(this).start();
     public void mouseDragged(MouseEvent e)     {}
     public void mouseMoved(MouseEvent e)     
          playerShipX = e.getX();
          playerShipY = e.getY();
     public void mouseReleased(MouseEvent e)     {}
     public void mousePressed(MouseEvent e)     {}
     public void mouseExited(MouseEvent e)     {}
     public void mouseEntered(MouseEvent e)  {}
     public void mouseClicked(MouseEvent e)     {}
     public void keyTyped(KeyEvent keyevent)     {}
     public void keyPressed(KeyEvent keyevent)
          keys[keyevent.getKeyCode()] = 1;
          System.out.println(keyevent.getKeyCode());
     public void keyReleased(KeyEvent keyevent)     
          keys[keyevent.getKeyCode()] = 0;
     public void run()
          generateStarfield();
          while (true)
               if (isVisible())
                    repaint();
                    updatePositions();
               }//end if
               try 
               { Thread.sleep(1000/frames); }
               catch (InterruptedException e)
               { e.printStackTrace(); }
          }//end while
     public void paint(Graphics g) {     /*empty etc*/ }
     public void updatePositions()
          if (keys[38] == 1 & keys[40] == 0) //IF UP IS PRESSED
          { playerShipY -= 3; }
          if (keys[40] == 1 & keys[38] == 0) //IF DOWN IS PRESSED
          { playerShipY += 6; }
          if (keys[37] == 1 & keys[39] == 0) //IF LEFT IS PRESSED
          { playerShipX -= 5; }
          if (keys[39] == 1 & keys[37] == 0) //IF RIGHT IS PRESSED
          { playerShipX += 5; }
          starFieldY = starFieldY + starFieldSpeed;
          if (starFieldY == 0)
               starFieldY = -600;
               System.out.println("dadssa");
     public int getRandom(int from, int to)
          if (from > to)
               int randTemp;
               randTemp = from;
               from = to;
               to = randTemp;
          return random.nextInt(to-from+1)+from;
     public void generateStarfield()
          starFieldObj.setColor(Color.black);
          starFieldObj.fillRect (0, 0, 800, 1200);
          for (int td=0; td < 900 ; td++) //draw 900x2 dots on 800x1200 image
                    starFieldObj.setColor(Color.white);
                    starFieldObj.fillRect (getRandom(1,800), getRandom(1,600), 1, 1);
                    starFieldObj.fillRect (getRandom(1,800) + 800, getRandom(1,600) + 800, 1, 1);
     public void update(Graphics g)
          if (ready)
               gfx.setColor(Color.black);
               //gfx.fillRect (0, 0, this.getSize().width, this.getSize().height);
               gfx.drawImage(starField,starFieldX,starFieldY,null);
               gfx.drawImage(playerShip,playerShipX,playerShipY,null);
               //gfx.drawString(Integer.toString(showFps),5,5);
               g.drawImage(buffer,0,0,null);
}