Beginner with a programming problem

Question

I am getting quite frustrated trying to figure out how to proceed with a Java program I've been assigned. I'm hoping that my problem is not too amorphous to warrant assistance.
The program is an exercise in Swing and creating GUI's, and at this stage, I only need to create an array of dice objects which can paint themselves on a JPanel. The instructor has given us two files -- one extending JPanel and containing a main method to test the classes, and another which represents a die.
Some issues are
1) I don't know where to call repaint.
2) When the overridden paintComponent method is called, it wipes out
the background color on my panel, which I set in the constructor.
3) I got null pointer exceptions when I ran the instructor 's main method... I moved the setVisible up in the chain and they went away (Though he stated not to alter the main method, I don't know enough about Java Graphics to know if it's his mistake or mine)
4) I'd just like to see some something appear.
I should add that I rewrote this whole thing in one file (without using an array) and was able to get the graphics to paint.
Sorry for posting copious amounts of code, but hopefully it will clear up what I'm trying to do. If anyone could just give me some pointers in the right direction, I would greatly appreciate it. Thank you for your time.
//      File:  DicePanel.java
public class DicePanel extends JPanel
  private int number_of_dice = 0;
  private DiceDrawn [] dice; 
  /** The x,y coordinates where the first die is drawn */
  private int xCoordinate = 5; //default
  private int yCoordinate = 5; //default
  /** Constructor with default properties */
  public DicePanel()
    this.setSize(getPreferredSize());
    this.setBackground(Color.blue);
    // set size and color
  /*  Set the Dice array  */
  public void setDice(DiceDrawn[] d)
  }// end setDice
  /** Return xCoordinate */
  public int getXCoordinate()
    return xCoordinate;
  /** Set a new xCoordinator */
  public void setXCoordinate(int x)
     xCoordinate = x;
  /** Return yCoordinator */
  public int getYCoordinate()
    return yCoordinate;
  /** Set a new yCoordinator */
  public void setYCoordinate(int y)
     yCoordinate = y;
  /** Paint the Dice Panel */
  protected void paintComponent(Graphics g)
    super.paintComponent(g);
    for (int i = 0; i < dice.length; i++ )
      dice.drawDie(g, xCoordinate, yCoordinate);
setXCoordinate(getXCoordinate() + 41);
// draw each die using default start coordinates,
// moving right each time
/** Override get method for preferredSize */
public Dimension getPreferredSize()
return new Dimension(200, 50);
// the main method to test DicePanel and DiceDrawn
// do not change this method.
public static void main(String [] args)
JFrame f = new JFrame();
// f.setVisible(true);
f.setTitle("Draw Dice");
f.setSize(500,400);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Container cp = f.getContentPane();
// show 6 dice initially
DiceDrawn [] dice = new DiceDrawn [6];
for(int i = 0; i < dice.length; i++)
dice[i] = new DiceDrawn(i+1);
DicePanel dicePanel = new DicePanel();
dicePanel.setDice(dice);
System.out.println(dice.length);
cp.add(dicePanel, BorderLayout.NORTH);
f.setVisible(true);
// show only one die for the rest of the program
dice = new DiceDrawn [1];
System.out.println("Dice Drawn Class Test");
int i = Io.readInt("Enter an integer between 1 and 6");
while( i > 0 )
dice[0] = new DiceDrawn(i);
dice[0].setSelected( i%2 == 0 ? true : false);
dicePanel.setDice(dice);
i = Io.readInt("Enter an integer between 1 and 6");
System.exit(0);
// File: DiceDrawn.java
public class DiceDrawn
private final int SIZE = 36;
private final int QUARTER = SIZE/4;
private int face; // face value
private boolean selected; // pressed = true
public DiceDrawn( int f)
setFace(f);
setSelected(true);
}// end DiceDrawn
public void setSelected( boolean s )
selected = s;
public boolean isSelected()
return selected;
public void setFace(int i)
face = i;
public int getFace()
return face;
// Draw a die starting at the given coordinates
public void drawDie(Graphics g, int xcoor, int ycoor)
g.setColor(Color.WHITE);
g.fillRect(xcoor,ycoor, SIZE, SIZE);
g.setColor(Color.BLACK);
switch(face)
case 1:
g.fillOval((xcoor-4)+(2*QUARTER), (ycoor-4)+(2*QUARTER), SIZE/8, SIZE/8);
break;
case 2:
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + QUARTER, SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+(3*QUARTER), (ycoor-4)+(3*QUARTER), SIZE/8, SIZE/8);
break;
case 3:
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + QUARTER, SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+(3*QUARTER), (ycoor-4)+(3*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+(2*QUARTER), (ycoor-4)+(2*QUARTER), SIZE/8, SIZE/8);
break;
case 4:
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + QUARTER, SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+(3*QUARTER), (ycoor-4)+(3*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + (3*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ (3*QUARTER), (ycoor-4)+ QUARTER, SIZE/8, SIZE/8);
break;
case 5:
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + QUARTER, SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+(3*QUARTER), (ycoor-4)+(3*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + (3*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ (3*QUARTER), (ycoor-4)+ QUARTER, SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+(2*QUARTER), (ycoor-4)+(2*QUARTER), SIZE/8, SIZE/8);
break;
case 6:
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + QUARTER, SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+(3*QUARTER), (ycoor-4)+(3*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4)+(2*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ (3*QUARTER), (ycoor-4)+(2*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ QUARTER, (ycoor-4) + (3*QUARTER), SIZE/8, SIZE/8);
g.fillOval((xcoor-4)+ (3*QUARTER), (ycoor-4)+ QUARTER, SIZE/8, SIZE/8);
break;
// draw a die using a rectangle of 36 by 36 pixels
// at the coordinates given.
// use a switch statement to draw the appropriate face.
}// end DrawDie
}// end DiceDrawn

Answer 1

Once you get beyond your initial problems, you'll find that there is another problem. When it displays the dice corresponding to the numbers you select, the x coordinate eventually becomes greater than the frame width. You need to make a change to the paintComponent method so it looks like:
  /** Paint the Dice Panel */
  protected void paintComponent(Graphics g)
    super.paintComponent(g);
    for (int i = 0; i < dice.length; i++ )
      dice.drawDie(g, xCoordinate, yCoordinate);
if(dice.length > 1)
setXCoordinate(getXCoordinate() + 41);
// draw each die using default start coordinates,
// moving right each time
Note that I have added an if above the setXCoordinate so that it will only add to the X coordinate when it is displaying the 6 dice initially. It won't add anything when it is displaying the single dice.