Thread: Problem changing the image in a JLabel

Hi,
I am having a problem with the following program. The program is supposed load four random card images and put them in them on four JLabels. The user is supposed to enter an expression using the four numbers provided that equals 24 and verify that the expression is correct. Then you can hit the refresh button and generate four new cards. It works fine the first time but when I hit refresh the images do not change. I've checked the numbers and the filenames the program generates after the refresh button is hit and they look good. I've tried using the setIcon method with the ImageIcon's but I get NullPointerExceptions. Any clue as to what I am doing wrong?

Casey

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class Exercise25_7 extends JFrame {
  private Integer[] numbersPicked = {0, 0, 0, 0};
  private int[] fileNumber = new int[4];
  private MyArrayList<Integer> list;
  private ImageIcon card1Icon, card2Icon, card3Icon, card4Icon;
  private JLabel jlblCard1, jlblCard2, jlblCard3, jlblCard4;
  private JButton jbtRefresh = new JButton("Refresh");
  private JButton jbtVerify = new JButton("Verify");
  private JTextField jtfExpression = new JTextField(10);
 // private int expressionNumber1, expressionNumber2, expressionNumber3, expressionNumber4, imageFileNumber1, imageFileNumber2, imageFileNumber3, imageFileNumber4;
  private String imageFile1 = "image/card/";
  private String imageFile2 = "image/card/";
  private String imageFile3 = "image/card/";
  private String imageFile4 = "image/card/";
 
  public Exercise25_7() {
    pickCards();
    JPanel p1 = new JPanel(new BorderLayout());
	p1.add(jbtRefresh, BorderLayout.EAST);
	JPanel p2 = new JPanel(new GridLayout(1, 4));
	p2.add(jlblCard1);
	p2.add(jlblCard2);
	p2.add(jlblCard3);
	p2.add(jlblCard4);
	JPanel p3 = new JPanel(new GridLayout(1, 3));
	p3.add(new JLabel("Enter an expression: "));
	p3.add(jtfExpression);
	p3.add(jbtVerify);
	add(p1, BorderLayout.NORTH);
	add(p2, BorderLayout.CENTER);
	add(p3, BorderLayout.SOUTH);
 
	jbtRefresh.addActionListener(new ActionListener() {
	  public void actionPerformed(ActionEvent e) {
	    clearData();
	    pickCards();
	  }
	});
 
	jbtVerify.addActionListener(new ActionListener() {
	  public void actionPerformed(ActionEvent e) {
	    int result = 0;
	    String expression = jtfExpression.getText();
		boolean correct = checkExpression(expression);
		if (correct) {
		  result = EvaluateExpression.evaluateExpression(expression);
		}
		else {
		  System.out.println("Expression includes invalid numbers");
		}
		if (result == 24)
		  System.out.println(expression + " = 24");
		else if ((result != 24) && correct)
		  System.out.println(expression + " does not equal 24");
	  }
	});
 
  }
 
  public boolean checkExpression(String expression) {
    list = new MyArrayList<Integer>(numbersPicked);
    java.util.StringTokenizer tokens = new java.util.StringTokenizer(expression, "()+-/*", true);
   // String tokens[] = expression.split("[ ()+-*/]");
	while (tokens.hasMoreTokens()) {
	  String token = tokens.nextToken().trim();
	  if (token.length() == 0)
	    continue;
	  else if (Character.isDigit(token.charAt(0))){
	    if (!checkNum(Integer.parseInt(token)))
		  return false;
	  }
	}
	return true;
  }
  public boolean checkNum(int num) {
 
    boolean found = false;
    for (int i = 0; i < list.size(); i++){
	  if (list.get(i) == num){
	    found = true;
		list.remove(i);
	  }
	}
	return found;
  }
 
  public void pickCards() {
    int picked = 0;
	//private boolean notPicked = true;
	while (picked < 4) {
	  int number = (int)(Math.random() * 13 + 1);
	  if (! isPicked(number)){
	    picked++;
	  }
	}
	for (int i = 0; i < numbersPicked.length; i++)
	  System.out.print(numbersPicked[i] + ", ");
	System.out.println();
	for (int i = 0; i < fileNumber.length; i++){
	  int set = (int)(Math.random() * 4 + 1);
	  if (set == 1)
	    fileNumber[i] = numbersPicked[i];
	  else if (set == 2)
	    fileNumber[i] = numbersPicked[i] + 13;
	  else if (set == 3)
	    fileNumber[i] = numbersPicked[i] + 26;
	  else
	    fileNumber[i] = numbersPicked[i] + 39;
	}
	for (int i = 0; i < fileNumber.length; i++)
	  System.out.print(fileNumber[i] + ", ");
	System.out.println();
 
	imageFile1 = imageFile1 + fileNumber[0] + ".png";
	imageFile2 = imageFile2 + fileNumber[1] + ".png";
	imageFile3 = imageFile3 + fileNumber[2] + ".png";
	imageFile4 = imageFile4 + fileNumber[3] + ".png";
 
	System.out.println("image file1 " + imageFile1);
	System.out.println("image file2 " + imageFile2);
	System.out.println("image file3 " + imageFile3);
	System.out.println("image file4 " + imageFile4);
 
	card1Icon = new ImageIcon(imageFile1);
	card2Icon = new ImageIcon(imageFile2);
	card3Icon = new ImageIcon(imageFile3);
	card4Icon = new ImageIcon(imageFile4);
 
	jlblCard1 = new JLabel(card1Icon);
	jlblCard2 = new JLabel(card2Icon);
	jlblCard3 = new JLabel(card3Icon);
	jlblCard4 = new JLabel(card4Icon);
 
  }
 
  public boolean isPicked(int num) {
    for (int i = 0; i < numbersPicked.length; i++){
	  if (numbersPicked[i] == num)
	    return true;
	  else if (numbersPicked[i] == 0) {
	    numbersPicked[i] = num;
		return false;
	  }
	}
	numbersPicked[0] = num;
	return false;
  }
 
  public void clearData() {
    for (int i = 0; i < numbersPicked.length; i++)
      numbersPicked[i] = 0;
	imageFile1 = "image/card/";
    imageFile2 = "image/card/";
    imageFile3 = "image/card/";
    imageFile4 = "image/card/";
	jtfExpression.setText("");
 
  }
 
  public static void main(String[] args) {
 
   Exercise25_7 frame = new Exercise25_7();
 
   frame.setTitle("Exercise 25.7");
   frame.setLocationRelativeTo(null);
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   //frame.setSize(400, 300);
   frame.pack();
   frame.setVisible(true);
 
  }
}

Never mind. I fixed it. Thanks.