Null pointer exception trouble

[Jason Rod]

I am working on a program that will grade tests and return the test scores, the average, highest, and lowest scores, and will sort the scores from highest to lowest. The program allows the user to input answers for only one of the students, the rest of the student answers are hard-coded. I am having trouble with a null pointer exception on the line that says:

char studAns[][] = new char[][]

I am also unsure about the sorting method that I used, I have been having problems trying to find a simple sorting algorithm that will allow the data in the array to maintain it's same index number while being sorted.

here is the code

  
 
import javax.swing.JOptionPane;
 
public class ScoreCalculator {
 
String userInput;
char userAnswer;
char answer[];
int i, j, k;
int correct[];
 
public void studentAnswers() {
for (int k = 1; k < 11; k++)
userInput = JOptionPane.showInputDialog
("Student answer for question " + k + ": ");
userAnswer = userInput.charAt(0);
userAnswer = Character.toUpperCase(userAnswer);
answer[k] = userAnswer;
}
 
char studAns[][] = new char[][]
{
        {answer[k]},
        {'C', 'B', 'C', 'D', 'A', 'A', 'C', 'D', 'B', 'C'},
        {'C', 'A', 'C', 'D', 'A', 'A', 'C', 'D', 'A', 'C'},   
                        {'A', 'C', 'C', 'D', 'A', 'A', 'B', 'D', 'D', 'C'},
        {'C', 'B', 'B', 'A', 'B', 'B', 'D', 'D', 'B', 'D'},
{'D', 'B', 'C', 'D', 'A', 'A', 'C', 'B', 'B', 'C'},
{'C', 'B', 'A', 'D', 'D', 'C', 'C', 'D', 'C', 'A'},
{'C', 'B', 'C', 'B', 'A', 'A', 'A', 'D', 'B', 'C'}
};
 
char[] answerKey = new char[]   
                        {'C', 'B', 'C', 'D', 'A', 'A', 'C', 'D', 'B', 'C'};
 
int score = 0;
public void gradeTest() {
for (int i = 0; i < 8; i++) {
for (int j = 0; j < 10; j++) {
while (studAns[i][j] == answerKey[j]) {
score++;
}
}
correct[i] = score;
score = 0;
}
}
 
public double getAvg() {
double avg = (correct[0] + correct[1] + correct[2] +                                                                    
                              correct[3] + correct[4] + correct[5] +
      correct[6] + correct[7] + correct[8] +    
                              correct[9]) / 10;
return avg;
}
 
public void printAvg() {
JOptionPane.showMessageDialog(null, "Average score was: " + getAvg());
}
 
public int getHighest() {
int highest = correct[0];
for (int i = 1; i < 8; i++) {
if (correct[i] > highest)
highest = correct[i];
}
return highest;
}
 
public void printHighest() {
JOptionPane.showMessageDialog(null, "Highest score was: " + getHighest());
}
 
public int getLowest() {
int lowest = correct[0];
for (int i = 1; i < 8; i++) {
if (correct[i] < lowest)
lowest = correct[i];
}
return lowest;
}
 
public void printLowest() {
JOptionPane.showMessageDialog(null, "Lowest score was: " + getLowest());
}
 
public int sortScores() {
for (int i = 0; i < 7; i++)
if (correct[i] > correct[i+1]) {
int temp = correct[i+1];
correct[i+1] = correct[i];
correct[i] = temp;
}
return correct[i];
}
 
}
 

[ May 17, 2006: Message edited by: Jason Rod ]

[ May 17, 2006: Message edited by: Jason Rod ]

[ May 17, 2006: Message edited by: Jason Rod ]
[ May 17, 2006: Message edited by: Jason Rod ]

[Keith Lynn]

Remember that an array reference is null until you instantiate it.
[ May 17, 2006: Message edited by: Keith Lynn ]

[Jason Rod]

Sorry, I'm new to using arrays, how would I instantiate it in this case?

[Garrett Rowe]

Jason Rod : Sorry, I'm new to using arrays, how would I instantiate it in this case?

Something like this:

final int ANSWER_ARRAY_LENGTH = 10;
answer = new char[ANSWER_ARRAY_LENGTH];

Also remember that arrays in Java are 0 based, so an array with length = 10 will have indices 0 - 9.

[Jason Rod]

oh ok, got it, thanks so much

[Srinivas Kalvala]

Please dont forget to post the entery point and order of the calls, next time. The program behavior depends on the order of the method calls.

Thank you.