Thread: NOOB: Array Help

After I ran DriverExamDemo, I got an error message in the command box:

HTML Code:

Exception in thread "main" java.lang.NullPointerException
        at DriverExam.<init>(DriverExam.java:16)
        at DriverExamDemo.main(DriverExamDemo.java:18)
Press any key to continue . . .

I don't know what I am doing wrong here.

public class DriverExamDemo
{
	public static void main(String[] args)
	{
		String[] answersGiven1 = {"B","D","B","A","C",
								"A","B","B","C","D",
								"B","C","C","A","D",
								"C","C","C","D","A"};
 
		String[] answersGiven2 = {"B","D","B","A","D",
								"A","B","B","C","D",
								"B","C","C","A","D",
								"E","C","C","D","A"};
 
		DriverExam myAttempt1 = new DriverExam(answersGiven1);
		DriverExam myAttempt2 = new DriverExam(answersGiven2);
 
		displayResults(myAttempt1,"Attempt 1:");
		displayResults(myAttempt2,"Attempt 2:");
	}
 
	public static void displayResults(DriverExam attempt, String title)
	{
		System.out.println("\n" + title);
		System.out.println("Passed? " + attempt.passed());
		System.out.println("# correct " + attempt.totalCorrect());
		System.out.println("# missed " + attempt.totalIncorrect());
 
		System.out.print("Questions missed: ");
 
		for (int missedQuestion : attempt.questionsMissed())
			System.out.printf("%2d ", missedQuestion);
 
		System.out.println("");
	}
}

public class DriverExam
{
	private String[] correctAnswers = {"B","D","B","A","C",
								"A","B","B","C","D",
								"B","C","C","A","D",
								"C","C","C","D","A"};
 
	private String[] studentAnswers;
 
 
	public DriverExam(String[] newAnswers)
	{
		for(int i = 0; i < correctAnswers.length; i++)
		studentAnswers[i] = newAnswers[i];
	}
 
	public boolean passed()
	{
		return (totalCorrect()>15);
	}
 
	public int totalCorrect()
	{
		int correctCount = 0;
		for (int i = 0; i < correctAnswers.length; i++)
		{
			if(correctAnswers[i].equals(studentAnswers[i]))
			correctCount++;
		}
		return correctCount;
	}
 
	public int totalIncorrect()
	{
		int totalMissed = 0;
		totalMissed = correctAnswers.length - totalCorrect();
		return totalMissed;
	}
 
	public int[] questionsMissed()
	{
		int[] missedQuestion = new int [totalIncorrect()];
 
		return missedQuestion;
	}
}

Oh, my bad.

private String[] studentAnswers = {"B","D","B","A","D",
								"A","B","B","C","D",
								"B","C","C","A","D",
								"E","C","C","D","A"};

I finally got it work. But the output is unexpected:

Attempt 1:
Passed? true
# correct 16
# missed 4
Questions missed: 0 0 0 0

Attempt 2:
Passed? false
# correct 14
# missed 6
Questions missed: 0 0 0 0 0 0
Press any key to continue . . .

The output is supposed to be like this:

Attempt 1:
Passed? true
# correct 16
# missed 4
Questions missed: 3 8 13 18

Attempt 2:
Passed? false
# correct 14
# missed 6
Questions missed: 3 5 8 13 16 18
Press any key to continue . . .


Any idea? I guess there's something wrong with the questionsMissed() part, but I don't see what I should fix.