title: null pointer exception string builder
body: <p>I am trying to use the setCharAt method in a StringBuilder but I am getting a null pointer exception.  Is there a way I can add values to the StringBuilder array I have made so I wont get these error.  </p>
  
  <p>From research I have found the .append() method but I'm not even sure how it works.</p>
  
  <pre><code>import java.util.*;  // Allows for the input of a scanner method.
  import java.io.*;    // Allows for the inputting and outputting of a data file.
  import java.lang.*;  // Allows for the use of String Methods.
  //////////////////////////////////////////////////////////////////////////////////////
  
  public class TESTY
  {
  static Scanner testanswers;
  static PrintWriter testresults;
  
  public static void main(String[] args) throws IOException
  {
      testanswers = new Scanner(new FileReader("TestInput.dat"));
      testresults = new PrintWriter("TestOutput.dat");
  
      String StudentID;
      String answers;
      // Reads first two lines first to know how many records there are.
      String answerKey = testanswers.nextLine();
      int count = Integer.parseInt(testanswers.nextLine());
  
      // Allocate the array for the size needed.
      String[][] answerArray = new String[count][];
  
      for (int i = 0; i &lt; count; i++)
      {
          String line = testanswers.nextLine();
          answerArray[i] = line.split(" ", 2);
      }
  
      for(int row = 0; row &lt; answerArray.length; row++)
      {
          for(int col = 0; col &lt; answerArray[row].length; col++)
          {
              System.out.print(answerArray[row][col] + " ");
          }
          System.out.println();
      }   
      gradeData(answerArray, answerKey);
  
  
  
  
      testanswers.close();
      testresults.close();
  
  }
  ///////////////////////////////////////////////////////////
  //Method: gradeData
  //Description: This method will grade testanswers showing 
  //what was missed, skipped, letter grade, and percentage.
  ///////////////////////////////////////////////////////////
  public static double gradeData(String[][] answerArray, String answerKey)
  {   
  
      String key = answerKey;
      double Points = 0;
      StringBuilder[] wrongAnswers = new StringBuilder[5];
      String studAnswers;
  
      for(int rowIndex = 0; rowIndex &lt; answerArray.length; rowIndex++)   /// Counting rows
      {
          studAnswers = answerArray[rowIndex][1].replace(" ", "S");  ///Counts rows, Col stay static index 1
          for(int charIndex = 0; charIndex &lt; studAnswers.length(); charIndex++) 
          {
              if(studAnswers.charAt(charIndex) == key.charAt(charIndex))
              {
                  Points += 2;
              }
              else if(studAnswers.charAt(charIndex) == 'S')
              {
                  Points --;
              }
              else if(studAnswers.charAt(charIndex) != key.charAt(charIndex))
              {
                  for(int i = 0; i &lt; wrongAnswers.length; i++)
                  {
  
                      wrongAnswers[i].setCharAt(charIndex, 'X');
                  }
                  Points -= 2;                        
              }
  
          }
          System.out.println(Points);
      }
  
      return Points;
  
  
  }
  </code></pre>
  
  <p>}</p>
  
  <p>The error is occurring on line 91 :</p>
  
  <pre><code>wrongAnswers[i].setCharAt(charIndex, 'X');
  </code></pre>