title: NullPointerException using java.util.Arrays.sort()
body: <p>This program reads the lines of an input file and stores them into the array words. Then each element in words[] is put into a character array and sorted alphabetically. Each sorted character array is assigned to a string and those strings populate another array sortedWords. I need to sort the elements in the sortedWords array. I get a NullPointerException when I use Arrays.sort(sortedWords).</p>
  
  <pre><code>public static void main(String[] args) throws FileNotFoundException {
      Scanner scanner = new Scanner(System.in);
      System.out.print("Enter a file name: ");
      System.out.flush();
  
      String filename = scanner.nextLine();
      File file = new File(filename);
  
      String[] words = new String[10];
      String[] sortedWords = new String[10];
  
      try {   
          FileReader fr = new FileReader(file);
          BufferedReader br = new BufferedReader(fr);
  
          String line = br.readLine();
          int i = 0;
  
          while(line != null) {
              words[i] = line.toString();    // assigns lines into the array
              line = br.readLine();    // this will eventually set line to null, terminating the loop
  
              String signature = words[i];                        
              char[] characters = signature.toCharArray();        
              Arrays.sort(characters);
              String sorted = new String(characters);               
  
              sortedWords[i] = sorted;    // assigns each signature into the array
              sortedWords[i] = sortedWords[i].replaceAll("[^a-zA-Z]", "").toLowerCase();    // removes non-alphabetic chars and makes lowercase
              Arrays.sort(sortedWords); 
  
              System.out.println(words[i] + " " + sortedWords[i]);
              i++;
          }     
      } 
  
      catch(IOException e) {
          System.out.println(e);
      }
  }
  </code></pre>