Having Trouble Using Insertion Sort with JSON

Question

public class ReverseList extends HttpServlet {

   public static void sort(int arr[]) {
      int N = arr.length;
      int i, j, temp;
      for (i = 1; i< N; i++) {
          j = i;
          temp = arr[i];
          while (j > 0 && temp < arr[j-1]) {
              arr[j] = arr[j-1];
              j = j-1;
          }
          arr[j] = temp;
      }
  }

  private void doService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
      BufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));
      String jsonStr = "";
      if(br != null){
          jsonStr = br.readLine();
      }
      StringReader strReader = new StringReader(jsonStr);
      JsonReader reader = Json.createReader(strReader);
      JsonObject obj = reader.readObject();
      JsonArray inArray = obj.getJsonArray("inList");
      int data [];
      for (int i = 0; i > inArray.size(); i++) {
         data = new int[inArray.getInt(i)];
      }
      long startTime = System.currentTimeMillis();
      this.sort(data);
      long stopTime = System.currentTimeMillis();
      long elapsedTime = stopTime - startTime;
      JsonArrayBuilder outArrayBuilder = Json.createArrayBuilder();
      for(int i = 0; i<data.length; i++) {
         outArrayBuilder.add(data[i]);
      }
      response.setContentType("application/json");
      PrintWriter out = response.getWriter();
      out.println("{ \"outList\" : " + outArrayBuilder.build().toString()+"\n");
      out.println("algorithim: Insertion Sort\n Execution time:"+ elapsedTime+"}");
  }
}

I am trying to convert the JSON Array into an int[], sort the int[], then output it as JSON with the time it took the sort to execute. The error I'm receiving is a NullPointerException.

 java.lang.NullPointerException   
 csi403.ReverseList.sort(ReverseList.java:23)   
 csi403.ReverseList.doService(ReverseList.java:99)   
 csi403.ReverseList.doPost(ReverseList.java:44) 

This is my first time using a servlet and I don't have an efficient way to make corrections and compile for errors. I am using a combination of Amazon Web Services- Elastic Beanstalk, Postman, and maven to create an HTTP endpoint. So, the question is twofold. Why am I running into this error? And, is there an easier way to compile this code and make corrections. I have heard about a tool called cURL but am unsure how it would be any different from what I am doing now. Any help is appreciated!

Answer 1

The way you create and populate your array before sorting may be near of:

int[] data = new int[inArray.size()];
for( int i = 0; i < inArray.size(); i++) {
   data[i] = inArray.getInt( i );
}

To sort an array, you may use java.util.Arrays.sort .

Arrays.sort( data, Collections.reverseOrder());

For the NPE: the condition i > inArray.size() is always false, so the array remains null, and in sort() arr.length; throw the NPE.