Arraylist of arraylist in method

Question

having a problem trying to get an arraylist containing arraylists to work in my program, Im getting a strange warning saying: add @SuppressWarnings "null" to processArray(), regardless of If I add this or not my program crashes, Is there anything obvious that Im doing wrong here? any help would go a long way thanks.

      private ArrayList<ArrayList<String>> processArray(ResponseList<Status> responses){

      ArrayList<ArrayList<String>> mainArray = null;
      ArrayList<String> innerArrays = null;

      for (Status response: responses ){
          String name, status, imgUrl, time;

          name = response.getUser().getName();
          status = response.getText();
          imgUrl = response.getUser().getProfileImageURL().toString();
          time = response.getCreatedAt().toString();

          ArrayList<String> rtLinks = checkLinks(response.getText());

          if(rtLinks != null){

              for (String tLink: rtLinks){

                  innerArrays.add(name);
                  innerArrays.add(status);
                  innerArrays.add(imgUrl);
                  innerArrays.add(time);
                  innerArrays.add(tLink);

                  mainArray.add(innerArrays);

              }

          }


      }
    return mainArray;

Answer 1

You never actually initialize either of those arraylists.

You want

ArrayList<ArrayList<String>> mainArray = new ArrayList<ArrayList<String>>(); // or new ArrayList<>() in java 7

and inside the inner for loop:

ArrayList<String> innerArrays = new ArrayList<String>(); // or new ArrayList<>() in java 7

Also, don't ever suppress warnings "just to make code work". Only suppress them when you know exactly why they appear and exactly why you're choosing to ignore them.

Answer 2

Your program is probably crashing due to a NullPointerException. This occurs when you try to access a variable whose value is null (known as dereferencing a null pointer). Before you access either of the two ArrayLists, you'll need to initialize it. Try changing the two lines to this:

  ArrayList<ArrayList<String>> mainArray = new ArrayList<ArrayList<String>>();
  ArrayList<String> innerArrays = new ArrayList<String>();

This should solve your immediate problem.

However, I'm not sure why you're using an innerArrays list at all. I would refactor the last bit to this:

for (String tLink: rtLinks){
    ArrayList<String> tempList = new ArrayList<String>();
    tempList.add(name);
    tempList.add(status);
    tempList.add(imgUrl);
    tempList.add(time);
    tempList.add(tLink);
    mainArray.add(tempList);
}

and remove the innerArrays variable. This way, the scope is more limited, and the code operates more logically.