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;

Learn to read errors, there is usually a name for the exception, and also a line number. — Hidde
– Hidde, Commented May 18, 2012 at 22:24

trutheality · Accepted Answer · 2012-05-18 22:19:54Z

5

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.

answered May 18, 2012 at 22:19

trutheality

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1 Comment

Edmund Rojas Over a year ago

wow I knew it was something totally stupid! thanks for the help!!

wchargin · Accepted Answer · 2012-05-18 22:26:17Z

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.

Hidde · Accepted Answer · 2012-05-18 22:23:52Z

1

You are trying to add the innerArrays variable into your main ArrayList, but you have declared your main ArrayList null in the beginning of the method. Initialize the main ArrayList as a new ArrayList of ArrayLists of Strings, and your code should work.

answered May 18, 2012 at 22:23

Hidde

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