Android（）上的Null指针异常

Question

I have a method that I use to recieve some information. I send to this method one string (the server direction), and two strings that contains the user and the password, and I recieve an String[] with all the information, but I have a NullPointerException at

values = request("http://myIp/XXX/post.php?request=1", user, password);

line. The complete code is the next:

String[] values;

@SuppressLint("InlinedApi")
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_USER_LANDSCAPE);
    setContentView(R.layout.activity_select_environment);   

    myBundle = getIntent().getExtras();

    user = myBundle.getString("user");
    password = myBundle.getString("password");

    values = request("http://myIp/XXX/post.php?request=1", user, password);  

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_expandable_list_item_1, android.R.id.text1, values);
}

The method is the next:

@SuppressWarnings("null")
public String[] request(String requesturl, String user, String password)
{
    String result[] = null; 

    try
    {
        HttpClient httpclient = new DefaultHttpClient(); 

        HttpPost httppost = new HttpPost(requesturl);

        String results = "";

        //Varibles POST
        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
        nameValuePairs.add(new BasicNameValuePair("user", user));
        nameValuePairs.add(new BasicNameValuePair("password", password));                        

        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

        //Execute HTTP Post Request
        HttpResponse response = httpclient.execute(httppost);
        response.getAllHeaders();
        response.getEntity();
        result[0] = "";                                                                         
        if(response.getStatusLine().getStatusCode() == HttpStatus.SC_OK)
        {
            results = EntityUtils.toString(response.getEntity());           
            if(!results.equals("-1"))
            {
                JSONObject jsonResponse = new JSONObject(results);

                JSONArray jsonMainNode = jsonResponse.optJSONArray("edificios");

                int lengthJsonArr =jsonMainNode.length();
                for(int i = 0; i < lengthJsonArr; i++)
                {
                    JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);   
                    result[i] = jsonChildNode.optString("nombre"); 
                }
            }                 
        }

        return result;
    }
    catch(Exception ex)
    {
        result[0] = "error";
        return result;
    }
}  

I recieved a NullPointerException on the "values=request("....");" line and I don't know why, because user and password have values and the other value is a string... What could be do?

Answer 1

You have declared your result String array but did not initialize it.

Try this:

@SuppressWarnings("null")
public String[] request(String requesturl, String user, String password)
{
    String result[] = new String[ <Insert value> ];
    // Code...
}

Let me explain further to avoid downvotes. This is why I think this:

You first initialize the array to null, so it can't be used yet. Further down the line you have this statement String results = "" but your array is still null . Then you do this result[0] = "" , oh dear. That is where the nullPointerException comes from, I think.

You have to initialize your array and you are right if you say that you need to know the size of your array up front. Use the maximum value you expect for the size or, better yet, use a list and not an array .

Here is a nice source for the list datastructure: List in Android Development .

There is also a nice discussion about list versus arrays here , it is for C# but the principles remain more or less the same.

Answer 2

You got the exception because you are trying to access the first element of the result , which is not even initialized.

So you need to initialize the array first and then use it

result = new String[<some size>];

If size of the array is not know in advance, then ArrayList can be a option for you.

ArrayList<String> result = new ArrayList<>();

usage

result.add(<some string value>);