Question

我想使用数组适配器在ListView中显示数据。当我运行程序时，我得到了NullPointerException。错误发生在myarray.add（contactid +＆＃34; - ＆＃34;）;

完整错误是：

java.lang.NullPointerException：尝试调用虚方法＆＃39; boolean java.util.ArrayList.add（java.lang.Object）＆＃39;在com.android.jsonparsingtest2.MainActivity $ JSONTask.doInBackground（MainActivity.java:82）上的空对象引用在com.android.jsonparsingtest2.MainActivity $ JSONTask.doInBackground（MainActivity.java:44）

MainActivity.java

public class MainActivity extends ActionBarActivity {

private ListView tvData;

private ArrayAdapter arrayAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button btnHit = (Button) findViewById(R.id.btnHit);

    btnHit.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            new JSONTask().execute("http://a.nextput.com/offers/19/o/a0f252064bd27339607c7166d270d82a/f?aff_id=56556_18&im=25");
        }
    });
}

public class JSONTask extends AsyncTask<String, String, ArrayList<String>> {
    ArrayList<String> myarray;
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected ArrayList<String> doInBackground(String... params) {
        HttpURLConnection connection = null;
        BufferedReader reader = null;
        try {
            URL url = new URL(params[0]);
            connection = (HttpURLConnection) url.openConnection();
            connection.connect();

            InputStream stream = connection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(stream));
            StringBuffer buffer = new StringBuffer();

            String line = "";
            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }

            String finalJson = buffer.toString();
            JSONObject parentObject = new JSONObject(finalJson);
            JSONArray parentArray = parentObject.getJSONArray("offers");

            //ArrayList<String> data = new ArrayList<>();
            StringBuffer finalBufferData = new StringBuffer();
            for (int i = 0; i < parentArray.length(); i++) {
                JSONObject finalObject = parentArray.getJSONObject(i);
                String contactid = finalObject.getString("app_name");
                //String custname = finalObject.getString("name");
                //String custemail = finalObject.getString("email");
                //String custgender = finalObject.getString("gender");
                //String mobileno = finalObject.getString("mobile");
                myarray.add(contactid+ " - ");
            }
            return myarray;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                connection.disconnect();
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onPostExecute(ArrayList<String> result) {
        super.onPostExecute(result);
        // tvData.setText(result);
        arrayAdapter = new ArrayAdapter(MainActivity.this, android.R.layout.simple_list_item_1, result);
        tvData = (ListView) findViewById(R.id.tvJsonItem);
        tvData.setAdapter(arrayAdapter);
    }
}

}

Answer 1

ArrayList<String> myarray;

是不够的（仍为空）。

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

应该有用。

Answer 2

您尚未初始化myarray。这样的事情会起作用：

ArrayList<String> myarray;
@Override
protected void onPreExecute() {
    super.onPreExecute();
    myarray = new ArrayList<>();
}

使用ArrayAdapter

2 个答案: