Question

虽然这个论坛已经多次询问过这个问题。我读过它们但至少没有一个能为我工作。我差不多2天了，但无法弄清楚如何初始化线性布局和逃避NPE。我没有使用任何片段，并且想要在活动中初始化布局管理器。

下面是我的代码的重要部分，其余的代码与适配器和xml相同，所以不要写在这里....但是如果需要它，我也会提供它。

public class MainNews extends AppCompatActivity {

    public RecyclerViewAdapter recyclerViewAdapter;
    Toolbar toolbar;
    private String[] inputStream;
    public RecyclerView recyclerView;
    //ArrayList<Data> dataList;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_news);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        recyclerView = (RecyclerView) findViewById(R.id.recyclerView);
        String newsMain = "http://microblogging.wingnity.com/JSONParsingTutorial/jsonActors";
        new JSONAsync(getApplicationContext()).execute(newsMain);

        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        RecyclerViewAdapter recyclerViewAdapter = new RecyclerViewAdapter(this, JSONAsync.dataArray);
        recyclerView.setAdapter(recyclerViewAdapter);
        recyclerView.setItemAnimator(new DefaultItemAnimator());


    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        return super.onOptionsItemSelected(item);
    }

    /////////////////////////////////////////////////////


    public static class JSONAsync extends AsyncTask<String, String, Boolean> {

        Context context;
        public static List<Data> dataArray;

        public JSONAsync(Context context) {
            this.context = context;
        }

        @Override
        protected Boolean doInBackground(String... params) {
            try {
                return downloadUrl(params[0]);
            } catch (IOException e) {
                return false;
            } catch (JSONException e) {
                e.printStackTrace();
            }

            return null;
        }

       public boolean downloadUrl(String myurl) throws IOException,JSONException {
            InputStream is = null;
            int response;

            try {
                URL url = new URL(myurl);
                HttpURLConnection conn = (HttpURLConnection)url.openConnection();
                conn.setRequestMethod("GET");
                conn.setDoInput(true);
                // Starts the query
                conn.connect();
                response = conn.getResponseCode();
                is = conn.getInputStream();
                if (response == 200) {
                    String responseBody = convertStreamToString(conn.getInputStream());
                    JSONArray jArray = new JSONArray(responseBody);
                    //  JSONArray jArray = new JSONArray(is);
                    for (int i = 0; i < jArray.length(); i++) {
                        JSONObject jobj = jArray.getJSONObject(i);
                        Data data = new Data();
                        data.setId(jobj.getInt("id"));
                        data.setHeading(jobj.getString("heading"));
                        data.setBrief(jobj.getString("brief"));
                        data.setDate(jobj.getString("date"));
                        dataArray.add(data);
                    }
                    return true;
                } else return false;

            } finally {
                if (is != null) {
                    is.close();
                }
            }

        }

        public String convertStreamToString(InputStream is) {
            BufferedReader reader = new BufferedReader(new InputStreamReader(is));
            StringBuilder sb = new StringBuilder();
            String line = null;
            try {
                while ((line = reader.readLine()) != null) {
                    sb.append(line).append("\n");
                }
            } catch (IOException e) {
            } finally {
                try {
                    is.close();
                } catch (IOException e) {
                }
            }

            return sb.toString();
        }

        @Override
        protected void onPostExecute(Boolean result) {

            if (result == true) {
                Toast.makeText(context, "Data Fetched, now do some work with it AH", Toast.LENGTH_LONG).show();
            } else
                Toast.makeText(context, "Unable to fetch data from server", Toast.LENGTH_LONG).show();

        }
       }
       }

这是logcat。

FATAL EXCEPTION: AsyncTask #1
    Process: admin.myproject, PID: 11032
    java.lang.RuntimeException: An error occurred while executing doInBackground()
    at android.os.AsyncTask$3.done(AsyncTask.java:309)
    at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
    at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
    at java.util.concurrent.FutureTask.run(FutureTask.java:242)
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
    at java.lang.Thread.run(Thread.java:818)
    Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference
    at admin.myproject.MainNews$JSONAsync.downloadUrl(MainNews.java:128)
    at admin.myproject.MainNews$JSONAsync.doInBackground(MainNews.java:90)
    at admin.myproject.MainNews$JSONAsync.doInBackground(MainNews.java:78)
    at android.os.AsyncTask$2.call(AsyncTask.java:295)
    at java.util.concurrent.FutureTask.run(FutureTask.java:237)
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
    at java.lang.Thread.run(Thread.java:818) enter code here

P.S：我已经使JSONAsync类静态/非静态，其他的东西都被改变但是没有用。

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Answer 1

dataArray未初始化。将其更改为

public static List<Data> dataArray = new Arraylist();

Answer 2

您尝试添加的数组列表从未初始化。在添加任何值之前，必须先将其初始化。

试试这个

public static List<Data> dataArray = new Arraylist();

或者你也可以这样做

List<Data> dataArray;

public boolean downloadUrl(String myurl) throws IOException,JSONException {
    InputStream is = null;
    int response;
    dataArray = new List<Data>();

    try {
        URL url = new URL(myurl);
        HttpURLConnection conn = (HttpURLConnection)url.openConnection();
        conn.setRequestMethod("GET");
        conn.setDoInput(true);
        // Starts the query
        conn.connect();
        response = conn.getResponseCode();
        is = conn.getInputStream();
        if (response == 200) {
            String responseBody = convertStreamToString(conn.getInputStream());
            JSONArray jArray = new JSONArray(responseBody);
            //  JSONArray jArray = new JSONArray(is);
            for (int i = 0; i < jArray.length(); i++) {
                JSONObject jobj = jArray.getJSONObject(i);
                Data data = new Data();
                data.setId(jobj.getInt("id"));
                data.setHeading(jobj.getString("heading"));
                data.setBrief(jobj.getString("brief"));
                data.setDate(jobj.getString("date"));
                dataArray.add(data);
            }
            return true;
        } else return false;

    } finally {
        if (is != null) {
            is.close();
        }
    }

循环器查看器线性布局管理器与空指针异常

2 个答案: