Question

我有课 LoadJSONTask 。在本课程中，我收到以下错误。我的应用程序正在运行但是当我尝试在代码中添加进度条时它无法正常工作。

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: in.kalakaaristudios.json.listviewmenu, PID: 3809
              java.lang.NullPointerException: Attempt to invoke interface method 'void in.kalakaaristudios.json.listviewmenu.LoadJSONTask$Listener.onLoaded(java.util.List)' on a null object reference
                  at in.kalakaaristudios.json.listviewmenu.LoadJSONTask$override.onPostExecute(LoadJSONTask.java:83)
                  at in.kalakaaristudios.json.listviewmenu.LoadJSONTask$override.access$dispatch(LoadJSONTask.java)
                  at in.kalakaaristudios.json.listviewmenu.LoadJSONTask.onPostExecute(LoadJSONTask.java:0)
                  at in.kalakaaristudios.json.listviewmenu.LoadJSONTask.onPostExecute(LoadJSONTask.java:21)
                  at android.os.AsyncTask.finish(AsyncTask.java:651)
                  at android.os.AsyncTask.access$500(AsyncTask.java:180)
                  at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:5441)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)

LoadJSONTask类

package in.kalakaaristudios.json.listviewmenu;

import android.os.AsyncTask;
import android.view.View;
import android.widget.ProgressBar;
import com.google.gson.Gson;
import com.google.gson.JsonSyntaxException;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.List;

public class LoadJSONTask extends AsyncTask<String, Void, Response>  {

private MainActivity activity;
private ProgressBar dwBar;

public LoadJSONTask(MainActivity activity) {
    this.activity = activity;
     dwBar = (ProgressBar) activity.findViewById(R.id.progress_bar);
}

public LoadJSONTask(Listener listener) {
    mListener = listener;
}

public interface Listener {
    void onLoaded(List<AndroidVersion> androidList);
    void onError();
}

private Listener mListener;

@Override
protected Response doInBackground(String... strings) {
    try {

        String stringResponse = loadJSON(strings[0]);
        Gson gson = new Gson();

        return gson.fromJson(stringResponse, Response.class);
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    } catch (JsonSyntaxException e) {
        e.printStackTrace();
        return null;
    }
}


@Override
protected void onPreExecute() {
    super.onPreExecute();
    MainActivity mn = new MainActivity();
    dwBar.setVisibility(View.VISIBLE);
}


@Override
protected void onPostExecute(Response response) {

    dwBar.setVisibility(View.GONE);

    if (response != null) {

        mListener.onLoaded(response.getAndroid());//getting error here

    } else {

        mListener.onError();
    }



}

private String loadJSON(String jsonURL) throws IOException {

    URL url = new URL(jsonURL);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setReadTimeout(10000);
    conn.setConnectTimeout(15000);
    conn.setRequestMethod("GET");
    conn.setDoInput(true);
    conn.connect();

    BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
    String line;
    StringBuilder response = new StringBuilder();

    while ((line = in.readLine()) != null) {

        response.append(line);
    }

    in.close();
    return response.toString();
}
}

MainActivity.java

package in.kalakaaristudios.json.listviewmenu;

import android.os.AsyncTask;
import android.view.View;
import android.widget.ProgressBar;

import com.google.gson.Gson;
import com.google.gson.JsonSyntaxException;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.List;

public class LoadJSONTask extends AsyncTask<String, Void, Response>  {

private MainActivity activity;
private ProgressBar dwBar;

public LoadJSONTask(MainActivity activity) {
    this.activity = activity;
     dwBar = (ProgressBar) activity.findViewById(R.id.progress_bar);
}


public LoadJSONTask(Listener listener) {
    mListener = listener;
}

public interface Listener {
    void onLoaded(List<AndroidVersion> androidList);
    void onError();
}

private Listener mListener;

@Override
protected Response doInBackground(String... strings) {
    try {

        String stringResponse = loadJSON(strings[0]);
        Gson gson = new Gson();

        return gson.fromJson(stringResponse, Response.class);
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    } catch (JsonSyntaxException e) {
        e.printStackTrace();
        return null;
    }
}


@Override
protected void onPreExecute() {
    super.onPreExecute();
    MainActivity mn = new MainActivity();
    dwBar.setVisibility(View.VISIBLE);
}


@Override
protected void onPostExecute(Response response) {

    dwBar.setVisibility(View.GONE);

    if (response != null) {

        mListener.onLoaded(response.getAndroid());

    } else {

        mListener.onError();
    }
}

private String loadJSON(String jsonURL) throws IOException {

    URL url = new URL(jsonURL);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setReadTimeout(10000);
    conn.setConnectTimeout(15000);
    conn.setRequestMethod("GET");
    conn.setDoInput(true);
    conn.connect();

    BufferedReader in = new BufferedReader(new        InputStreamReader(conn.getInputStream()));
    String line;
    StringBuilder response = new StringBuilder();

    while ((line = in.readLine()) != null) {

        response.append(line);
    }

    in.close();
    return response.toString();
}
}

Answer 1

我认为您的问题就在这一行

dwBar = (ProgressBar) activity.findViewById(R.id.progress_bar);

因为您正在使用活动类实例来获取对ProgressBar视图窗口小部件的引用

您应该对包含ProgressBar小部件的布局进行扩充，并使用find来获取ProgressBar的实例

View view = inflater.inflate(R.Layout_that_contains_progress_bar, null);
dwBar = (ProgressBar) activity.findViewById(R.id.progress_bar);

或者，您可以通过编程方式创建ProgressBar，如下所示

dwBar = new ProgressBar(activity);

尝试在空对象引用上调用接口方法

1 个答案: