Question

问题是我打电话给Initialize()

public void Initialize(){
        question.setText(test.get("question").get(index));
        answer_1.setText(test.get("answer_1").get(index));
        answer_2.setText(test.get("answer_2").get(index));
        answer_3.setText(test.get("answer_3").get(index));
        answer_4.setText(test.get("answer_4").get(index));
        answer_5.setText(test.get("answer_5").get(index));
        correct = test.get("answer_5").get(index);
        index++;
    }

在Sql_bridge asyncTask

@Override
    protected void onPostExecute(HashMap<String,List<String>> stringList) {
       if(!stringList.isEmpty()) {
            test test = new test();
            test.setTest(stringList);
            test.Initialize();
        }
        else
            Log.i("test","empty");
    }

问题是test.setTest(stringList)工作正常这是完整的代码 test.java

package com.example.littledev.gson;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.HashMap;
import java.util.List;

public class test extends AppCompatActivity {

    private HashMap<String, List<String>> test;
    TextView question;
    Button answer_1, answer_2, answer_3, answer_4, answer_5;
    String correct;
    int index;
    int score;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test);
        index = 0;
        score = 0;
        correct ="";
        question = (TextView) findViewById(R.id.question);
        answer_1 = (Button) findViewById(R.id.answer_1);
        answer_2 = (Button) findViewById(R.id.answer_2);
        answer_3 = (Button) findViewById(R.id.answer_3);
        answer_4 = (Button) findViewById(R.id.answer_4);
        answer_5 = (Button) findViewById(R.id.answer_5);
        Sql_bridge bridge = new Sql_bridge();
        bridge.execute("english");

    }

    public void onAnswer(View view){

        if (index == test.get("question").size()) {

            if (view.getTag().equals(correct))
                score++;
            Initialize();
        }
        else {
            //startActivity(new Intent(test.this, Result.class));
            Result result = new Result();
            result.setScore(100*score/5);
        }

    }
    public void Initialize(){
        question.setText(test.get("question").get(index));
        answer_1.setText(test.get("answer_1").get(index));
        answer_2.setText(test.get("answer_2").get(index));
        answer_3.setText(test.get("answer_3").get(index));
        answer_4.setText(test.get("answer_4").get(index));
        answer_5.setText(test.get("answer_5").get(index));
        correct = test.get("answer_5").get(index);
        index++;
    }
    public void setTest(HashMap<String, List<String>> map){
        test = map;
    }
}

Sql_bridge.java

package com.example.littledev.gson;

import android.os.AsyncTask;
import android.util.Log;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;


class Sql_bridge extends AsyncTask<String, Void, HashMap<String,List<String>>> {


    @Override
    protected HashMap<String,List<String>> doInBackground(String... params) {

        String reg_url = "http://e-shops.hol.es/app_test";
        String response = "";
        List<String> questions = new ArrayList<>();
        List<String> answer_1 = new ArrayList<>();
        List<String> answer_2 = new ArrayList<>();
        List<String> answer_3 = new ArrayList<>();
        List<String> answer_4 = new ArrayList<>();
        List<String> answer_5 = new ArrayList<>();
        List<String> correct = new ArrayList<>();
            try {
                URL url = new URL(reg_url);
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                httpURLConnection.setRequestMethod("POST");
                httpURLConnection.setDoOutput(true);
                httpURLConnection.setDoInput(true);
                OutputStream outputStream = httpURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String data = URLEncoder.encode("test", "UTF-8") + "=" + URLEncoder.encode(params[0], "UTF-8");
                bufferedWriter.write(data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();
                InputStream inputStream = httpURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String line;
                while ((line = bufferedReader.readLine()) != null) {
                    response += line;
                }
                bufferedReader.close();
                inputStream.close();
                httpURLConnection.disconnect();
            } catch (IOException e) {
                e.printStackTrace();
            }
        JSONObject jsonResponse;
        try {
            jsonResponse = new JSONObject(response);
            JSONArray jsonMainNode = jsonResponse.optJSONArray("result");
            int lengthJsonArr = jsonMainNode.length();
            Log.i("test",lengthJsonArr + " length");
            for(int i=0; i < lengthJsonArr; i++)
            {
                JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);
                questions.add(jsonChildNode.optString("question"));
                answer_1.add(jsonChildNode.optString("answer_1"));
                answer_2.add(jsonChildNode.optString("answer_2"));
                answer_3.add(jsonChildNode.optString("answer_3"));
                answer_4.add(jsonChildNode.optString("answer_4"));
                answer_5.add(jsonChildNode.optString("answer_5"));
                correct.add(jsonChildNode.optString("correct"));

            }

        } catch (JSONException e) {
            e.printStackTrace();
        }

        HashMap<String, List<String>> test = new HashMap<>();
        test.put("question",questions);
        test.put("answer_1",answer_1);
        test.put("answer_2",answer_2);
        test.put("answer_3",answer_3);
        test.put("answer_4",answer_4);
        test.put("answer_5",answer_5);
        test.put("correct",correct);
        return test;
    }


    @Override
    protected void onPostExecute(HashMap<String,List<String>> stringList) {
        if(!stringList.isEmpty()) {
            test test = new test();
            test.setTest(stringList);
            test.Initialize();
        }
        else
            Log.i("test","empty");
    }

}

logcat的

04-09 20:15:38.832 10470-10470/com.example.littledev.gson E/AndroidRuntime: FATAL EXCEPTION: main
                                                                            Process: com.example.littledev.gson, PID: 10470
                                                                            java.lang.NullPointerException
                                                                                at com.example.littledev.gson.test.Initialize(test.java:57)
                                                                                at com.example.littledev.gson.Sql_bridge.onPostExecute(Sql_bridge.java:104)
                                                                                at com.example.littledev.gson.Sql_bridge.onPostExecute(Sql_bridge.java:25)
                                                                                at android.os.AsyncTask.finish(AsyncTask.java:632)
                                                                                at android.os.AsyncTask.access$600(AsyncTask.java:177)
                                                                                at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                at android.os.Looper.loop(Looper.java:136)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5433)
                                                                                at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                at java.lang.reflect.Method.invoke(Method.java:515)
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)
                                                                                at dalvik.system.NativeStart.main(Native Method)

Answer 1

您正在尝试使用构造函数创建Activity，这是不允许的。如果你继续这样做，你将永远得到NullPointerException，因为你的Activity将不会被创建，因此对Initialize方法内的视图的引用为null。请看一下how to start an Activity

您应该做的是创建Intent，将Activity中所需的字符串放在Intent内，然后使用Activity启动context.startActivity(intent) }

Answer 2

另一种可能性是为您的AsyncTask提供对您的活动的回调。

public class Sql_bridge extends AsyncTask<String, Void, HashMap<String,List<String>>>
{
    public interface SqlBridgeListener
    {
        /**
         * Called when your query is completed.
         * @param stringList Result list.
         */
        void onQueryCompleted(HashMap<String,List<String>> stringList);
    }

    /**
     * Store listener as a weak reference to not leak the resource.
     */
    private WeakReference<SqlBridgeListener> mListener;

    public SqlBridge(SqlBridgeListener listener)
    {
        mListener = new WeakReference<SqlBridgeListener>(listener);
    }

    @Override
    protected HashMap<String, List<String>> doInBackground(String... strings)
    {
        // Same code as your post, not shown for brevity.
    }

    @Override
    protected void onPostExecute(HashMap<String,List<String>> stringList) {
        if(!stringList.isEmpty())
        {
            SqlBridgeListener listener = mListener.get();
            if(listener != null)
            {
                // Notify listener with results.
                listener.onQueryCompleted(stringList);
            }
        }

    }
}

然后关于你的活动：

public class test extends AppCompatActivity implements Sql_bridge.SqlBridgeListener {

private HashMap<String, List<String>> test;
TextView question;
Button answer_1, answer_2, answer_3, answer_4, answer_5;
String correct;
int index;
int score;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_test);
    index = 0;
    score = 0;
    correct ="";
    question = (TextView) findViewById(R.id.question);
    answer_1 = (Button) findViewById(R.id.answer_1);
    answer_2 = (Button) findViewById(R.id.answer_2);
    answer_3 = (Button) findViewById(R.id.answer_3);
    answer_4 = (Button) findViewById(R.id.answer_4);
    answer_5 = (Button) findViewById(R.id.answer_5);
    // Provide activity as a listener.
    Sql_bridge bridge = new Sql_bridge(this);
    bridge.execute("english");

}

@Override
public void onQueryCompleted(HashMap<String,List<String>> stringList)
{
  // Update your views with the result.
}
// Rest of the activity not shown for brevity.

Answer 3

感谢@quiro我做到了。我将上下文作为参数传递

private Context ctx;
    Sql_bridge(Context ctx) {
        this.ctx = ctx;
    }

然后使用

if(ctx instanceof Test){
                ((Test) ctx).Initialize();
            }

感谢您的帮助。

在AsyncTask Android中调用方法时，应用程序崩溃

3 个答案: