HTTP請求Android-嘗試調用虛擬方法

Question

所以我有一個問題。 我一直在嘗試使http請求工作一段時間，並且我創建了一個新線程，以便該請求永遠不會在主線程上運行。 我已經成功創建了HTTP請求，並且在請求線程中創建了HTTP請求，因此它記錄了它從http請求中獲取的數據。 現在，如果我想在方法之外使用此jsonObject，則會給我錯誤。 救命？

LogCat：

02-23 22:48:08.612    4239-4239/com.tamrefrank.www.listviewandroidstudio E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.tamrefrank.www.listviewandroidstudio, PID: 4239
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tamrefrank.www.listviewandroidstudio/com.tamrefrank.www.listviewandroidstudio.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String org.json.JSONArray.toString()' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String org.json.JSONArray.toString()' on a null object reference
            at com.tamrefrank.www.listviewandroidstudio.MainActivity.onCreate(MainActivity.java:38)
            at android.app.Activity.performCreate(Activity.java:5933)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
02-23 22:48:09.332    4239-4254/com.tamrefrank.www.listviewandroidstudio I/log_tag﹕ [{"Name":"Cabinet","Location":"In Storage","Latitude":"40.83990100","Longitude":"14.25185000","pk":19587,"image_id":"2006AJ6728"},{"Name":"Panel","Location":"Medieval and Renaissance, room 8, case WS EXP","Latitude":"48.87287900","Longitude":"4.63710000","pk":11371,"image_id":"2006AP3163"},{"Name":"Wallpaper","Location":"British Galleries, room 122f, case 1","Latitude":"53.47962200","Longitude":"-2.24881000","pk":19492,"image_id":"2006AU6265"},{"Name":"Portrait miniature","Location":"British Galleries, room 58c, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":9491,"image_id":"2006AH0271"},{"Name":"Caricature","Location":"In Storage","Latitude":"53.02513900","Longitude":"-2.18676000","pk":29979,"image_id":"2006AE9142"},{"Name":"Card game","Location":"In Storage","Latitude":"52.88328900","Longitude":"-1.97685000","pk":26483,"image_id":"2006AU5916"},{"Name":"Tile","Location":"Islamic Middle East, room 42, case WW9","Latitude":"39.76586200","Longitude":"64.42234000","pk":30220,"image_id":"2006AY5617"},{"Name":"Sculpture","Location":"Sculpture, room 22, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":36026,"image_id":"2006AM7780"},{"Name":"Oil painting","Location":"Prints & Drawings Study Room, room 315, case 24, shelf R","Latitude":"52.88328900","Longitude":"-1.97685000","pk":6905,"image_id":"2006AU6480"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10895,"image_id":"2006AU8783"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10902,"image_id":"2006AU9234"}]
02-23 22:48:09.332    4239-4254/com.tamrefrank.www.listviewandroidstudio I/log_tag﹕ [{"Name":"Cabinet","Location":"In Storage","Latitude":"40.83990100","Longitude":"14.25185000","pk":19587,"image_id":"2006AJ6728"},{"Name":"Panel","Location":"Medieval and Renaissance, room 8, case WS EXP","Latitude":"48.87287900","Longitude":"4.63710000","pk":11371,"image_id":"2006AP3163"},{"Name":"Wallpaper","Location":"British Galleries, room 122f, case 1","Latitude":"53.47962200","Longitude":"-2.24881000","pk":19492,"image_id":"2006AU6265"},{"Name":"Portrait miniature","Location":"British Galleries, room 58c, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":9491,"image_id":"2006AH0271"},{"Name":"Caricature","Location":"In Storage","Latitude":"53.02513900","Longitude":"-2.18676000","pk":29979,"image_id":"2006AE9142"},{"Name":"Card game","Location":"In Storage","Latitude":"52.88328900","Longitude":"-1.97685000","pk":26483,"image_id":"2006AU5916"},{"Name":"Tile","Location":"Islamic Middle East, room 42, case WW9","Latitude":"39.76586200","Longitude":"64.42234000","pk":30220,"image_id":"2006AY5617"},{"Name":"Sculpture","Location":"Sculpture, room 22, case 2","Latitude":"52.88328900","Longitude":"-1.97685000","pk":36026,"image_id":"2006AM7780"},{"Name":"Oil painting","Location":"Prints & Drawings Study Room, room 315, case 24, shelf R","Latitude":"52.88328900","Longitude":"-1.97685000","pk":6905,"image_id":"2006AU6480"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10895,"image_id":"2006AU8783"},{"Name":"Snuff bottle","Location":"In Storage","Latitude":"36.89445100","Longitude":"104.16564900","pk":10902,"image_id":"2006AU9234"}]
02-23 22:48:32.712    4239-4239/com.tamrefrank.www.listviewandroidstudio I/Process﹕ Sending signal. PID: 4239 SIG: 9

碼：

package com.tamrefrank.www.listviewandroidstudio;
//All the imports happen when you auto complete. Often you may have erros when you copy paste things directly because libraries may not get imported
import android.os.AsyncTask;
import android.os.StrictMode;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;
import java.io.InputStream;


public class MainActivity extends ActionBarActivity {
    private String [] drinks;
    JSONArray jsonObject;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //set the content view to activity_main.xml
        setContentView(R.layout.activity_main);

        JSONArray object = new JSONArray();
        object = getData();
        Log.i("log_tag", jsonObject.toString());

//        System.out.print("success: " + object.toString());


        drinks = new String[3];
        drinks[0] = "hi";
        drinks[1] = "5";
        drinks[2] = "3";
 /**
  * Lists require adapters-Adapters are the place that we store the various data items we want
  to appear in the Lists. This will be handled in an adapter.
  */

        ArrayAdapter<String> adapter;

//We will create the array adapter and reference it to our individual List item from the ListView and then to the array we just created- Drinks.
        adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,drinks);

//We finally will connect the adapter to the List View
        ListView drinkLists = (ListView) findViewById(R.id.listView);

        //Set the drinks array to the adapter below
        drinkLists.setAdapter(adapter);
    }

    public JSONArray getData(){
        new Thread(new Runnable() {
                public void run() {

                    HttpGet httpGet = new HttpGet("https://stampwatch.herokuapp.com/");
                    HttpClient client = new DefaultHttpClient();
                    HttpResponse response;
                    StringBuilder stringBuilder = new StringBuilder();

                    try {
                        response = client.execute(httpGet);
                        HttpEntity entity = response.getEntity();
                        InputStream stream = entity.getContent();
                        int b;
                        while ((b = stream.read()) != -1) {
                            stringBuilder.append((char) b);
                        }
                    } catch (ClientProtocolException e) {
                    } catch (IOException e) {
                        System.out.println(e);
                    }

                    try {
                        jsonObject = new JSONArray(stringBuilder.toString());
                        Log.i("log_tag", jsonObject.toString());
;
                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                    Log.i("log_tag", jsonObject.toString());
                    }

            }).start();
            return jsonObject;
        }


    @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();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }
         return super.onOptionsItemSelected(item);
    }
}

Answer 1

這里：

 Log.i("log_tag", jsonObject.toString());

NPE因為jsonObject為null並嘗試使用null對象為JSONObject調用toString()方法。

但是，如果我向該對象提出請求，為什么該對象為null？

因為jsonObject對象在getData();初始化getData(); 線程用於從api獲取數據的方法。 表示：

Log.i("log_tag", jsonObject.toString());

在啟動Thread之后立即執行的代碼行，而不是等到線程完成為止。