连接到外部数据库Android由：java.lang.NullPointerException

Question

I'm in the process of building my first android app and my goal of this app is to connect to an external database. The syntax below is giving me a java.lang.NullPointerException error message. Below is all of my syntax and logcat information. What am I doing wrong?

MainActivity

package com.example.connecttoexternaldatabase;

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

import android.support.v7.app.ActionBarActivity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

private TextView responseTextView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    responseTextView = (TextView) findViewById(R.id.responseTextView);

    new GetAllCustomerTask().execute(new ApiConnector());
}

public void setTextToTextView(JSONArray jsonArray){
    String theTextToDisplayOnPhone = "";
    for(int i=0; i<jsonArray.length(); i++){
        JSONObject json = null;
        try{
            json = jsonArray.getJSONObject(i);
            theTextToDisplayOnPhone = theTextToDisplayOnPhone +
                    "Name : " +json.getString("FirstName")+" "+ json.get("LastName")+"\n"+
                    "Age : "+json.getInt("Age")+"\n"+
                    "Mobile Using : "+json.getString("Mobile")+"\n\n";
        } catch (JSONException e){
            e.printStackTrace();
        }
    }

    responseTextView.setText(theTextToDisplayOnPhone);
}

private class GetAllCustomerTask extends AsyncTask<ApiConnector, Long, JSONArray>{

    @Override
    protected JSONArray doInBackground(ApiConnector... params) {
        // is it executed on the background thread
        return params[0].GetAllCustomers();
    }

    @Override
    protected void onPostExecute(JSONArray jsonArray){
        // it is executed on the main thread
        setTextToTextView(jsonArray);
    }

}

}

ApiConnector

package com.example.connecttoexternaldatabase;

import java.io.IOException;
import java.net.HttpURLConnection;

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

import android.util.Log;

public class ApiConnector {

    public JSONArray GetAllCustomers(){
        String url = "http://localhost/app/getAllCustomers.php";

        HttpEntity httpEntity = null;

        try{
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpGet httpGet = new HttpGet(url);

            HttpResponse httpResponse = httpClient.execute(httpGet);
            httpEntity = httpResponse.getEntity();

        } catch (ClientProtocolException e){
            e.printStackTrace();
        } catch (IOException e){
            e.printStackTrace();
        }

        JSONArray jsonArray = null;

        if(httpEntity != null){
            try{
                String entityResponse = EntityUtils.toString(httpEntity);
                Log.e("Entity Response: ", entityResponse);
                jsonArray = new JSONArray(entityResponse);
            } catch (JSONException e){
                e.printStackTrace();
            } catch (IOException e){
                e.printStackTrace();
            }
        }

        return jsonArray;
    }
}

Logcat Information

01-03 13:59:19.599: E/AndroidRuntime(762): FATAL EXCEPTION: main
01-03 13:59:19.599: E/AndroidRuntime(762): java.lang.NullPointerException
01-03 13:59:19.599: E/AndroidRuntime(762):  at com.example.connecttoexternaldatabase.MainActivity.setTextToTextView(MainActivity.java:31)
01-03 13:59:19.599: E/AndroidRuntime(762):  at com.example.connecttoexternaldatabase.MainActivity$GetAllCustomerTask.onPostExecute(MainActivity.java:58)
01-03 13:59:19.599: E/AndroidRuntime(762):  at com.example.connecttoexternaldatabase.MainActivity$GetAllCustomerTask.onPostExecute(MainActivity.java:1)
01-03 13:59:19.599: E/AndroidRuntime(762):  at android.os.AsyncTask.finish(AsyncTask.java:417)
01-03 13:59:19.599: E/AndroidRuntime(762):  at android.os.AsyncTask.access$300(AsyncTask.java:127)
01-03 13:59:19.599: E/AndroidRuntime(762):  at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)
01-03 13:59:19.599: E/AndroidRuntime(762):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-03 13:59:19.599: E/AndroidRuntime(762):  at android.os.Looper.loop(Looper.java:130)
01-03 13:59:19.599: E/AndroidRuntime(762):  at android.app.ActivityThread.main(ActivityThread.java:3683)
01-03 13:59:19.599: E/AndroidRuntime(762):  at java.lang.reflect.Method.invokeNative(Native Method)
01-03 13:59:19.599: E/AndroidRuntime(762):  at java.lang.reflect.Method.invoke(Method.java:507)
01-03 13:59:19.599: E/AndroidRuntime(762):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:870)
01-03 13:59:19.599: E/AndroidRuntime(762):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)
01-03 13:59:19.599: E/AndroidRuntime(762):  at dalvik.system.NativeStart.main(Native Method)

Answer 1

You are setting JSONObject to null in your loop:

JSONObject json = null;

But I don't see anywhere where you create your JSONObject. You should have this before your loop:

JSONObject json = new JSONObject();