AsncTask在android中的致命错误

Question

我正在尝试构建一个应用程序，它以json数据的形式从webservices获取数据，处理它并在应用程序屏幕上显示选择性数据。但是在android studio 1.5.1上进行模拟时我得到了错误。

代码

<android.support.v4.widget.NestedScrollView 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:src="@drawable/logoprefeitura"/>


</android.support.v4.widget.NestedScrollView>

错误列表

package com.example.mrugesh.myjweats2;

import android.content.Context;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.view.inputmethod.InputMethodManager;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

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

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLEncoder;

public class MainActivity extends AppCompatActivity {
    EditText cityName;
    TextView resultTextView;

    public void findWeather(View view) {
        Log.i("cityName", cityName.getText().toString());
        InputMethodManager mgr = (InputMethodManager)
        getSystemService(Context.INPUT_METHOD_SERVICE);
        mgr.hideSoftInputFromWindow(cityName.getWindowToken(), 0);

        try {
            String encodedCityName = URLEncoder.encode(cityName.getText().toString(), "UTF-8");
            DownloadTask task = new DownloadTask();
            task.execute("http://api.openweathermap.org/data/2.5/weather?q=" + encodedCityName );
        }

        catch (UnsupportedEncodingException e) {
            e.printStackTrace();
            Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        cityName = (EditText) findViewById(R.id.cityName);
        resultTextView = (TextView) findViewById(R.id.resultTextView);
    }

    public class DownloadTask extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... urls) {

            String result = "";
            URL url;
            HttpURLConnection urlConnection = null;

            try {
                url = new URL(urls[0]);
                urlConnection = (HttpURLConnection) url.openConnection();

                InputStream in = urlConnection.getInputStream();
                InputStreamReader reader = new InputStreamReader(in);

                int data = reader.read();

                while (data != -1) {
                    char current = (char) data;
                    result += current;
                    data = reader.read();
                }

                return result;
            } catch (Exception e) {
                Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
            }

            return null;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);

            try {

                String message =" ";

                JSONObject jsonObject = new JSONObject(result);
                String weatherInfo = jsonObject.getString("weather");
                Log.i("Weather content", weatherInfo);

                JSONArray arr = new JSONArray(weatherInfo);
                for (int i = 0; i < arr.length(); i++) {
                    JSONObject jsonPart = arr.getJSONObject(i);

                    String main = "";
                    String description = "";

                    main = jsonPart.getString("main");
                    description = jsonPart.getString("description");

                    if (main != "" && description != "") {
                        message += main + ": " + description + "\r\n";
                    }
                }

                if (message != "") {
                    resultTextView.setText(message);
                } else {
                    Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
                }
            } catch (JSONException e) {
                Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
            }
        }
    }

    @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);
    }
}