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android - 使用默認的ArrayAdapter構(gòu)建ListView時拋出異常NullPointerException
天蓬老師
天蓬老師 2017-04-17 14:54:14
[Android討論組]

本人Android新手在看第一行代碼，最后一個大程序的時候出現(xiàn)了NullPointerException，希望大家能夠幫忙解決一下，謝謝

代碼如下：

ChooseAreaActivity.java

package com.example.asuka.coolweather.activity;

import android.app.Activity;
import android.app.ProgressDialog;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import com.example.asuka.coolweather.R;
import com.example.asuka.coolweather.db.CoolWeatherDB;
import com.example.asuka.coolweather.model.City;
import com.example.asuka.coolweather.model.County;
import com.example.asuka.coolweather.model.Province;
import com.example.asuka.coolweather.util.HttpCallbackListener;
import com.example.asuka.coolweather.util.HttpUtil;
import com.example.asuka.coolweather.util.Utility;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by asuka on 15-10-7.
 */
public class ChooseAreaActivity extends Activity {

    public static final int LEVEL_PROVINCE = 0;

    public static final int LEVEL_CITY = 1;

    public static final int LEVEL_COUNTY = 2;

    private ProgressDialog progressDialog;
    private TextView titleText;
    private ListView listView;
    private ArrayAdapter<String> adapter;
    private CoolWeatherDB coolWeatherDB;
    private List<String> dataList = new ArrayList<String>();

    /**
     * 省列表
     */
    private List<Province> provinceList;

    /**
     * 市列表
     */
    private List<City> cityList;

    /**
     * 縣列表
     */
    private List<County> countyList;

    /**
     * 選中的省份
     */
    private Province selectedProvince;

    /**
     * 選中的城市
     */
    private City selectedCity;

    /**
     * 當前選中的級別
     */
    private int currentLevel;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.choose_area);
        listView = (ListView) findViewById(R.id.list_view);
        titleText = (TextView) findViewById(R.id.title_text);
        adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, dataList);
        listView.setAdapter(adapter);
        coolWeatherDB = CoolWeatherDB.getInstance(this);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?>arg0, View view, int index, long arg3) {
                if (currentLevel == LEVEL_PROVINCE) {
                    selectedProvince = provinceList.get(index);
                    queryCities();
                } else if (currentLevel == LEVEL_CITY) {
                    selectedCity = cityList.get(index);
                    queryCounties();
                }
            }
        });
        queryProvinces();
    }

    /**
     * 查詢?nèi)珖械氖。瑑?yōu)先從數(shù)據(jù)庫查詢，如果沒有查詢到再去服務(wù)器上查詢
     */
    private void queryProvinces() {
        provinceList = coolWeatherDB.loadProvinces();
        if (provinceList.size() > 0) {
            dataList.clear();
            for (Province province : provinceList) {
                dataList.add(province.getProvinceName());
            }
            adapter.notifyDataSetChanged();
            listView.setSelection(0);
            titleText.setText("China");
            currentLevel = LEVEL_PROVINCE;
        } else {
            queryFromServer(null, "province");
        }
    }

    /**
     * 查詢選中省所內(nèi)所有的市，優(yōu)先從數(shù)據(jù)庫查詢，如果沒有查詢到再去服務(wù)器上查詢
     */
    private void queryCities() {
        cityList = coolWeatherDB.loadCities(selectedProvince.getId());
        if (cityList.size() > 0) {
            dataList.clear();
            for (City city : cityList) {
                dataList.add(city.getCityName());
            }
            adapter.notifyDataSetChanged();
            listView.setSelection(0);
            titleText.setText(selectedProvince.getProvinceName());
            currentLevel = LEVEL_CITY;
        } else {
            queryFromServer(selectedProvince.getProvinceCode(), "city");
        }
    }

    /**
     * 查詢選中市內(nèi)所有的縣，有先從數(shù)據(jù)庫查詢，如果沒有查詢到再去服務(wù)器上查詢
     */
    private void queryCounties() {
        countyList = coolWeatherDB.loadCounties(selectedCity.getId());
        if (countyList.size() > 0) {
            dataList.clear();
            for (County county : countyList) {
                dataList.add(county.getCountyName());
            }
            adapter.notifyDataSetChanged();
            listView.setSelection(0);
            titleText.setText(selectedCity.getCityName());
            currentLevel = LEVEL_COUNTY;
        } else {
            queryFromServer(selectedCity.getCityCode(), "county");
        }
    }

    /**
     * 根據(jù)傳入的代號和類型服務(wù)器上查詢省市縣數(shù)據(jù)
     */
    private void queryFromServer(final String code, final String type) {
        String address;
        if (!TextUtils.isEmpty(code)) {
            address = "http://www.weather.com.cn/data/list3/city" + code + ".xml";
        } else {
            address = "http://www.weather.com.cn/data/list3/city.xml";
        }
        showProgressDialog();
        HttpUtil.sendHttpRequest(address, new HttpCallbackListener() {
            @Override
            public void onFinish(String response) {
                boolean result = false;
                if ("province".equals(type)) {
                    result = Utility.handleProvinceResponse(coolWeatherDB, response);
                } else if ("city".equals(type)) {
                    result = Utility.handleCitiesResponse(coolWeatherDB, response, selectedCity.getId());
                } else if ("county".equals(type)) {
                    result = Utility.handleCountiesResponse(coolWeatherDB, response, selectedCity.getId());
                }
                if (result) {
                    //通過runOnUiThread方法回到主線程邏輯處理
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                           closeProgressDialog();
                            if ("province".equals(type)) {
                                queryProvinces();
                            } else if ("city".equals(type)) {
                                queryCities();
                            }
                        }
                    });
                }

            }

            @Override
            public void onError(Exception e) {
                //通過runOnUiThread()方法回到主線程處理邏輯
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        closeProgressDialog();
                        Toast.makeText(ChooseAreaActivity.this, "加載失敗", Toast.LENGTH_SHORT).show();
                    }
                });
            }
        });
    }

    /**
     * 顯示進度對話框
     */
    private void showProgressDialog() {
        if (progressDialog == null) {
            progressDialog = new ProgressDialog(this);
            progressDialog.setMessage("正在加載..");
            progressDialog.setCanceledOnTouchOutside(false);
        }
        progressDialog.show();
    }

    /**
     * 關(guān)閉進度對話框
     */
    private void closeProgressDialog() {
        if (progressDialog != null) {
            progressDialog.dismiss();
        }
    }

    /**
     * 捕獲Back按鍵，根據(jù)當前的級別來判斷，此時應(yīng)該返回市列表、省列表、還是直接退出
     */
    @Override
    public void onBackPressed() {
        if (currentLevel == LEVEL_COUNTY) {
            queryCities();
        } else if (currentLevel == LEVEL_CITY) {
            queryProvinces();
        } else {
            finish();
        }
    }
}

choose_area.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="#484E61">

        <TextView
            android:id="@+id/title_text"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerInParent="true"
            android:textColor="#fff"
            android:textSize="24sp"/>
    </RelativeLayout>

    <ListView
        android:id="@+id/list_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </ListView>

</LinearLayout>

logcat提示的錯誤信息

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
            at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:401)
            at android.widget.ArrayAdapter.getView(ArrayAdapter.java:369)
            at android.widget.AbsListView.obtainView(AbsListView.java:2346)
            at android.widget.ListView.makeAndAddView(ListView.java:1875)
            at android.widget.ListView.fillSpecific(ListView.java:1354)
            at android.widget.ListView.layoutChildren(ListView.java:1674)
            at android.widget.AbsListView.onLayout(AbsListView.java:2148)
            at android.view.View.layout(View.java:16630)
            at android.view.ViewGroup.layout(ViewGroup.java:5437)
            at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1743)
            at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1586)
            at android.widget.LinearLayout.onLayout(LinearLayout.java:1495)
            at android.view.View.layout(View.java:16630)
            at android.view.ViewGroup.layout(ViewGroup.java:5437)
            at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
            at android.view.View.layout(View.java:16630)
            at android.view.ViewGroup.layout(ViewGroup.java:5437)
            at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1743)
            at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1586)
            at android.widget.LinearLayout.onLayout(LinearLayout.java:1495)
            at android.view.View.layout(View.java:16630)
            at android.view.ViewGroup.layout(ViewGroup.java:5437)
            at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
            at com.android.internal.policy.PhoneWindow$DecorView.onLayout(PhoneWindow.java:2678)
            at android.view.View.layout(View.java:16630)
            at android.view.ViewGroup.layout(ViewGroup.java:5437)
            at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2171)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1931)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
            at android.view.Choreographer.doCallbacks(Choreographer.java:670)
            at android.view.Choreographer.doFrame(Choreographer.java:606)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:148)
            at android.app.ActivityThread.main(ActivityThread.java:5417)
            at java.lang.reflect.Method.invoke(Native Method)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
天蓬老師
天蓬老師

歡迎選擇我的課程，讓我們一起見證您的進步~~

全部回復(fù)(4)
伊謝爾倫

錯誤提示信息告訴你問題出在這：

at android.widget.ArrayAdapter.getView(ArrayAdapter.java:369)

你需要了解默認的getView方法的實現(xiàn)：

在adapter的構(gòu)造方法中指定的布局(android.R.layout.simple_list_item_1)是Android SDK 提供的預(yù)定義布局資源。該布局擁有一個TextView根元素。
而默認的ArrayAdapter.getView(...)實現(xiàn)方法依賴于toString()方法。它首先生成布局視圖,然后找到指定位置的對象并對其調(diào)用toString()方法,最后得到字符串信息并傳遞給TextView。
《Android權(quán)威編程指南 P157，158》

所以，我猜問題應(yīng)該是你傳給Adapter的dataList包含null對象。
你應(yīng)該著重檢查更改adapter數(shù)據(jù)的代碼，比如：

dataList.add(); // 確定添加的不是null嗎？
adapter.notifyDataSetChanged();
天蓬老師

我也遇到同樣的問題，樓上的說的很對，按照樓上說的解決了

伊謝爾倫

手動還原了一下你出現(xiàn)的情況，

 List<String> list = new ArrayList<String>();
        String a = null;
        list.add(a);

打印出的錯誤日志和你出現(xiàn)的問題是一樣的，也就是說dataList里面出現(xiàn)的null值
應(yīng)該是后面的數(shù)據(jù)庫和服務(wù)器拉取數(shù)據(jù)時有null值被加進去了，建議加一個數(shù)據(jù)檢測
判斷一下取回的數(shù)據(jù)是否為null值，如果是的話賦一個缺省值或者"".

迷茫

我也出現(xiàn)一樣的問題。我的具體情況是，可以選擇省，選擇市，到縣的時候閃退。錯誤原因在于API放回的關(guān)于縣的數(shù)據(jù)中解析不對，一行代碼寫錯了。應(yīng)該是county.setCountyName(array[1]);寫成了county.setCountyCode(array[1]);。

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