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對於Android和Java我還是很新的。正在學習Android Listviews，並通過修改本教程中的代碼來實現此目的：http://w2davids.wordpress.com/android-listview-with-iconsimages-and-sharks-with-lasers/使用Android列表視圖時拋出異常java.lang.NullPointerException
但是，當進行stockName.setText(stock.name)時，我遇到了拋出java.lang.NullPointerException的問題。
這裏是我的代碼：
Stocks.java
package sg.jon.stocks;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.ResponseHandler;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.BasicResponseHandler;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.app.Activity;
import android.content.Context;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
public class Stocks extends Activity {
static final String STOCK_API_URL = "<removed>";
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
new UpdateStockData().execute();
}
private class UpdateStockData extends AsyncTask<Void, Integer, JSONObject>{
protected JSONObject doInBackground(Void... params) {
Log.v("stocks", "Starting HTTP request");
HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet(STOCK_API_URL);
String response = null;
try {
ResponseHandler<String> responseHandler=new BasicResponseHandler();
response = httpclient.execute(httpget, responseHandler);
} catch (ClientProtocolException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Log.v("stocks", "Finished HTTP request");
Log.v("stocks", "Starting to parse JSON");
JSONObject jsonResult = null;
try {
jsonResult = new JSONObject(response.replace("{}&& ",""));
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Log.v("stocks", "JSON parsing complete");
return jsonResult;
}
protected void onPostExecute(JSONObject jsonResult){
String[] stockNames = null;
String[] stockPrices = null;
List<Stock> stocks = null;
String label = null;
try {
// label => As at dd-mm-yyyy h:mm AM/PM
label = jsonResult.getString("label");
JSONArray stocksJSONArray = jsonResult.getJSONArray("items");
stockNames = new String[stocksJSONArray.length()];
stockPrices = new String[stocksJSONArray.length()];
stocks = new ArrayList<Stock>();
for(int i=0; i<stocksJSONArray.length(); i++){
Stock stock = new Stock();
stock.name = stocksJSONArray.getJSONObject(i).getString("N");
stock.price = stocksJSONArray.getJSONObject(i).getString("LT");
stocks.add(stock);
stockNames[i] = stocksJSONArray.getJSONObject(i).getString("N");
stockPrices[i] = stocksJSONArray.getJSONObject(i).getString("LT");
}
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Toast.makeText(Stocks.this, label, Toast.LENGTH_LONG).show();
ListView stocksListView = (ListView) findViewById(R.id.stocksListView);
stocksListView.setAdapter(new StockArrayAdapter(
getApplicationContext(), R.layout.list_row, stocks)
);
}
}
public class Stock
{
public String name;
public String price;
public Stock()
{
// TODO Auto-generated constructor stub
}
public Stock(String name, String price)
{
this.name = name;
this.price = price;
}
@Override
public String toString()
{
return this.name;
}
}
public class StockArrayAdapter extends ArrayAdapter<Stock> {
private Context context;
private TextView stockName;
private TextView stockPrice;
private List<Stock> stocks = new ArrayList<Stock>();
public StockArrayAdapter(Context context, int textViewResourceId,
List<Stock> objects) {
super(context, textViewResourceId, objects);
this.context = context;
this.stocks = objects;
}
public int getCount() {
return this.stocks.size();
}
public Stock getItem(int index) {
return this.stocks.get(index);
}
public View getView(int position, View convertView, ViewGroup parent) {
View row = convertView;
if (row == null) {
// ROW INFLATION
Log.d("xml", "Starting XML Row Inflation ... ");
LayoutInflater inflater = (LayoutInflater) this.getContext()
.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
row = inflater.inflate(R.layout.list_item, parent, false);
Log.d("xml", "Successfully completed XML Row Inflation!");
}
// Get item
Stock stock = getItem(position);
Log.v("stk",stock.name);
Log.v("stk",stock.price);
stockName = (TextView) row.findViewById(R.id.stockName);
stockPrice = (TextView) row.findViewById(R.id.stockPrice);
stockName.setText(stock.name);
stockPrice.setText(stock.price);
return row;
}
}
}
main.xml中
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<ListView
android:id="@+id/stocksListView"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
</ListView>
</LinearLayout>
list_item.xml
<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:padding="10dp"
android:textSize="16sp"
android:textColor="#fff">
</TextView>
list_row.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<TextView
android:id="@+id/stockName"
android:paddingLeft="10dip"
android:layout_weight="0.5"
android:layout_gravity="center"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/stockPrice"
android:layout_gravity="right"
android:paddingRight="10dip"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</LinearLayout>
這是在日誌中顯示的錯誤：
04-30 12:07:16.835: W/dalvikvm(1657): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
04-30 12:07:16.883: E/AndroidRuntime(1657): FATAL EXCEPTION: main
04-30 12:07:16.883: E/AndroidRuntime(1657): java.lang.NullPointerException
04-30 12:07:16.883: E/AndroidRuntime(1657): at sg.jon.stocks.Stocks$StockArrayAdapter.getView(Stocks.java:191)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.AbsListView.obtainView(AbsListView.java:2033)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.ListView.measureHeightOfChildren(ListView.java:1244)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.ListView.onMeasure(ListView.java:1155)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.View.measure(View.java:12723)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1369)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.LinearLayout.measureVertical(LinearLayout.java:660)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.LinearLayout.onMeasure(LinearLayout.java:553)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.View.measure(View.java:12723)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.View.measure(View.java:12723)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.LinearLayout.measureVertical(LinearLayout.java:812)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.LinearLayout.onMeasure(LinearLayout.java:553)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.View.measure(View.java:12723)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
04-30 12:07:16.883: E/AndroidRuntime(1657): at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2092)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.View.measure(View.java:12723)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1064)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2442)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.os.Handler.dispatchMessage(Handler.java:99)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.os.Looper.loop(Looper.java:137)
04-30 12:07:16.883: E/AndroidRuntime(1657): at android.app.ActivityThread.main(ActivityThread.java:4424)
04-30 12:07:16.883: E/AndroidRuntime(1657): at java.lang.reflect.Method.invokeNative(Native Method)
04-30 12:07:16.883: E/AndroidRuntime(1657): at java.lang.reflect.Method.invoke(Method.java:511)
04-30 12:07:16.883: E/AndroidRuntime(1657): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
04-30 12:07:16.883: E/AndroidRuntime(1657): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
04-30 12:07:16.883: E/AndroidRuntime(1657): at dalvik.system.NativeStart.main(Native Method)
感謝（：
什麼是Stocks.java的第191行？ – MikeIsrael