好的我想尝试创建progressDialog但是当我运行我的代码时会出现NullPointerException。现在在良好的旧logcat的帮助下，我可以确认我的对话框为空但是我无法弄清楚为什么会这样。继承我的代码
public class asyncTask extends AsyncTask<String, Integer, String> {
ProgressDialog dialog;
@Override
protected void onPreExecute() {
dialog = new ProgressDialog(PetrolPriceActivity.this);
dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
dialog.setMax(100);
dialog.show();
}
@Override
protected String doInBackground(String...parmans){
{
for(int i = 0; i < 20; i++){
Log.v(TAG, "index=" + "loop");
publishProgress(5);
Log.v(TAG, "index=" + dialog);
}
String urlString = petrolPriceURL;
String result = "";
InputStream anInStream = null;
int response = -1;
URL url = null;
try {
url = new URL(urlString);
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
return null;
}
URLConnection conn = null;
try {
conn = url.openConnection();
} catch (IOException e) {
// TODO Auto-generated catch block
return null;
}
// Check that the connection can be opened
if (!(conn instanceof HttpURLConnection))
try {
throw new IOException("Not an HTTP connection");
} catch (IOException e) {
// TODO Auto-generated catch block
return null;
}
try
{
// Open connection
HttpURLConnection httpConn = (HttpURLConnection) conn;
httpConn.setAllowUserInteraction(false);
httpConn.setInstanceFollowRedirects(true);
httpConn.setRequestMethod("GET");
httpConn.connect();
response = httpConn.getResponseCode();
// Check that connection is OK
if (response == HttpURLConnection.HTTP_OK)
{
// Connection is OK so open a reader
anInStream = httpConn.getInputStream();
InputStreamReader in= new InputStreamReader(anInStream);
BufferedReader bin= new BufferedReader(in);
// Read in the data from the RSS stream
String line = new String();
while (( (line = bin.readLine())) != null)
{
result = result + "\n" + line;
}
}
}
catch (IOException ex)
{
try {
throw new IOException("Error connecting");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return result;
}
}
@Override
protected void onProgressUpdate(Integer...progress){
dialog.incrementProgressBy(progress[0]);
}
@Override
protected void onPostExecute(String result) {
// Get the data from the RSS stream as a string
dialog.dismiss();
response = (TextView)findViewById(R.id.error);
response = (TextView)findViewById(R.id.title);
try
{
// Get the data from the RSS stream as a string
result = doInBackground(petrolPriceURL);
response.setText(result);
Log.v(TAG, "index=" + result);
}
catch(Exception ae)
{
// Handle error
response.setText("Error");
// Add error info to log for diagnostics
errorText.setText(ae.toString());
}
}
当publishProgress（5）;发生我的logcat告诉我变量对话框之后仍然为null所以我意识到我的错误必须与onPreExecute和对话框的初始化有关，或者我在onPostUpdate中增加了一些问题但这似乎不太可能。这也是我的logcat，它说明了我的问题，并显示我的对话框为空。
08-13 15:35:01.976: V/AreaURL(1676): index=http://www.petrolprices.com/feeds/averages.xml?search_type=&search_value=
08-13 15:35:03.136: V/PetrolPrice(1676): index=loop
08-13 15:35:03.136: V/PetrolPrice(1676): index=null
08-13 15:35:03.136: V/PetrolPrice(1676): index=loop
08-13 15:35:03.146: V/PetrolPrice(1676): index=null
08-13 15:35:03.146: V/PetrolPrice(1676): index=loop
08-13 15:35:03.146: V/PetrolPrice(1676): index=null
08-13 15:35:03.146: V/PetrolPrice(1676): index=loop
08-13 15:35:03.146: V/PetrolPrice(1676): index=null
08-13 15:35:03.146: V/PetrolPrice(1676): index=loop
08-13 15:35:03.146: V/PetrolPrice(1676): index=null
08-13 15:35:03.146: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.156: V/PetrolPrice(1676): index=loop
08-13 15:35:03.156: V/PetrolPrice(1676): index=null
08-13 15:35:03.286: D/AndroidRuntime(1676): Shutting down VM
08-13 15:35:03.286: W/dalvikvm(1676): threadid=1: thread exiting with uncaught exception (group=0xb3a82ba8)
08-13 15:35:03.326: E/AndroidRuntime(1676): FATAL EXCEPTION: main
08-13 15:35:03.326: E/AndroidRuntime(1676): Process: org.me.myandroidstuff, PID: 1676
08-13 15:35:03.326: E/AndroidRuntime(1676): java.lang.NullPointerException
08-13 15:35:03.326: E/AndroidRuntime(1676): at org.me.myandroidstuff.PetrolPriceActivity$asyncTask.onProgressUpdate(PetrolPriceActivity.java:140)
08-13 15:35:03.326: E/AndroidRuntime(1676): at org.me.myandroidstuff.PetrolPriceActivity$asyncTask.onProgressUpdate(PetrolPriceActivity.java:1)
08-13 15:35:03.326: E/AndroidRuntime(1676): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:648)
08-13 15:35:03.326: E/AndroidRuntime(1676): at android.os.Handler.dispatchMessage(Handler.java:102)
08-13 15:35:03.326: E/AndroidRuntime(1676): at android.os.Looper.loop(Looper.java:136)
08-13 15:35:03.326: E/AndroidRuntime(1676): at android.app.ActivityThread.main(ActivityThread.java:5017)
08-13 15:35:03.326: E/AndroidRuntime(1676): at java.lang.reflect.Method.invokeNative(Native Method)
08-13 15:35:03.326: E/AndroidRuntime(1676): at java.lang.reflect.Method.invoke(Method.java:515)
08-13 15:35:03.326: E/AndroidRuntime(1676): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
08-13 15:35:03.326: E/AndroidRuntime(1676): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
08-13 15:35:03.326: E/AndroidRuntime(1676): at dalvik.system.NativeStart.main(Native Method)
感谢您的帮助
答案 0 :(得分：1)
如果你在onPreExecute（）之外实例化你的对话框会发生什么。像那样：
ProgressDialog dialog = new ProgressDialog(PetrolPriceActivity.this);
我不知道为什么会这样。可能是你没有进入onPreExecute x）添加日志以查看那个:)