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我想添加一個初始屏幕到我的Android Rss閱讀器應用程序，以便我可以檢查網絡連接並根據連接性確定用戶首先看到哪個屏幕。初始屏幕導致主活動中的空指針與AsyncTask
沒有SplashScreen，MainActivity.java可以正常工作，但是當我將SplashScreenActivity設置爲Manifest中的啓動程序時，MainActivity中的Asynctask在運行時會引發空指針。有人知道這可能是什麼原因嗎？
謝謝你的時間！
SplashScreenActivity.java
enter code here
public class SplashScreenActivity extends MainActivity {
boolean alreadyExecuted = false;
private static String TAG = SplashScreenActivity.class.getName();
private static long SLEEP_TIME = 5;
@Override
public void onCreate(Bundle savedInstanceState) {
this.requestWindowFeature(Window.FEATURE_NO_TITLE); // Removes title bar
this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN); // Removes notification bar
super.onCreate(savedInstanceState);
setContentView(R.layout.splash_screen);
// Start timer and launch main activity
IntentLauncher launcher = new IntentLauncher();
launcher.start();
}
private class IntentLauncher extends Thread {
@Override
/**
* Sleep for some time and than start new activity.
*/
public void run() {
try {
// Sleeping
Thread.sleep(SLEEP_TIME*2000);
} catch (Exception e) {
Log.e(TAG, e.getMessage());
}
//If this method is called once, set boolean alreadyExecuted to true
if(!alreadyExecuted)
{
Check();
alreadyExecuted=true;
}
}
}
//Check if network is available. If so go to MainActivity screen, which requires internet
//If not, go to MyFeeds screen
public void Check()
{
if (NetworkCheck.getInstance(this).isOnline()) {
Intent intent = new Intent(SplashScreenActivity.this, MainActivity.class);
SplashScreenActivity.this.startActivity(intent);
SplashScreenActivity.this.finish();
}
else
{
Intent intent = new Intent(SplashScreenActivity.this, MyFeedsScreen.class);
SplashScreenActivity.this.startActivity(intent);
SplashScreenActivity.this.finish();
}
}}
MainActivity.java
public class MainActivity extends Activity {
private MainActivity local;
private DatabaseHandler db;
//Method to create main application view
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//set view
requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.main);
//**************************create a button to move to the user's saved feeds screen*****************************
Button myFeedsButton = (Button) findViewById(R.id.myFeedsButton);
myFeedsButton.setOnClickListener(new View.OnClickListener() {
public void onClick(View v)
{
startActivity(new Intent(MainActivity.this, MyFeedsScreen.class));
}
});
//*****************************************************************************************************************
//Create new instance of database handler
db = new DatabaseHandler(this);
//set local ref to this activity
local = this;
//Calls the doInBackground method
GetRSSDataTask task = new GetRSSDataTask();
//start download Rss task - execute method calls the
task.execute("http://pitchfork.com/rss/reviews/best/albums/");
//debug thread name
Log.d("RssReaderApp", Thread.currentThread().getName());
}
//*******************************************************************************************************************
private class GetRSSDataTask extends AsyncTask<String, Void, List<RssItem>>
{
@Override
protected List<RssItem> doInBackground(String... urls) {
//debug task thread name
Log.d("RssReaderApp", Thread.currentThread().getName());
try {
//create a new reader
RssReader rssReader = new RssReader(urls[0]);
//Parse RSS, get items
return rssReader.getItems();
} catch (Exception e) {
Log.e("RssReaderApp", e.getMessage());
}
return null;
}//doInBackground
//is invoked on UI thread after background tasks are complete.
// Results of background task are passed here as a parameter
@Override
protected void onPostExecute(List<RssItem> result)
{
//Gets listview from main.xml
final ListView listItems = (ListView) findViewById(R.id.listMainView);
//Creates a new list adapter - displays an array of strings in listview
ArrayAdapter<RssItem> adapter = new ArrayAdapter<RssItem>(local, android.R.layout.simple_list_item_1, result);
//Set list adapter for listView
listItems.setAdapter(adapter);
//OnItemClick listener set to allow user to access content from title
listItems.setOnItemClickListener(new ListListener(result, local));
//*******************************LONG CLICK FUNCTIONALITY******************************************
//Set new long click listener which should allow item to be stored to db
listItems.setLongClickable(true);
listItems.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
try {
db.open();
RssItem fromList = (RssItem) listItems.getItemAtPosition(position);
RssItem item = new RssItem();
item.title = fromList.title;
item._id = fromList._id;
//item.completeTextLink = fromList.completeTextLink;
//item.mainBody = fromList.mainBody;
item.link = fromList.link;
item.description = fromList.description;
// item.page = fromList.page;
db.insertRssItem(item);
db.close();
} catch (SQLException e) {
e.printStackTrace();
}
Toast.makeText(getBaseContext(), "Item saved in My Feeds!", Toast.LENGTH_SHORT).show();
return true;
}
});
}//onPostExecute
}//getRssTaskClass}//class
Android清單，與SplashScreenActvity，而不是現在MainActivity設置爲啓動：
<?xml version="1.0" encoding="utf-8"?>
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application
android:label="@string/app_name">
<activity android:name="com.example.rory.RssReaderApp.MainActivity">
</activity>
<activity android:name="com.example.rory.RssReaderApp.MyFeedsScreen"></activity>
<activity android:name=".DetailsActivity"></activity>
<activity android:name="com.example.rory.RssReaderApp.SplashScreenActivity">
android:icon="@drawable/newspaper"
android:label="@string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
logcat的錯誤：
08-19 02:23:52.467 15372-15372/com.example.rory.readerapp E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.rory.readerapp, PID: 15372
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.example.rory.RssReaderApp.MainActivity$GetRSSDataTask.onPostExecute(MainActivity.java:113)
at com.example.rory.RssReaderApp.MainActivity$GetRSSDataTask.onPostExecute(MainActivity.java:79)
at android.os.AsyncTask.finish(AsyncTask.java:632)
at android.os.AsyncTask.access$600(AsyncTask.java:177)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5312)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
LogCat錯誤？向我們顯示錯誤 –
通過您的logcat錯誤的外觀...出於某種原因，您的listview將會變爲空，並且您正在將適配器設置爲空對象。 – Nate