无论我如何修改预执行类或ProgressBar的声明,我都会在上下文中获得空指针异常。我已经尝试了其他人为修复错误而实现的几个解决方案,但没有任何效果。
我的应用程序应在CatalogActivity中单击按钮时导出CSV。
已经花了几天时间......我的帮助非常感谢。
CatalogActivity:
@Override
public boolean onOptionsItemSelected(MenuItem item) {
//export data to CSV using method in InventoryProvider via separate java class ExportDatabaseCSVTask
case R.id.export_to_csv:
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
new ExportDatabaseCSVTask().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
} else {
new ExportDatabaseCSVTask().execute("");
}
ExportDatabaseCSVTask:
public class ExportDatabaseCSVTask extends AsyncTask<String, String, Boolean> {
private Context context;
private ProgressDialog dialog;
InventoryProvider iProvider;
@Override
protected void onPreExecute() {
super.onPreExecute();
dialog = new ProgressDialog(context); ---ERROR HERE
this.dialog.setMessage("Saving. Please Wait...");
this.dialog.show();
}
@TargetApi(Build.VERSION_CODES.O)
protected Boolean doInBackground(final String... args) {
File exportDir = new File(Environment.getExternalStorageDirectory(), "/codesss/");
if (!exportDir.exists()) { exportDir.mkdirs(); }
File file = new File(exportDir, "inventory.csv");
try {
file.createNewFile();
CSVWriter csvWrite = new CSVWriter(new FileWriter(file));
Cursor curCSV = iProvider.raw(CONTENT_URI);
csvWrite.writeNext(curCSV.getColumnNames());
while(curCSV.moveToNext()) {
String arrStr[]=null;
String[] mySecondStringArray = new String[curCSV.getColumnNames().length];
for(int i=0;i<curCSV.getColumnNames().length;i++)
{
mySecondStringArray[i] =curCSV.getString(i);
}
csvWrite.writeNext(mySecondStringArray);
}
csvWrite.close();
curCSV.close();
return true;
} catch (IOException e) {
return false;
}
}
protected void onPostExecute(final Boolean success) {
if (this.dialog.isShowing()) { this.dialog.dismiss(); }
if (success) {
Toast.makeText(CatalogActivity.getApplicationContext, "this is my Toast message!!! =)", Toast.LENGTH_LONG).show();
ShareFile();
} else {
Toast.makeText(CatalogActivity.getApplicationContext, "Export failed", Toast.LENGTH_SHORT).show();
}
}
private void ShareFile() {
File exportDir = new File(Environment.getExternalStorageDirectory(), "/codesss/");
String fileName = "myfile.csv";
File sharingGifFile = new File(exportDir, fileName);
Intent shareIntent = new Intent(android.content.Intent.ACTION_SEND);
shareIntent.setType("application/csv");
Uri uri = Uri.fromFile(sharingGifFile);
shareIntent.putExtra(Intent.EXTRA_STREAM, uri);
CatalogActivity.getApplicationContext.startActivity(Intent.createChooser(shareIntent, "Share CSV"));
}
logcat的:
2019-03-02 21:05:16.109 7122-7122 / com.example.android.name E / AndroidRuntime:FATAL EXCEPTION:main进程:com.example.android.stockpile,PID:7122 java.lang.NullPointerException:Attempt to在android.app.AlertDialog上的android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:224)上的空对象引用上调用虚拟方法'android.content.res.Resources $ Theme android.content.Context.getTheme()'。 (AlertDialog.java:201)在android.app.AlertDialog。(AlertDialog.java:197)android.app.AlertDialog。(AlertDialog.java:142)在android.app.ProgressDialog。(ProgressDialog.java:94)at位于com.example.android.stockpile.CatalogActivity.onOptionsItemSelected的android.os.AsyncTask.executeOnExecutor(AsyncTask.java:648)中的com.example.android.stockpile.ExportDatabaseCSVTask.onPreExecute(ExportDatabaseCSVTask.java:40)(CatalogActivity.java: 199)在android.app.Activity.onMenuItemSelected(Activity.java:3435)的android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:436)at在android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:109)android.support.v7.app.AppCompatDelegateImpl.onMenuItemSelected的android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:196) AppCompatDelegateImpl.java:888)在android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:840)在android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:158)在android位于android.support.v7.widget.ActionMenuView的android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:981)上的.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:991)位于android.view的android.view.View.performClick(View.java:6256)的android.support.v7.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:151)上的.invokeItem(ActionMenuView.java:625)。在android.os.Handler.dispatchMessage(Handler.java)的android.os.Handler.handleCallback(Handler.java:789)上查看$ PerformClick.run(View.java:24701) 98)在android.app.Looper.loop(Looper.java:164)的android.app.ActivityThread.main(ActivityThread.java:6541)at com.android的java.lang.reflect.Method.invoke(Native Method) .internal.os.Zygote $ MethodAndArgsCaller.run(Zygote.java:240)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)2019-03-02 21:05:16.114 1694-4875 / system_process W / ActivityManager:强制完成活动com.example.android.name/.CatalogActivity
如果我尝试将以下内容添加到ExportDatabaseCSVTask:
public ExportDatabaseCSVTask(Context context) {
this.context = context;
}
我在CatalogActivity中收到以下错误:
case R.id.export_to_csv:
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
new ExportDatabaseCSVTask().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR); - ERROR
} else {
new ExportDatabaseCSVTask().execute(""); --ERROR
}
错误:类ExportDatabaseCSVTask中的构造函数ExportDatabaseCSVTask不能应用于给定类型; new ExportDatabaseCSVTask()。executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR); ^ required:找到上下文:无参数原因:实际和形式参数列表的长度不同F:\ Android projects \ Stockpile \ app \ src \ main \ java \ com \ example \ android \ stockpile \ CatalogActivity.java:203:错误:类ExportDatabaseCSVTask中的构造函数ExportDatabaseCSVTask不能应用于给定类型; new ExportDatabaseCSVTask()。execute(“”); ^ required:找到的上下文:无参数原因:实际和形式参数列表的长度不同
您的上下文将始终为null,因为他没有初始化。通过构造函数初始化上下文
private Context context;
public ExportDatabaseCSVTask(Context context) {
this.context = context;
}
这个例子如何在添加构造函数后调用:
new ExportDatabaseCSVTask(this)
还要注意在ExportDatabaseCSVTask类中传递上下文,通过WeakReference更好地包装上下文。