android edittext获得文本,如何从android中的edittext获取文本？

我使用以下代码从文本框中获取值。但它不起作用它给出异常并强制关闭。如何获取文本框的值？任何人都可以帮助我吗？

public class alertwithlogin extends Activity {

/** Called when the activity is first created. */

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

showDialog(1);

}

@Override

protected Dialog onCreateDialog(int id) {

LayoutInflater factory = LayoutInflater.from(this);

final View textEntryView = factory.inflate(R.layout.alertedit, null);

return new AlertDialog.Builder(alertwithlogin.this)

.setTitle(R.string.alert_dialog_text_entry)

.setView(textEntryView)

.setPositiveButton(R.string.alert_dialog_ok, new DialogInterface.OnClickListener() {

public void onClick(DialogInterface dialog, int whichButton) {

EditText edittext = (EditText) findViewById(R.id.username_edit);

Toast.makeText(getApplicationContext(),edittext.getText(), Toast.LENGTH_SHORT).show();

}

})

.setNegativeButton(R.string.alert_dialog_cancel, new DialogInterface.OnClickListener() {

public void onClick(DialogInterface dialog, int whichButton) {

}

})

.create();

}

}

alertedit.xml

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:orientation="vertical">

android:id="@+id/username_view"

android:layout_height="wrap_content"

android:layout_width="wrap_content"

android:layout_marginLeft="20dip"

android:layout_marginRight="20dip"

android:text="@string/alert_dialog_username"

android:gravity="left"

android:textAppearance="?android:attr/textAppearanceMedium" />

android:id="@+id/username_edit"

android:layout_height="wrap_content"

android:layout_width="fill_parent"

android:layout_marginLeft="20dip"

android:layout_marginRight="20dip"

android:scrollHorizontally="true"

android:autoText="false"

android:capitalize="none"

android:gravity="fill_horizontal"

android:textAppearance="?android:attr/textAppearanceMedium" />