我正在尝试从我的SharedPreference加载字符串对象但是我得到了一个NullPointerException ..我对此非常新，所以我可能没有正确使用它。
错误在
loadPreferences
at
SharedPreferences sharedPreferences = getPreferences（MODE_PRIVATE）;
这是我的代码，
public class OfflineActivity extends Activity implements NetworkObserver {
public GeoPoint ourLocationGeoPoint;
public List<PointOfInterest> pointList = new ArrayList<PointOfInterest>();
SharedPreferences pointsToAddToServer;
int index;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.offline_mode_layout);
//Load preferences
loadPreferences();
// Register for network status updates
if (!NetworkStatus.isConnectedToInternet(getApplicationContext())) {
NetworkStatus networkStatus = new NetworkStatus(this);
networkStatus.addObserver(new OfflineActivity());
Thread thread = new Thread(networkStatus);
thread.start();
}
// Acquire a reference to the system Location Manager
LocationManager locationManager = (LocationManager) this
.getSystemService(Context.LOCATION_SERVICE);
// Define a listener that responds to location updates
LocationListener locationListener = new LocationListener() {
public void onLocationChanged(Location location) {
// Called when a new location is found by the network location
// provider.
/**
* Save Location to geopoint
*/
int lat = (int) (location.getLatitude() * 1E6);
int lng = (int) (location.getLongitude() * 1E6);
ourLocationGeoPoint = new GeoPoint(lat, lng);
}
public void onStatusChanged(String provider, int status,
Bundle extras) {
}
public void onProviderEnabled(String provider) {
}
public void onProviderDisabled(String provider) {
}
};
for (String provider : locationManager.getAllProviders()) {
// Register the listener with the Location Manager to receive
// location updates
locationManager.requestLocationUpdates(provider, 0, 0,
locationListener);
}
}
/**
* Add new location to database
*
* @param view
*/
public void addCurrentLocation(View view) {
if (ourLocationGeoPoint != null) {
LayoutInflater inflater = LayoutInflater.from(this);
AlertDialog.Builder builder = new AlertDialog.Builder(this);
view = inflater.inflate(R.layout.new_location_dialog, null);
builder.setView(view);
// Set fields
final EditText titleBox = (EditText) view.findViewById(R.id.title);
final EditText descriptionBox = (EditText) view
.findViewById(R.id.description);
final RatingBar ratingBar = (RatingBar) view
.findViewById(R.id.ratingBar);
final Spinner categoryDropdownMenu = (Spinner) view
.findViewById(R.id.categoryDropDown);
// When User clicks Save
builder.setPositiveButton("Save",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int button) {
String title = titleBox.getText().toString();
String description = descriptionBox.getText()
.toString();
String category = ("" + categoryDropdownMenu
.getSelectedItem())
.toLowerCase(Locale.ENGLISH);
int ratingNum = (int) ratingBar.getRating();
Log.d("User Setting title / description to: ",
title + " : " + description);
// Create new Private Data field containing title /
// rating
PrivateField privateField = new PrivateField(
"Michael", title, ratingNum);
// Create new point of Interest
PointOfInterest point = new PointOfInterest(
category, description, ourLocationGeoPoint,
privateField);
// Add to list to update later
Gson gson = new Gson();
String pointOfInterest = gson.toJson(point);
savePreferences(String.valueOf(index), pointOfInterest);
// Increase our index
index++;
Log.d("OfflineActivity", "Saving point "
+ point.latitude + ":" + point.longitude);
Toast.makeText(
getBaseContext(),
"Will add point \""
+ point.getTitle()
+ "\" when network connection has been made.",
Toast.LENGTH_SHORT).show();
}
});
// When User clicks cancel
builder.setNegativeButton("Cancel",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
Log.d("Cancel setting Title & adding point", "");
return;
}
});
builder.show();
} else {
Toast.makeText(this, "Could not determine location :(",
Toast.LENGTH_SHORT).show();
}
}
@Override
public void updateStatus() {
loadPreferences();
SharedPreferences sharedPreferences = getPreferences(MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
for (PointOfInterest poi : pointList) {
UploadPointOfInterest uploadPointOfInterest = new UploadPointOfInterest(poi);
uploadPointOfInterest.execute();
}
// Clear preferences, reset index
editor.clear();
editor.commit();
index = 0;
}
/**
* Save preferences
* @param key
* @param value
*/
private void savePreferences(String key, String value) {
SharedPreferences sharedPreferences = getPreferences(MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString(key, value);
editor.commit();
}
/**
* Load saved Preferences
*/
private void loadPreferences() {
SharedPreferences sharedPreferences = getPreferences(MODE_PRIVATE); <-- NullPointer here
PointOfInterest temp;
PointOfInterest point;
Gson gson = new Gson();
/**
* Load all points from shared prefs
*/
for(int x =0; ;x++) {
index = x;
if(sharedPreferences.contains(String.valueOf(x))){
temp = gson.fromJson(sharedPreferences.getString(String.valueOf(x), null), PointOfInterest.class);
point = new PointOfInterest(temp.getType(),temp.getDescription(),temp.getGeoPoint(),temp.getPrivateField());
Log.d("Offline activity", "Loading point from sharedpref: " + point.getTitle());
pointList.add(point);
}
else {
break;
}
}
}
}
和堆栈跟踪，
02-01 03:32:53.046: E/AndroidRuntime(30325): Uncaught handler: thread Thread-14 exiting due to uncaught exception
02-01 03:32:53.046: E/AndroidRuntime(30325): java.lang.NullPointerException
02-01 03:32:53.046: E/AndroidRuntime(30325): at android.content.ContextWrapper.getPackageName(ContextWrapper.java:120)
02-01 03:32:53.046: E/AndroidRuntime(30325): at android.app.Activity.getLocalClassName(Activity.java:3413)
02-01 03:32:53.046: E/AndroidRuntime(30325): at android.app.Activity.getPreferences(Activity.java:3447)
02-01 03:32:53.046: E/AndroidRuntime(30325): at com.example.mapproject.OfflineActivity.loadPreferences(OfflineActivity.java:207)
02-01 03:32:53.046: E/AndroidRuntime(30325): at com.example.mapproject.OfflineActivity.updateStatus(OfflineActivity.java:177)
02-01 03:32:53.046: E/AndroidRuntime(30325): at com.example.mapproject.NetworkStatus.notifyObservers(NetworkStatus.java:58)
02-01 03:32:53.046: E/AndroidRuntime(30325): at com.example.mapproject.NetworkStatus.run(NetworkStatus.java:68)
02-01 03:32:53.046: E/AndroidRuntime(30325): at java.lang.Thread.run(Thread.java:1096)
02-01 03:32:53.226: E/SemcCheckin(30325): Get crash dump level : java.io.FileNotFoundException: /data/semc-checkin/crashdump
答案 0 :(得分：4)
由于每个人都没有阅读评论，OP的问题与偏好保存没有关系，它与他们实例化另一个Activity的事实有关。 Android 不可以很好地使用它，因为通过构造函数显式实例化的Activity不完整。所有操作都必须做的是改变
networkStatus.addObserver(new OfflineActivity());
到
networkStatus.addObserver(this);
因此Android不会创建活动的新（坏）实例。
每当你看到类似的东西时：
java.lang.NullPointerException 02-01 03：32：53.046： E / AndroidRuntime（30325）：at
<强> android.content.ContextWrapper.getPackageName（ContextWrapper.java:120）
它通常与幕后操作（如何getPackageName()如此为空？）由显式实例化引起。
答案 1 :(得分：0)
public void savePreference(String stringToSave){
Editor custom_editor = custom_editor.edit();
custom_editor.putString(NameToSaveAs, stringToSave);
custom_editor.commit();
}
public String getPreferences(){
return yourPreferenceName.getString(NameToSaveAs, null);
}
您可以使用其他待办事项修改此内容。
答案 2 :(得分：0)
问题出在这里，
// Register for network status updates
if (!NetworkStatus.isConnectedToInternet(getApplicationContext())) {
NetworkStatus networkStatus = new NetworkStatus(this);
networkStatus.addObserver(new OfflineActivity()); <----
Thread thread = new Thread(networkStatus);
thread.start();
}
应该是：
// Register for network status updates
if (!NetworkStatus.isConnectedToInternet(getApplicationContext())) {
NetworkStatus networkStatus = new NetworkStatus(this);
networkStatus.addObserver(this); <----
Thread thread = new Thread(networkStatus);
thread.start();
}