[英]NullPointerException on SharedPreferences (Seems to be the context)
我剛開始使用Android Studio,目前正在嘗試制作一個簡單的應用程序,讓您做筆記並保存筆記,也許還有一些“待辦事項”功能。 我正在嘗試使用Json / Gson將它們存儲在SharedPreferences中。 遵循了一些在線教程,但似乎無法使其正常工作。 這里類似的問題也沒有幫助。 到目前為止,這是我的代碼:
MainActivity.java:
public class MainActivity extends AppCompatActivity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
NoteHandler noteHandler = new NoteHandler();
noteHandler.newNote("test", 1);
}
}
Note.java:
public class Note {
public enum State { TODO, DONE };
private String content;
private State state;
private Date date;
private int id;
private Note parent;
public Note(String content, State state, Date date, Note parent, int id) {
this.content = content;
this.state = state;
this.date = date;
this.parent = parent;
this.id = id;
}
/* Getters and setters */
}
NoteHandler.java:
public class NoteHandler {
NotePrefs notePrefs = new NotePrefs(ContextGetter.getAppContext());
void newNote(String content, int id) {
newNote(content, null, id);
}
void newNote(String content, Note parent, int id) {
Note newNote = new Note(content, Note.State.TODO, new Date(), parent, id);
notePrefs.saveNote(newNote);
}
Note getNote(int id) {
return notePrefs.loadNote(id);
}
}
NotePrefs.java:
public class NotePrefs {
private final String PREFS_NAME = "notes.NotePrefs";
private static SharedPreferences settings;
private static SharedPreferences.Editor editor;
private static Gson gson = new Gson();
public NotePrefs(Context ctx) {
if(settings == null) {
settings = ctx.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
}
editor = settings.edit();
}
public static void saveNote(Note note) {
editor.putString("" + note.getId(), gson.toJson(note));
editor.apply();
}
public static Note loadNote(int id) {
String noteJson = settings.getString("" + id, "");
return gson.fromJson(noteJson, Note.class);
}
public static List<Note> loadAllNotes() {
return null;
}
}
ContextGetter.java:
public class ContextGetter extends Application {
private static Context context;
public void onCreate(){
super.onCreate();
context = getApplicationContext();
}
public static Context getAppContext() {
return context;
}
}
例外:
java.lang.RuntimeException:無法啟動活動ComponentInfo {notes / notes.MainActivity}:java.lang.NullPointerException:嘗試調用虛擬方法'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)'在空對象引用上
似乎仍然無法將我的腦袋纏在上下文中。 任何幫助表示贊賞!
另外,什么是實現loadAllNotes()的好方法?
干杯
您應該添加name=".ContextGetter"
以在<application>
標記中顯示。
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