java - SQLite 游标处的 NullPointerException

Question

我在我的 Google Play 控制台中收到了这份崩溃报告。我从来没有在自己的设备和模拟器上遇到过这样的崩溃。

Caused by: java.lang.NullPointerException: 
  at android.preference.PreferenceManager.getDefaultSharedPreferencesName (PreferenceManager.java:498)
  at android.preference.PreferenceManager.getDefaultSharedPreferences (PreferenceManager.java:487)
  at DictionaryDB2.getHistoryWords (DictionaryDB2.java)
  or                     .hasObject (DictionaryDB2.java)

这是getHistoryWords

public List<Bean> getHistoryWords() {
        SharedPreferences SP = PreferenceManager.getDefaultSharedPreferences(MyApp.getAppContext());
        String historyNum = SP.getString("history","50");

        SQLiteDatabase db = initializer.getReadableDatabase();

        String sql = "SELECT * FROM " + HISTORY_NAME +
                " WHERE " + STATUS + " ORDER BY " + STATUS + " DESC LIMIT " + historyNum ;

        Cursor cursor = db.rawQuery(sql, null);

        List<Bean> wordList = new ArrayList<Bean>();
        while(cursor.moveToNext()) {
            int id = cursor.getInt(0);
            String english = cursor.getString(1);
            String malay = cursor.getString(2);
            wordList.add(new Bean(id, english, malay));
        }

        cursor.close();
        db.close();
        return wordList;
    }

这是MyApp:

public class MyApp extends Application {  

    private static Context context;

    public void onCreate() {
        super.onCreate();
        MobileAds.initialize(this, "my admob ");
        MyApp.context = getApplicationContext();
    }

    public static Context getAppContext() {
        return MyApp.context;
    }
}

如何解决这个问题？

Answer 1

与其在 getHistoryWords() 中使用对应用程序上下文的静态引用，我建议您添加一个上下文参数，并从主 Activity 中传递该参数。

public List<Bean> getHistoryWords(Context context) {
    SharedPreferences SP = PreferenceManager.getDefaultSharedPreferences(context);
    ...
}

然后在主 Activity 中...

public void onCreate( Bundle savedInstanceState ) {
    setContentView( ... );
    ...
    List<Bean> beans = getHistoryWords( getApplicationContext() );
    ...
}