Android SQLiteDatabase空指针异常

我刚刚开始为Android开发。到目前为止，它令人惊讶地一帆风顺，但现在我在实现SQLite数据库时遇到了一些问题。我的代码基于Google的记事本示例（运行得非常好），但我不断收到向我抛出的空指针异常，它似乎与getWritableDatabase（）方法有关

冒犯者是QuizDbAdapter：

package com.mypackage.myapp;

import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class QuizDbAdapter {
   public static final String KEY_ROWID =         "_id";
   public static final String KEY_QUESTION =      "question";
   public static final String KEY_ANSWER1 =      "answer1";
   public static final String KEY_ANSWER2 =      "answer2";
   public static final String KEY_ANSWER3 =      "answer3";
   public static final String KEY_ANSWER4 =      "answer4";
   public static final String KEY_CORRECT_ANSWER =   "correct_answer";
   public static final String KEY_IMAGE =         "image";
   public static final String KEY_CATEGORIES =      "categories";
   public static final String KEY_IMAGEQUESTION =   "image_question";
   public static final String KEY_EXPLANATION =   "explanation";

   private static final String TAG = "QuizDbAdapter";
   private DatabaseHelper databaseHelper;
   private SQLiteDatabase database;

   private static final String DATABASE_CREATE =
      "create table questions (_id integer primary key autoincrement, "
        + "question text not null, answer1 text not null, answer2 text not null,"
        + "answer3 text not null, answer4 text not null, correct_answer integer not null,"
        + "image text not null, categories text not null, image_question boolean not null,"
        + "explanation text not null);"
        + "create table roadsigns (_id integer primary key autoincrement, "
        + ");";

   private static final String DATABASE_NAME = "mydb.sqlite";
   private static final int DATABASE_VERSION = 1;
   private static final String TABLE_QUESTIONS = "questions";

   private final Context context;

   private static class DatabaseHelper extends SQLiteOpenHelper {
      DatabaseHelper(Context c) {
         super(c, DATABASE_NAME, null, DATABASE_VERSION);
      }

      @Override
      public void onCreate(SQLiteDatabase db) {
         db.execSQL(DATABASE_CREATE);
      }

      @Override
      public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
         db.execSQL("DROP TABLE IF EXISTS questions");
            onCreate(db);
      }
   }

   public QuizDbAdapter(Context c) {
      this.context = c;
   }

   public QuizDbAdapter open() throws SQLException {
      databaseHelper = new DatabaseHelper(context);
      database = databaseHelper.getWritableDatabase();
      return this;
   }

   public void close() {
      databaseHelper.close();
   }

   public Cursor fetchAllQuestions() {
      return database.query(TABLE_QUESTIONS, new String[] { KEY_ROWID, KEY_QUESTION,
            KEY_ANSWER1, KEY_ANSWER2, KEY_ANSWER3, KEY_ANSWER4, KEY_CORRECT_ANSWER,
            KEY_IMAGE, KEY_CATEGORIES, KEY_IMAGEQUESTION, KEY_EXPLANATION },
            null, null, null, null, null);
   }

   public Cursor fetchQuestion(long rowId) throws SQLException {
      Cursor cursor = database.query(true, TABLE_QUESTIONS, new String[] { KEY_ROWID,
            KEY_QUESTION, KEY_ANSWER1, KEY_ANSWER2, KEY_ANSWER3, KEY_ANSWER4,
            KEY_CORRECT_ANSWER, KEY_IMAGE, KEY_CATEGORIES, KEY_IMAGEQUESTION,
            KEY_EXPLANATION }, KEY_ROWID + "=" + rowId, null, null, null, null, null);
      if (cursor != null)
         cursor.moveToFirst();
      return cursor;
   }
}

每当我尝试使用打开数据库时

QuizDbHelper databaseHelper = new QuizDbHelper(this);
databaseHelper.open(); // <-- X(

---

quick.newquick

是静态方法吗？如果是这种情况，

此

不可用（因此为空指针）。

是

quick.newquick

静态方法吗？如果是这种情况，

此

不可用（因此为空指针）。

确保数据库正确打开，就像您可以创建一些适配器类一样，您可以随时调用它

public SupplierAdapter open() throws SQLException {
        dbHelper = new DatabaseHelper(context);
        database = dbHelper.getWritableDatabase();
        return this;
    }

---

确保数据库正确打开，就像您可以创建一些适配器类一样，您可以随时调用它

public SupplierAdapter open() throws SQLException {
        dbHelper = new DatabaseHelper(context);
        database = dbHelper.getWritableDatabase();
        return this;
    }

---

抱歉，已将堆栈跟踪添加到原始帖子…抱歉，已将堆栈跟踪添加到原始帖子…遗憾的是，没有。为了更好的衡量，我记录了“this”，它返回了“com.mypackage.myapp”。Quiz@43e461d8“在我看来，这是一个有效的内存地址。忽略上面的内容。事实证明，poke是对的。事后看来很明显。对于将来发现此问题的任何人：确保作为上下文传递给SQliteOpenHelper的类实际上扩展了活动。我的问题是，这个测验是一个单件测验，这导致它或多或少地忽略了“扩展活动”部分。不幸的是，不是。为了更好的衡量，我记录了“this”，它返回了“com.mypackage.myapp”。Quiz@43e461d8“在我看来，这是一个有效的内存地址。忽略上面的内容。事实证明，poke是对的。事后看来很明显。对于将来发现此问题的任何人：确保作为上下文传递给SQliteOpenHelper的类实际上扩展了活动。我的问题是测验是一个单件，这导致它或多或少忽略了“扩展活动”部分。